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A novel fusion-based deep learning model for sentiment analysis of COVID-19 tweets

Undoubtedly, coronavirus (COVID-19) has caused one of the biggest challenges of all times. The ongoing COVID-19 pandemic has caused more than 150 million infected cases and one million deaths globally as of May 5, 2021. Understanding the sentiment of people expressed in their social media comments can help in monitoring, controlling, and ultimately eradicating the disease. This is a sensitive matter as the threat of infectious disease significantly affects the way people think and behave in various ways. In this study, we proposed a novel method based on the fusion of four deep learning and one classical supervised machine learning model for sentiment analysis of coronavirus-related tweets from eight countries. Also, we analyzed coronavirus-related searches using Google Trends to better understand the change in the sentiment pattern at different times and places. Our findings reveal that the coronavirus attracted the attention of people from different countries at different times in varying intensities. Also, the sentiment in their tweets is correlated to the news and events that occurred in their countries including the number of newly infected cases, number of recoveries and deaths. Moreover, common sentiment patterns can be observed in various countries during the spread of the virus. We believe that different social media platforms have great impact on raising people’s awareness about the importance of this disease as well as promoting preventive measures among people in the community.


Introduction
The sudden spread of COVID-19 virus observed in animal and/or bird populations which became infectious and damaging in humans has become a cause of concern for scientists and policymakers. The COVID-19 called by the World Health Organization as a pandemic has endangered human lives all over the world [1]. Some geographies have few cases, others with early community transmission have a few hundred, and those with uncontrolled, pervasive transmission have tens of thousands. To minimize the number of infected people, several strategies including social distancing measures, travel bans, self-quarantines, and business closures have been implemented, which rapidly transform the structure of societies all around the world. This situation progresses day by day; therefore, speedy and on-time information release is a top priority for disease control and prevention. Besides, the threat of infectious disease significantly affects how people think and behave in various ways. Some of the observed changes over the last months are stockpiling and increased fear has substantial consequences on societal well-being, health, and the global economy [2]. In response to this unprecedented situation, governments have initiated extraordinary public-health and economic responses. We are observing a remarkable restructuring of the economic and social order in which business and society have traditionally operated.
Over the last few months, some epidemiological studies have been done to characterize and control the coronavirus disease. In a study conducted by Holshue et al. [3], described the identification, diagnosis, clinical course, and management of the first case of 2019-nCoV infection confirmed in the United States. Their findings emphasized the importance of direct management between clinicians and public health authorities at the local, state, and federal levels, as well as the need for rapid broadcasting of both hygiene information to prevent more spread of the disease and clinical information to care patients with this evolving infection. In another study published by Perlman [4] highlighted the importance of collecting many temporally and geographically unrelated clinical isolations to evaluate the extent of virus transformation and measure whether these transformations specify adaptation to the human host. One of the topics mentioned worth studying is the impact of physical-distancing measures. We know that meticulous, at-scale physical-distancing measures can result in substantial reduction in the number of new COVID-19 cases. Though, the range of methods in use and level of severity with which they are being employed to learn about what exactly works and how long it takes requires more investigation. Many studies are being conducted to find out more about the epidemiological impacts of this disease and much still needs to be learned about this infection.
The other key concern during this unprecedented time is the resilience of economy in response to this pandemic. In a study conducted by FitzGerald et al. [5], COVID-19 pandemic is coined as ''instant economic crisis'', and they investigated how deep and long the economic crisis will last. They discussed the cautious reopening of social and economic lives. Also, it studied the effects of restrictions applied to populations to halt the fast spread of the virus, including quarantines, stay-at-home orders, business closures, and travel prohibitions which led to an enormous decline in world economy. The data to quantify these impacts are still coming; however, early statistics show that 30 million Americans so far have applied for unemployment aids in the week ending on May 9th. To deal with this situation, they proposed 5 stages of strategies, including resolve, resilience, return, re-imagination, and reform. Cadena et al. [6] carried out a study on how to restart national economies during the coronavirus crisis. They indicated that the world must act on both controlling the virus and alleviating the negative impact on citizens' businesses at the same time. The improvement we make on those fronts will help to shape the economic recovery. Illanes et al. [7] investigated the impact of coronavirus on higher education and how they respond. They proposed an integrated nerve center which is a simple, flexible, multidisciplinary construct intended to adjust to fast-changing conditions. It contained four types of actions including discover, decide, design, and deliver. The main goal of the proposed integrated nerve center is for the institution to be efficient in getting ahead of events and react timely, cleverly, and tactically. Qiu et al. [8] scrutinized the role of several socioeconomic factors in facilitating the transmissions of COVID-19 at the local and cross-city levels in China. A machine learning method is used to choose critical parameters that robustly can predict virus transmission among the rich exogenous weather characteristics.
