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Meta-analysis of the effects of ambient temperature and relative humidity on the risk of mumps

Many studies have shown that the relationship between ambient temperature, relative humidity and mumps has been highlighted. However, these studies showed inconsistent results. Therefore, the goal of our study is to conduct a meta-analysis to clarify this relationship and to quantify the size of these effects as well as the potential factors. Systematic literature researches on PubMed, Embase.com, Web of Science Core Collection, Cochrane library, Chinese BioMedical Literature Database (CBM) and China National Knowledge Infrastructure (CNKI) were performed up to February 7, 2022 for articles analyzing the relationships between ambient temperature, relative humidity and incidence of mumps. Eligibility assessment and data extraction were conducted independently by two researchers, and meta-analysis was performed to synthesize these data. We also assessed sources of heterogeneity by study region, regional climate, study population. Finally, a total of 14 studies were screened out from 1154 records and identified to estimate the relationship between ambient temperature, relative humidity and incidence of mumps. It was found that per 1 °C increase and decrease in the ambient temperature were significantly associated with increased incidence of mumps with RR of 1.0191 (95% CI: 1.0129–1.0252, I2 = 92.0%, Egger’s test P = 0.001, N = 13) for per 1 °C increase and 1.0244 (95% CI: 1.0130–1.0359, I2 = 86.6%, Egger’s test P = 0.077, N = 9) for per 1 °C decrease. As to relative humidity, only high effect of relative humidity was slightly significant (for per 1 unit increase with RR of 1.0088 (95% CI: 1.0027–1.0150), I2 = 72.6%, Egger’s test P = 0.159, N = 9). Subgroup analysis showed that regional climate with temperate areas may have a higher risk of incidence of mumps than areas with subtropical climate in cold effect of ambient temperature and low effect of relative humidity. In addition, meta-regression analysis showed that regional climate may affect the association between incidence of mumps and cold effect of ambient temperature. Our results suggest ambient temperature could affect the incidence of mumps significantly, of which both hot and cold effect of ambient temperature may increase the incidence of mumps. Further studies are still needed to clarify the relationship between the incidence of mumps and ambient temperature outside of east Asia, and many other meteorological factors. These results of ambient temperature are important for establishing preventive measures on mumps, especially in temperate areas. The policy-makers should pay more attention to ambient temperature changes and take protective measures in advance.


Methods
We carried out this meta-analysis in accordance with the Preferred Reporting Items for Systematic review and Meta-Analysis (PRISMA) statement 31 . Search strategy. Systematic research was performed in both English and Chinese database, including Pub-Med, Embase.com, Web of Science Core Collection, Cochrane library, Chinese Biomedical Literature Database (CBM) and the China National Knowledge Infrastructure (CNKI) for relevant studies up to February 7, 2022. Medical Subject Headings (MeSH) Term and free words strategy were applied to maximize the output. The search strategy, which was referenced to Qiang Cheng et al. 18 , was listed as follows: ("temperature" OR "humidity" OR "rainfall" OR "precipitation" OR "atmospheric pressure" OR "air pressure" OR "barometric pressure" OR "climate" OR "Meteorolog*" OR "weather" OR "wind speed" OR "wind velocity" OR "sunshine duration") AND (mumps OR epidemic parotitis).
After deleting duplicates, all abstracts and titles were filtered independently by two reviewers to remove the irrelevant articles (T.W. Wang and J.J. Wang). Two authors evaluated potential publications by checking their titles and abstracts and then procured the most relevant publications for further full-text examination. Bibliographies section of retrieved articles were also reviewed for additional pertinent studies that were possibly missed in the initial search. If any agreement can't be solved by discussion, a third reviewer (J.H. Zhang) was requested to make arbitration.
Inclusion and exclusion criteria. Studies included for further analysis should fulfill the rules as follows: (i) the outcome measure was incidence of mumps at a monthly, weekly or daily resolution; (ii) the exposure of interest was mean ambient temperature and/or relative humidity; (iii) the estimated parameter was the rate of incidence (RR) of mumps associated with each 1 °C increase/decrease in ambient temperature and each 1% increase/decrease in relative humidity, or which can be converted to standardized RR using the following formula 18,32 : RR standardised = RR Increment(1)/Increment(Original) (Original) , or studies could provide effect estimates of approximate value, such as the excess risk (ER), percentage change(PC), and then convent to RR using the following rule: ER(%) = (RR−1) × 100% 33 , or RR = 1 + PC 17 ; and (iv) provided the corresponding 95% confidence interval(CI); (v) the study incorporated at least 4 year of continuous data and controlled for potential confounding factors.