Nowadays, many methods to share information have been incorporated by massive social media platforms in high speed, and penetration. More than 2.9 billion individuals use social media frequently, and a lot of them for long periods of time. Sharma et al. [9] designed a dashboard to analyze the misinformation in Twitter conversations during these unprecedented times. This dashboard provided visibility to the social media discussions around coronavirus and the quality of information shared on the platform as the situation progressed. They also designed a detection method to find misleading, deceiving and clickbait contents from Twitter information flow. In another study, Merchant and Lurie [10] investigated the role of social media as a key tool in handling the on-going pandemic along with changing features of preparedness and response for future disasters. They emphasized that social media should be utilized to circulate accurate information about different critical topics during the pandemic, like when to test, what to do with the results, and where to receive care. In addition to the mentioned problems, few studies addressed the problem of event detection [11]. For example, Doulamis et al. [11], classified twitter analysis methods based on tasks, types of events, and the orientation of tweets' content. In a similar study, Farzindar and Khreich [12] reviewed event detection in Twitter data based on the types of events, features, detection methods, and tasks. Recently, Saeed et al. [13] classified existing methodologies for Twitter event detection and highlighted their limitations. They also proposed few solutions to address the shortcomings of existing methodologies.
The people are being forced out of public spaces, much discussion about Coronavirus now happens online, e.g., on social media platforms such as Twitter. As an increasing number of people rely on the social media platform, now the question is how we can leverage social media data to help with mitigation and control of COVID-19 pandemic. However, there is no study yet to understand the public's perception and opinion about COVID-19 pandemic. The goal of this study is to track people's sentiments to gauge how their expectations, perceptions, and behaviors change throughout the crisis across multiple countries over time. This study aims to answer the following questions related to coronavirus: • Is there a difference in the sentiment intensities of Twitter users shown in the coronavirus-related tweets in eight countries?
• Is there a meaningful correlation between the number of newly infected cases, new recoveries, and deaths with tweets' sentiment in different dates and countries?
To answer the above-mentioned research questions, we proposed a new method based on the fusion of deep and classical supervised learning methods. In this approach, four deep learning and one classical supervised classification models are first trained on a large labeled dataset of tweets and then, as a late fusion method, a meta learner is trained on the features obtained using five classification methods. This trained learner is finally used to classify the unlabeled coronavirus-related tweets. The reason for applying both classical and deep classification models is that different types of test samples can be classified using these models [14]. Also, it has been shown that the fusion method can achieve higher performance than simple machine learning methods in different applications [15][16][17][18][19].
In addition to the two above-mentioned coronavirus-related questions, we aim to answer the following research question: • Does the proposed fusion model improve the performance of sentiment classification of tweets?
Understanding the dynamics of public responses to such events under uncertainty is necessary to avoid unnecessary panic and provide a proper response. This study used geotagged Twitter data from eight English-speaking countries including the US, UK, Canada, and Australia as well as some non-English-speaking counties including Iran, China, Spain, and Italy to collect a wide range of public opinion data and track the public reaction to coronavirus. The results of this study will help to identify factors that shape the public reactions to coronavirus in the context of this rapidly unfolding public health emergency. The study findings will also help to provide guidance to the public health authorities to communicate well with people and help to provide public health responses to those who are most susceptible to the virus.
The main contributions of this study are as follows: • Proposes a new fusion model for sentiment analysis of tweets by combining state-of-the-art transformer-based deep models. This model is trained and validated using large-scale Twitter dataset.
• A new real-world large-scale Twitter dataset of coronavirusrelated tweets of people in 8 highly affected countries is collected and analyzed to develop the deep model.
• Collected Twitter dataset, Google trend data from 8 countries are analyzed to find meaningful patterns related to coronavirus.
• Analysis of coronavirus-related tweets and Google trend data for the 8 countries is presented to find the sentiment of people in different time intervals and countries.
• Results of this study may help social scientists and governments in finding the temporal and geographical sentiment of people regarding the coronavirus.

Existing research on the analysis of COVID-19 in microblog texts
Few studies on Twitter analysis of COVID-19 have been reported recently. In this section, we briefly discussed the most related studies and compared them with the current study.
Zhu et al. [20] analyzed 8 topics extracted from Weibo microblog data from spatiotemporal perspectives. There are four major differences between this study and our study. First, they analyzed Weibo text while we analyzed Twitter data. Second, they focused on different spatial zones of China while we analyzed tweets from 8 different countries. Third, they analyzed microblog texts from spatiotemporal view while we analyzed tweets from an emotional point (i.e., sentiment analysis). Fourth, they addressed classical machine learning methods for extracting and analyzing topics in microblog texts while we employed both deep learning and classical machine learning methods for sentiment classification of tweets.
Zhao and Xu [21] discussed the public attention in two-month time intervals to COVID-19-related events in China through Sina Microblog. They analyzed the hot topics and sentiment trends in this time interval and reported ''the emotional tendency of public toward the COVID-19 epidemic-related hot topics has changed from negative to neutral, with negative emotions weakening and positive emotions increasing as a whole'' [21]. The main differences between this study and ours are as follows. First, they analyzed Sina microblog texts while we analyzed Twitter data. Second, they focused only on China while we analyzed tweets for 8 different countries. Third, they analyzed a short two-month period while we analyzed a six-month time interval. Four, they focused on the public attention toward different aspects of COVID-19, while we focused on the sentiment change over time.
Saleemi [22] analyzed the relationship between cost-based market liquidity and investors' sentiment analysis of Twitter data. The results of this study showed that ''Twitter sentiment indicators are relevant in the forecasting of market liquidity and execution cost at market level'' [22]. The main differences between this study and ours are as follows. One, they analyzed the Australian Securities Exchange (ASX) related tweets while we analyzed COVID-19 Twitter data. Second, they focused only on Australia while we analyzed tweets for 8 different countries. Third, they focused on the relation between market and sentiment of investors' tweets, while we addressed the problem of sentiment analysis of the COVID-19 related tweets.
Barkur et al. [23] proposed a model to analyze the sentiment of Indians through lockdown announcements. They analyzed 24000 tweets and showed that ''Even though there is negativity, fear, disgust, and sadness about the lockdown, the positive sentiments stood out'' [23]. The main differences between this study and ours are as follows. First, they only analyzed Indian related tweets while we analyze tweets for 8 different countries. Second, they only analyzed tweets for a short time interval from March 25th to 28th, 2020 while we analyzed tweets published in a six-month time interval. Thirdly, they focused on emotions like fear, disgust, and sadness, while we addressed the problem of sentiment analysis of COVID-19 related tweets. Finally, they only considered two keywords namely, #India Lockdown and #India fights Corona for collecting tweets while we used more than 20 COVID-19 keywords.
Wang et al. [24] compared public opinion towards COVID-19 between California and New York. They reported that ''COVID-19 tweets from California had more negative sentiment scores than New York'' [24]. Also, they showed similar trend over time in both states. The main differences between this study and ours are as follows. First, they only analyzed two states of USA tweets while we analyzed tweets for 8 different countries. Second, they only analyzed tweets for a short time interval from March 5, 2020, to April 2, 2020, while we analyzed tweets published in a six-month time interval. Third, they employed the VADER tool for sentiment analysis while we proposed a fusion of deep and classical machine learning models for sentiment analysis of tweets.
Chen et al. [25] analyzed the use of neutral and controversial terms for COVID-19 on Twitter. They showed substantial difference between the emotion in tweets containing controversial and non-controversial terms. The main differences between this study and ours are as follows. First, they analyzed a few specific controversial and non-controversial terms while we analyzed general COVID-19 terms. Second, they only analyzed tweets for a short time interval from March 5, 2020, to April 5, 2020, while we analyzed tweets published in a six-month time interval. Third, they employed LIWC2015 that is a dictionary-based linguistic analysis tool for sentiment analysis while we proposed a fusion of deep and classical machine learning models for sentiment analysis of tweets. Four, they focused on emotions like fear, disgust, and sadness, while we addressed the problem of sentiment analysis of COVID-19 related tweets.
Jelodar et al. [26] proposed to use a recurrent deep learning method for sentiment classification of microblogging texts related to COVID-19. Specifically, they proposed a model to extract COVID-19 related topics. There are few major differences between this study and our study. First, they analyzed reddit text while we analyze Twitter data. Second, they did not consider country-specific comments while we analyzed tweets for 8 different countries. Third, they used long short-term memory (LSTM) as a deep learning method while we employed both deep learning and classical machine learning methods for sentiment classification of tweets. Finally, they focused on the extraction of topics while we focused on the extraction of sentiment intensity of Twitter users toward COVID-19.
In summary, the existing works on the analysis of COVID-19 microblog texts are proposed for spatiotemporal analysis [20], topic analysis [21,26], market liquidity [22], event analysis [23], term analysis [25]. In contrast to the existing methods, this study focused on the extraction of sentiment intensity of Twitter users in 8 countries toward COVID-19. In addition to this difference, this work presents a new COVID-19 Twitter dataset extracted for a duration of four months. We showed the relationship between a number of infected cases and death due to COVID-19 and the change in the sentiment intensity of Twitter users.