The exclusion criteria of our literature were listed as follows: (i) the full text of the article could not be found; (ii) the format of the paper is not a research article; (iii) articles do not contain data for meta-analysis; and (iv) duplicate publication, or the data was covered by another study included completely. www.nature.com/scientificreports/ Data extraction. The following information was extracted from the studies that met the inclusion criteria: first author, publication year; location, study period; population, ages, exposure meteorological factor, data sources, measures of exposure (95% CI) and outcome, statistical model and so on. Due to some studies providing different hysteresis patterns to estimate the delayed effects of ambient temperature and relative humidity 18 , such as single-day hysteresis and cumulative hysteresis 34 , we chose the estimate effect with the following rules. If only one lag estimate was provided (either because only one was analyzed or only one was reported from the study), this estimate was recorded. If multiple lags were reported, we chose one based on the following criteria: (1) the lag that the investigators focused on or stated as a priority, (2) the lag that was statistically significant, (3) the lag with the largest effect estimate 35 .
In addition, if the threshold effect existed in the studies, and two effect values be given, we pooled them separately, such as ambient temperature with hot effect and cold effect. Where multiple estimates were reported, we selected the final model as specified by the authors or, if a final model was not specified, the model with the greatest number of relevant covariates based on the Cochrane Collaboration guidelines 36 . If more than one place were reported in one study, we selected the overall effect or pooled it as one single study to avoid over weight any one study.
Quality assessment of studies in meta-analysis. A new domain-based risk of bias (RoB) assessment tool was adopted to assess the RoB of included studies, which was developed by experts convened by the WHO 37 . This tool provides a description of the instrument devised to assess the risk of bias in the individual studies included in the systematic reviews of adverse health effects informing the guidelines. A total of 13 items are grouped into six domains, which include confounding, selection bias, exposure assessment, outcome measurement, missing data, and selective reporting in the instrument. Each item could be evaluated as low, moderate, and high risk of bias. The results for each domain were analyzed separately, without considering a single result for the whole article. If only one item of the same domain was judged as having high RoB, the entire domain was classified as having "High Risk". If all domains in one study were low risk of bias, the study could be classified as "Low Risk" study; if at least two domains were high risk of bias, the study would be classified as "High Risk" study; Otherwise, the study was classified as "Moderate Risk". Further detailed assessments were referred to in similar articles [38][39][40] .

Meta-analysis.
For standardized results obtained from each study included, we assessed the heterogeneity of the relationship between ambient temperature, relative humidity and incidence of mumps before got the pooled results 36 . Statistical heterogeneity was evaluated by Cochran's Q test (with P < 0.10 indicating statistically significant heterogeneity) and the I 2 statistic 41 . An I 2 from 0 to 40% was treated as an unimportant heterogeneity, I 2 from 30 to 60% was treated as moderate heterogeneity, I 2 from 50 to 90% was treated as substantial heterogeneity and I 2 from 75 to 100% was treated as considerable heterogeneity 42 . A pooled model was chosen based on the heterogeneity: if obvious heterogeneity existed, a random effects model was adopted; otherwise, a fixed effects model was adopted. In addition, subgroup analysis and meta-regression were also performed to explore the potential source of heterogeneity if obvious heterogeneity was found. Furthermore, sensitivity analysis was performed by two means, (i) by converting the pooled results from a random effects model to a fixed effects model or from a fixed effects model to a random effects model; (ii) "leave-one-out" analyses, which was performed by sequentially removing individual studies and evaluating the effect on the overall estimate. Publication bias was tested by funnel plot, Begg's and Egger's test 43 . All statistical tests for the meta-analysis were carried out using R software version 4.1.2 44 (with the package "meta" version 4.18-0 45 ). A P value of ≤ 0.05 was considered to indicate statistical significance for all tests and models.

Results
Study selection and characteristics. A total of 1154 records were obtained through electric searches.
Risk of bias assessment. The summary of the risk of bias assessment is shown in summary plot (Fig. 2).
In three out of six domains (selection bias, missing data, selective reporting), the risk of bias was found to be only low, and the risk of bias of two domains (exposure assessment, outcome measurement) was found to be only low or moderate. But in the domain of confounding, we found a variable proportion of articles having high and moderate risk of bias, of which 35.7%(5) articles were classified as high risk of bias on confounding. The main reason for the high risk of bias in the confounding domain was the lack of adjusting critical confounders (seasonality, long-term trends, day of the week) and/or additional confounders (holidays). Based on the rules of the RoB assessment for a study, five studies were classified as "Moderate Risk", and the rests were classified as "Low Risk". In this study, especially for studies conducted in China mainland, the data of exposure assessment and outcome measurement were mainly from the local Center for Disease Control and Prevention, and the local Meteorological Bureau. However, some studies didn't clearly state how to collect and manage the data, and define the cases, which made two domains of exposure assessment and outcome measurement of some studies were classified as "Moderate Risk". In total, based on our assessment results, the quality of included studies was medium to high quality. www.nature.com/scientificreports/ Overall effects analysis. As the effects expressed in original studies, both ambient temperature and relative humidity showed high and low effect, hot/cold effect in ambient temperature and high/low values in relative humidity. The sizes of the effects obtained from each dataset are presented in the forest plots along with the overall results of the meta-analysis (Figs. 3 and 4). The pooled results of meta-analysis with 14 studies indicated that per 1 °C increase and decrease in the ambient temperature were statistically associated with incidence of mumps, with RR(95% CI) of 1.0191 (1.0129-1.0252) for per 1 °C increase (Fig. 3A) and 1.0244 (1.0130-1.0359) for per 1 °C decrease (Fig. 3B). At the same time, for per 1 unit change of relative humidity, high relative humidity was significant (1.0088 (1.0027-1.0150)), while low relative humidity not (1.0036 (0.9987-1.0085)). Nevertheless, significant heterogeneity was also observed between the studies in both ambient temperature and relative humidity (Figs. 3 and 4), indicating that subgroup analysis was necessary.

Subgroup and meta-regression analyses.
We conducted subgroup analysis based on the following variables that we could obtain to explore the potential source of heterogeneity: study region (Province level vs City level), regional climate (Subtropical vs Temperate), study population (All vs Children only). As shown in Tables 2 and 3, of the three factors we explored, only two subgroups showed statistically significant association with mumps, that were regional climate in cold effect of ambient temperature and low relative humidity. In the cold effect of ambient temperature, people lived in temperate region had higher relative risk than in subtropical region along with decrease of ambient temperature. Similar situation happened in low relative humidity effect, the relative risk was significantly higher in temperate region along with the decrease of relative humidity, while no significant difference was found in subtropical region.
To better understand the influence of factors, we also performed meta-regression analysis based on the information we obtained. As shown in Table 4, we found that regional climate could affect the association between hot effect of ambient temperature and incidence of mumps, which was also consistent with the results of subgroup analysis. Sensitivity analysis. Two types of sensitivity analyses were performed. Firstly, we removed any single study to verify whether the overall effect was affected. The results showed that no significant difference was found except for the effect of relative humidity. After removing the study of Yang 55 , the result become significant with RR (95%) of 1.0054 (1.0005, 1.0103) in low relative humidity. Secondly, sensitivity analysis was performed by The results exhibited no big differences before and after pooling of ambient temperature effect, indicating stability in the pooled results. However, similar results were not found in the low effect of relative humidity. Overall, the results of low effects of relative humidity were not stable.
Publication bias. We plotted the funnel plot and performed Begg's and Egger's tests to check whether publication bias exists. As shown in Fig. 5 and Table 5, publication bias may exist in ambient temperature, especially in hot effect of ambient temperature. www.nature.com/scientificreports/  www.nature.com/scientificreports/

Discussion
Although vaccination was widely used in mumps prevention, dramatic increases in mumps cases have received much attention in recent years 57 . China, as one of the countries including the MMR into the immunization programs, also has a relative high morbidity of mumps among adolescents 58 . To date, this study was the first meta-analysis to examine the relationship between ambient temperature, relative humidity and incidence of mumps. In the 14 articles included in this paper, we identified significant positive relationship between ambient  www.nature.com/scientificreports/ temperature and incidence of mumps, and high relative humidity, while no relationship was found between low relative humidity and incidence of mumps. We also found that there was also significant heterogeneity in the estimated association, and subgroup analysis also showed that people living in temperate regions had a higher risk of incidence of mumps than in subtropical regions in cold effect of ambient temperature and low effect of relative humidity, which was also confirmed by meta-regression in cold effect of ambient temperature. To our surprise, relative humidity is found to be slightly associated with mumps in our study, only high relative humidity with RR of 1.0088 (1.0027-1.0150). In previous study, humidity was found to be associated with many diseases, such as hand, foot, and mouth disease 59,60 , dengue outbreaks 61 , COVID-19 62 , et al. Many reasons may be used to explain this phenomenon. First, compared with the other pathogen, the pathogenicity of mumps virus may have different effect on humidity. The effect of mumps may not be affected by humidity significantly. Second, as to humidity, the publication bias was found, leading to that the pooled results may be affected by unpublished studies. Third, obvious heterogeneity also existed. Compared with ambient temperature, the effect of humidity is relative smaller per 1 unit, and could be affected by many factors. Fourth, even some factors were adjusted, such as school holidays, long-term trend, and seasonality, the confounding factor may not be fully explained, and the number of studies included for humidity is not large enough, only 11 studies. In addition, sensitivity analysis showed that the results of relative humidity were not stable. Therefore, the results may be unstable, indicating further exploring may be still needed.