Motivation
Sentiment analysis [27,28] aims to find people's opinions and attitudes from their comments in social media toward different aspects of products and events [28,29]. It has been studied in different languages [30][31][32][33][34] and performed on the comments of different social media [35][36][37]. Sentiment analysis of different Twitter data has been studied in the literature such as critical events [38], drug reviews [14], classifying Twitter streams during outbreaks [39], prediction of the early impact of literature [40], Table 1 List of keywords used to crawl the tweets and retrieve the normalized Google Trends.

Corona
Coronavirus halal products on Twitter [41], consumer sentiment towards a particular brand [42,43], and many other subjects [42][43][44][45][46]. In the past few decades, advances in machine learning and particularly deep learning technologies/methods, sentiment analysis of variety of resources have gained significant attention [47,48]. Hence, in this study, we have investigated the public opinion towards COVID-19 in eight countries mentioned in 4.1 to 4.8 using artificial intelligence techniques. We have made an effort to understand to seriousness of people to pandemic during the specified period. It may be noted that in this study, we are presenting the opinion of a group of people in these eight countries as case studies. Our results reflect the opinion and sentiments of citizens of these countries who tweeted their opinion about COVID-19 using the keywords shown in Table 1.
As discussed earlier, sentiment analysis is a meticulous method used to achieve public opinion of subjects. With the onset of coronavirus in late 2019 and its spread in early 2020, we have decided to focus on analyzing people's opinions and their thinking about the impacts of COVID-19 in eight countries which are severely affected by this disease. Examining past studies along with our previous experience shows that analyzing the people's general opinion can reveal how seriously people have taken the disease. This can provide insight to the governments and organizations about their involvement with this disease to develop better solutions to the disease for their citizens.

Data
In this study, we employed two sources of data, namely Google Trends and Twitter data. Google Trends data was used to analyze the interest of people to obtain information about COVID-19 disease using Google searches of related keywords. This type of data has been used for different applications including predicting the stock market [49], detecting emerging political topics on Twitter [50], and predicting the speed of national spread coronavirus [51]. Twitter data was used to analyze the change of sentiment in peoples' tweets over time. The same keywords and time intervals were used to obtain both Google Trends and Twitter data. Specifically, we crawled the data from 2020-01-01 to 2020-04-21 and filtered the initial dates for which there was no tweets associated with the keywords. This shifted the beginning of the time frame to 2020-01-24. Keywords listed in Table 1 were used to crawl the tweets and retrieve Google Trends. In addition to the above keywords, we also considered the hashtag sign at the beginning of keywords in the crawling process.

Google Trend data
Google Trends is a popular service offered by Google to obtain a normalized count of users' searches for a given phrase in a specific time frame. This service returns a number in the range 0 (lowest number of searches)-100 (highest number of searches) for the given keywords in the provided time frame. The normalized search number of Google Trends service used to compare ''Coronavirus'' and ''corona'' in a three-month time frame from 2020-01-24 is shown in Fig. 1.
In this study, we did not use the Google Trends website directly, instead, we used Pytrends 1 library written in the Python 1 https://pypi.org/project/pytrends/. language to have more flexibility in obtaining relevant data needed for our analysis.

Tweets
All keywords shown in Table 1 were used to filter the tweets extracted from 8 countries including United States, China, Iran, Italy, Spain, Australia, England, and Canada from 01/24/2020 to 04/21/2020: Twint 2 library of Python was used to collect geo-tagged tweets for the specified keywords and time frame. For each country, we selected only English tweets. Table 2 summarized the number of tweets, average length, and average number of words.

Proposed model
To evaluate the overall performance of the sentiment of tweets, a new fusion model is proposed in this study. The overall architecture of the proposed model is shown in Fig. 2. The first layer of the proposed method consists of four deep learning and one classical supervised classification models. The reason for applying both classical and deep classification models is that different types of test samples can be classified using these models [14]. In fact, according to the theory of three-way decisions [52], data samples that belong to the low-confidence decision region of one classifier may belong to the high-confidence decision region of another. Hence, using different types of classifiers may improve the overall confidence of the classification system and improve the accuracy of the results.
In the next subsections, each classification method is described.

Naïve Bayes Support Vector Machines (NBSVM)
This model is a text classification method proposed by Wang and Manning [53] takes the classical support vector machine (SVM) and combines it with Bayesian probabilities. In this model, word count features are replaced by Naïve Bayes log-count ratios: where, p and q are word count vectors for a binary classification problem with the label y (i) ∈ {−1, 1} as follows:  Here, f (i) ∈ R |V | is the feature count vector for training sample i and V is the set of features. This model is shown to be not only fast but also accurate for different text classification problems.
In the current study, we implemented a simple neural implementation of NBSVM which consists of two embedding layers followed by a sigmoid activation layer. The input document to this model is represented as a sequence of words IDs. The reason for using representation is that this model trains much faster than one using term-document matrix because the look-up mechanism of the embedding layer reduces the number of parameters and words. In the NBSVM model, the first embedding layer stores the Naïve Bayes log-count ratios which are the probability of a word appearing in a document in one class versus another. The second layer stores learned coefficients for each word in the document. The final prediction of the model is simply the dot product of these two vectors. The main benefit of this simple neural implementation is that its training process is very fast as it is using a graphics processing unit (GPU).

Convolutional Neural Network (CNN)
The 1D-CNN model used in this study is shown in Fig. 3(a) [54]. This model consists of an embedding layer which is used for mapping vocabulary indices into an embedding space. Specifically, given a sentence s composed of M words {w 1 , w 2 , . . . , w M }, we first transform each word w m into a word embedding r w m . Given a word w, its embedding r w is obtained using the following matrix-vector product: Global max pooling is used as the next operation to downsample feature maps and to reduce the parameter size of the following fully-connected layers. Max-pooling is a popular pooling strategy to select the most significant feature msf of the feature map M as follows: The outputs of the global max-pooling layer are concatenated and sent to the next layer as a pooled feature vector. A dense (fully connected) layer is used as the hidden layer of the network followed by a dropout layer. Finally, the output layer consisting of a dense layer with two Softmax cells is used for classification. The parameter setting of the CNN model is shown in Table 3.

Bidirectional Gated Recurrent Network (BiGRU)
To model long dependencies in the text, gated recurrent network (GRU) is used [55]. Bidirectional GRU (BiGRU) is a special type of GRU which extracts sequential dependencies in both forward and backward directions. The BiGRU enables the model to consider both the preceding and succeeding contexts. This is an important feature because considering the context of sentiment words is an important problem in sentiment analysis applications [56]. In the proposed system, the BiGRU model shown in Fig. 3(b) is used. Similar to the CNN model described in 3.3.2, BiGRU model consists of an embedding layer for mapping vocabulary indices into an embedding space followed by a dropout layer. However, this model employed pre-trained word vectors trained on ''common crawl'' and ''Wikipedia'' using fastText model [57]. This word vector is trained using CBOW with position-weights. After the dropout layer, a BiGRU layer is applied to extract both the forward and backward contexts. This layer consists of GRU cells which employ two gates, an update gate r combining forget and input gates used in standard Long Short-Term Memory cells (LSTMS), and a reset gate z. The update and reset mechanisms are based on the following functions: where, δ is the logistic sigmoid function, U and W are the weight matrices of gates, x t and h t are input and hidden state, and b shows the bias vector. The output of the cell is computed using the value of the input and the hidden state. The hidden state of the cell is computed using the following functions: To extract both the future and preceding contexts, two hidden layers ⃗ h t and ← − h t , are combined in the BiGRU layer. This provides the temporal information flow in both directions.
In addition to the global max pooling, a global average pooling layer is also applied to the output of the BiGRU model to obtain different feature maps. The outputs of the pooling layers are then concatenated and sent to a fully connected dense output layer. The parameter setting of the BiGRU model is shown in Table 4.

fastText
The fastText model used in this study (see Fig. 3) is similar to the original fastText model [58] and has similar embedding, spatial dropout, and global max-pooling layers as described in 3.3.2 and 3.3.3. Moreover, the fastText model uses batch normalization layer to improve the training speed and performance of the model.
The input to the fastText model is a bag-of-words representation. This representation is then fed to a lookup layer, where the embeddings are computed for every single word. In the next step, word embeddings are averaged to attain a single embedding for the whole text. The hidden layer has Max_features × Embedding_ dim parameters. Finally, the averaged vector is fed to a linear classifier which is a softmax function in this study. The parameter setting of the fastText model is shown in Table 5.

DistilBERT
DistilBERT [58] is proposed to reduce the size and improve the training speed of original bidirectional encoder representations from transformers (BERT) model [59]. The DistilBERT model is pre-trained by concatenating ''English Wikipedia'' and ''Toronto book corpus'' [60]. The original BERT model has 24 transformer blocks, a hidden size of 1024, and 16 self-attention heads with 340 million parameters. In the DistilBERT model, the number of layers is reduced by a factor of two. This makes the DistilBERT model 60% faster than the original BERT model [59]. As pointed out in [59], ''DistilBERT was trained on 8 16GB V100 GPUs for approximately 90 hours''. This is much faster and less memory intensive than similar architectures such as RoBERTa [61] model which requires one day of training on 1024 32GB V100.

Meta learner
To fuse the five base learners (see 3.3.1 to 3.3.5), several strategies can be used. In this study, we selected a stacked generalization mechanism. Stack generalization is a fusion method used to train a new learner (meta learner) to fuse the output of base learners, trained on the same training data set [60,62]. Stack generalization conditionally assigns different weights to its inputs (i.e., the outputs of base learners) [63]. The pseudo-code   of stack generalization mechanism is shown in Algorithm 1 [48]. The input to the algorithm is a training dataset D, a test vector X ′ , a set of base learners L 1 , . . . , L n , and a meta learner L. The result of applying algorithm to the test vector X ′ is a prediction vector Y ′ which specifies the labels of samples in X ′ . The first step in the algorithm is to split D into S almost equal folds. Then, each learner is applied to all folds except one (D −i ) and produce a temporary prediction vector h i . Finally, a new training dataset D ′ for the meta learner is created by concatenating the temporal predictions. This new dataset is used to train the meta learner. When using stack generalization, the base learners must be accurate and diverse [60]. Hence, we selected five different classification methods described in 3.3.1 to 3.3.5. Specifically, all the base learners are accurate as they have lower classification errors than the random classifier and they are expected to be diverse as their structures are totally different. The XGBoost method [64] is used in this study as the meta learner. This method is a computationally efficient implementation of an iterative gradient boosting algorithm to create a fusion of weak regression trees [65].

Training the model
The Twitter data downloaded (see 3.2.2) has no sentiment label and hence cannot be used to train the models. Our proposed model is a deep learning model, needs a large training dataset to train the model. To address this problem, we used the Stanford Sentiment140 dataset [66] containing 1,600,000 tweets labeled as positive or negative. During the training process, we randomly selected 80% of the tweets for training the models and 20% for validation. Table 2, more than one million Twitter posts are collected for 8 countries during these three months. In order to analyze these data, we applied the six above-mentioned deep learning methods to the tweets of the Stanford Sentiment140 dataset [66]. This is because we aim to evaluate the performance of these algorithms on such well-known labeled data. To do so, as discussed earlier, we employed the following algorithms: CNN, BiGRU, fastText, NBSVM, DistilBERT, and the proposed fusion model. Also, to evaluate the performance of these methods, we used both accuracy and F1-score in the testing stage as described in Eqs. (11) to (14).

As shown in
Accuracy = TP + TN TP + FP + TN + FN (14) where, TP, TN, FP, and FN are true positive, true negative, false positive, and false negative, respectively. The obtained results are shown in Table 6.
It can be noted from Table 6 that, the proposed method showed better performance than CNN, BiGRU, NBSVM, and fast-Text. Also, yielded slightly better performance than DistilBERT. This is promising as DistilBERT is a state-of-the-art method which is widely used in many text mining applications [58].
The normalized number of corona-related searches and the daily percentage of positive and negative sentiments in 8 different countries from 2020-01-24 to 2020-04-23 are discussed in the following subsections.

China
The graph of a normalized number of corona-related keywords (see Table 1) searches versus the time duration (from 2020-01-24 to 2020-04-23) in China is shown in Fig. 4.
As shown in Fig. 4, the number of searches from 2020-01-24 to 2020-02-21 is less than the rest of the time frame. This may be explained by considering the total number of cases and total deaths chart for China, as shown in Fig. 5. As can be seen in the figure, from 2020-02-21 the total number of cases remained almost constant and the number of deaths has reached a new steady state. This has led to more people paying attention to corona-related topics.
The overall trend of positive and negative sentiment toward the corona-related keywords for China is shown in Fig. 6. As shown in this figure, the lowest negative sentiment is about 35% of the maximum on 2020-03-08 while the highest is about 70% of the maximum on 2020-03-28. Most of the positive days are from 2020-02-28 to 2020-03-08. This interval is a special time frame with respect to the number of recoveries. As shown in Fig. 7, the highest number of recoveries occurred during this period.

Italy
The graph of normalized number of the corona-related keyword (see Table 1) searches versus time duration (from 2020-01-24 to 2020-04-23) for Italy is shown in Fig. 8.
Similar to China, a sudden growth in the number of searches occurred from 2020-02-21 for Italy. This may be due to two reasons; First, searches are influenced by global statistics. Second, as shown in Fig. 9, new patients in Italy have been reported since 2020-02-21.
The overall trend of positive and negative sentiment toward the corona-related keywords for Italy is shown in Fig. 10. It can be seen that the amount of negative sentiment in Italy is higher than China (the maximum of negative sentiment is about 75% and the minimum is about 55%). This may be due to the fact that China is the first country to experience COVID-19 and the reaction of people in social media especially on Twitter to this virus is less affected by the global news. Hence, less negative news affected the people who tweeted about COVID-19.

England
The graph of normalized number of corona-related keywords (see Table 1) searches versus time duration (from 2020-01-24 to 2020-04-23) for England is shown in Fig. 11. As shown in the figure, the major growth in the number of corona related searches occurred in 2020-03-10 which is 20 days later than similar change for Italy and China. This may be the result of observing the first patients in 2020-03-10 for England (see Fig. 12).
The overall trend of positive and negative sentiment towards the corona-related keywords for England is shown in Fig. 13. In England, between 12th March to the end of March, the percentage of negative sentiments is high compared to other days and remained almost constant during this time period. However, a small fall is seen on 5th April and a jump is observed after this which may be due to the increase in the number of deaths in England in this period.

United States
The graph of normalized number of corona-related keywords (see Table 1) searches for the United States is shown in Fig. 14. As shown in the figure, there is a steady increase in the number of searches from 2020-03-10 which may be due to new cases (see Fig. 15).
The overall trend of positive and negative sentiments towards the corona-related keywords for US is shown in Fig. 16. A relatively constant trend is observed in the US during this study period. This may be due to spread of new cases in the US compared to the countries analyzed in previous sections. The percentage of negative tweets started increase from March 12th due to the rise of new cases in the US. The negative rate is high for the following two weeks and remained stable. The peak in the US occurred on 27th March, which is close to 0.58. This is the start of sharp increase in the number of total cases and total deaths in the US as shown in Fig. 17.

Canada
The graph of normalized number of corona-related keywords (see Table 1) searches versus time period (from 2020-01-24 to 2020-04-23) for Canada is shown in Fig. 18. As displayed in the figure, most of the searches started from 2020-03-11 which is the starting point of new cases as shown in Fig. 19. The peak of the chart in Fig. 19 is at 2020-03-18 which corresponds to the start of a more increased rate in newly infected patients is shown in Fig. 20.
A very similar trend to the US is seen in Canada (see Fig. 21). The percentage of negative tweets remained high for approximately two weeks with a peak of 0.68 on 27th March. However, compared to the US, the valley point of negative sentiment is relatively lower for Canada which is about 0.35. This occurred in the first few days of the time frame and may be due to few numbers of reported cases in those days.

Australia
The graph of normalized number of corona-related keyword (see Table 1) searches versus time duration (from 2020-01-24 to 2020-04-23) for Australia is shown in Fig. 22. As shown in the figure, the time frame may be divided into two parts. The first part is from the beginning of the time frame to 2020-03-02 corresponding to the first part of Fig. 23 during which the number of daily new cases is almost zero. The second part begins from 2020-03-02 which continues till the end of the time frame.
The overall trend of positive and negative sentiment toward the corona-related keywords for Australia is shown in Fig. 24. The

Iran
The graph of a normalized number of corona-related keywords versus time duration from 2020-01-24 to 2020-04-23) for Iran   (see Table 1) is presented in Fig. 25. As illustrated in the figure, the normalized number of searches during the period 2020-01-24 to 2020-02-19 is less than the rest of the time frame. This        The overall trend of positive and negative sentiment toward the corona-related keywords for Iran is shown in Fig. 26. In contrast to most of the aforementioned countries, the negative sentiment for Iran is not constant and fluctuates from 25% to 80%. This can be explained by observing the fluctuation in the number of newly infected cases versus the number of recovered and discharged patients, as shown in Fig. 27.
The maximum negative sentiment for Iran is on 2020-04-02 which corresponds to the maximum number of active cases (see Fig. 28).

Spain
The normalized number of corona-related keywords (see Table 1) searches for Spain is shown in Fig. 29. As shown in the figure, the number of searches has increased from 2020-02-20. This is similar to China, Italy, and Iran and maybe due to international news about COVID-19. In 2020-03-08, we observed a sharp increase in the number of searches. As shown in Fig. 30, the number of total cases (674 on 2020-03-08 and 1231 on 2020-03-09) and total deaths (17 on 2020-03-08 and 30 on 2020-03-09) have increased during this period.
The overall trend of positive and negative sentiment towards the corona-related keywords for Spain is shown in Fig. 31.
As shown in the figure, the minimum negative sentiment values (about 40%) are observed from 2020-02-28 to 2020-03-09. This time period corresponds to the valley point of charts in Fig. 30. Moreover, as shown in Fig. 32, new cases and new recoveries are reported after this time period. The maximum negative sentiment values are seen around 2020-03-20 when new cases started as shown in Fig. 32.

Discussions
This section discusses the results obtained in eight countries (Sections 4.1 to 4.8) and summarized the answers to the research questions listed in Section 1.
The availability of abundant textual data from different online sources can be utilized to better recognize the growth, nature, and spread of COVID-19. As discussed in the previous section (see Section 3), we leveraged the tweets of users from eight countries who have been seriously impacted by COVID19 to explore their positive or negative opinions about the disease. In this study, the positive tweets show that the users are somewhat optimistic about the disease, while negative tweets mean that they are worried or disappointed about the impacts it might have on their life. The reaction and opinion of people about COVID-19 depend on various factors like socioeconomic, health condition, the economy of their country, culture, etc. which are beyond the scope of this study and will be discussed in our future work. However, few obvious differences are observed in the sentiment of tweets obtained from the eight countries. For example, in Iran, China, and Canada more positive tweets are observed, while in Italy and Spain more negative tweets are prevailing. Another difference is observed between English and non-English speaking countries. There is less fluctuation in the percent of tweets' sentiments in English-speaking countries.
While answering the first research question, one can see the difference in the intensities of sentiments of Twitter users about coronavirus-related tweets in eight countries (Figs. 6, 10, 13, 16, 21, 24, 26 and 31). Overall, it may be noted that the increase in positive sentiment correlates with the increase in the number of recoveries. However, the percent of positive tweets differ in different countries. For example, the most positive tweets belong to Iran, China, and Canada (80%, 65%, and 65%) while the most negative tweets belong to Iran, Italy, and Spain (80%, 75%, and 75%, respectively). For English speaking countries, there is less fluctuations in the percent of tweets' sentiments. This may be due to the fact that the words are chosen more accurately by the users in those countries, while non-English users unintentionally may have used the sentiment words incorrectly to express in their tweets. Therefore, different sentiment pattern exists in different countries and hence there is a meaningful difference in English and non-English speaking countries.
While answering the second research question, one can see a correlation between the number of newly infected cases, new recoveries, and deaths with tweets' sentiment in different dates and countries, in addition to the figures mentioned (Figs. 7, 9, 12, 15, 19, 20, 23, 27, 28, 30 and 32). As shown in these figures, the maximum negative sentiment values are seen either around the rise of new and active cases or the number of death charts. Similarly, the maximum positive sentiment values are obtained for time frames for which the highest number of recoveries occurred. This, answers the third research question conclusively.
Finally, to answer the last research question which addresses the performance of different models, the collected coronavirusrelated dataset has no gold-standard labels and hence cannot be used to assess the performance of the models. Therefore, as stated in Section 3.2, a large-scale Twitter dataset containing 1.6 million tweets is used. As shown in Table 6, the performance of the proposed model is better than four other deep models described in Sections 3.3.1 to 3.3.5 and slightly better than DistilBERT which is a state-of-the-art model for many text mining applications [58].
Hence, our proposed model has performed better than the stateof-the-art techniques for Twitter sentiment analysis and hence, can be used to accurately classify the coronavirus Twitter dataset collected for sentient analysis of users' tweets in eight countries in this study.
The results of this study can be used for governors, social policymakers, and healthcare managers to better understand the effect of COVID-19 on the overall sentiment of society. This may be used to design better strategies for improving public mood during different phases of pandemic. For example, when the increasing trend is detected in the active cases chart or number of death chart, the social policymakers can devise suitable policies and decisions to control the public sentiment and decrease the effects of a pandemic on the emotions of society. Another application of the obtained results of the current study is to use similar trend occurred in other countries to predict the effect of changes in the COVID-19 charts on the sentiment charts. In addition to these applications, researchers can use the results of the current study to compare the public mood of different countries during the pandemic. This may be of concerns for social scientists as well as global healthcare agencies.

Conclusion
The coronavirus or COVID-19 disease is one of the biggest human challenges of the last few decades. All governments and researchers are relentlessly trying to reduce the deadly effects of this pervasive disease. This study is conducted to find the general sentiment (opinion) of people in eight countries around the world. To do so, we collected the tweets from those eight countries between 2020-01-24 to 2020-04-21 and also Google Trends users by using coronavirus-related keyword searches from 2020-01-24 to 2020-04-23. For sentiment analysis, we proposed a new hybrid fusion model using five deep classifiers and then combined them to improve the final output using a meta learning method. To assess the performance of the proposed method for Twitter sentiment analysis, we evaluated all five base models and proposed model using Stanford Sentiment140 Twitter dataset with 1600,000 labeled tweets. Our findings show that the proposed fusion models can classify the sentiments with promising performance and can be used to analyze the collected coronavirus-related dataset accurately. Three main observations of this study are: (i) rise in information about coronavirus is correlated with the reported first infected case, (ii) every country has unique sentiment patterns, (iii) maximum negative sentiment values happened either around the rise of new and active cases or a number of death charts. Similarly, the highest number of recoveries happened during the time period of maximum positive sentiment values. In addition to these findings, the results show that different sentiment pattern exists in different countries and it may be concluded that there are meaningful differences in English and non-English speaking countries. We hope that these findings will have a significant impact on controlling the disease. The limitation of our study is ignoring the effects of global COVID-19 news and statistics on the overall sentiment of other countries. Another limitation is the keyword selection for information seeking and filtering tweets are independent of the country for which we are extracting the dataset. In future, this work can be extended by focusing on the tweets published by special communities or target societies to find its impact on the public sentiment and mood.