In detail, the ambient temperature affected the incidence of mumps in both hot and cold effect, indicating hotter and colder ambient temperature had higher risk than the thresholds, which was corresponding to the fact that non-linear relationships of meteorological factors with morbidity and mortality 47 . The pathogen of Table 3. Subgroup analysis of the incidence of mumps with relative humidity.  www.nature.com/scientificreports/ mumps has clear tolerance of ambient temperature in biological mechanism, which had bad tolerance of high ambient temperature and relative better tolerance of low tolerance. However, as the difference in environmental and climate factors, which led to the different thresholds of meteorological factors, such as threshold of ambient temperature in our study, ranging from about 10 °C and 25 °C, mainly focusing on 20 °C to 25 °C, which may also contribute to the heterogeneity. What's more, the non-linear relationships were also complex. In our studies included, several types were found, such as inverse "S" 46 , inverse "V" 49,51 , "V" 50 , and even "M" 50 , et al. However, in our study, based on the information provided in the included studies, we only obtained the main part result, such as only "V" part of "M" shape. Previous studies indicated a slightly lower pooled value and narrower confidence interval than the studies of weekly resolution, suggesting it may be more reasonable to use the daily exposure measures in future research  www.nature.com/scientificreports/ designs, especially for those diseases with shorter incubation period 18 . In our studies, 12 studies reported cases with resolution of daily, 3 of weekly and 1 of monthly. As only 3 with weekly data and 1 with monthly data, we didn't perform subgroup analysis based on time resolution. As mumps is an acute infectious disease, research based on daily data may be more suitable for disease control and prevention by providing more timely information. In the future, similar studies may perform sensitivity analysis by changing the resolution to confirm the effect of resolution and the stability of overall and specific results. In our subgroup analysis, only the regional climate was found to affect the relationship between meteorological factors and mumps. Human activities may be one of the most important reasons. Temperate climate may be more suitable for outdoor activities 25 , especially for children and adolescents. Mumps is a disease mainly happens in younger people, which also increase the risk for children and adolescents from temperate region. It is reported that a total of about 500 thousand cases was reported in 2018 63 , and half of them happened in China 64 . However, in our study, all studies are from Chinese mainland, China Taiwan and Japan, which may not represent other places, such as tropical and frigid. Therefore, researchers may pay more attention to other climate regions in the future.
Distributed Lag Non-linear Models, which was achieved with "dlnm" package constructed by Gasparrini A 65 within the statistical environment R, provided much convenience for analysis between meteorological factors and disease. In the meta-analysis, most of include studies were performed with this package. However, due to complexity between meteorological factors and disease, and no standard report guideline, all of these led to the difficulty for meta-analysis. To solve this situation, the author has built a package "mvmeta" within the statistical environment R to pool the results from multiple cities [66][67][68] . However, this two-stage analysis is based on the individual data, which is hard to obtain for meta-analysis. Therefore, if the authors provided the RR (or OR, PC, ER et al.) per 1 unit in each line trend range in both single lag day and accumulative lagged effect, meta-analysis of this type analysis may be easier in the future.
Although this was the first meta-analysis with 16 studies to examine the relationship between ambient temperature, relative humidity and incidence of mumps, our meta-analysis has several limitations that should be recognized when interpreting the results. Firstly, the heterogeneity is obvious. Although we have performed subgroup and meta-regression analysis, and found the great importance of climate, obvious heterogeneity still existed. In addition, when available, adjusted estimates were used in preference to unadjusted estimates. Even though the adjusted estimates may be closer to the true effect for adjusted results could control confounding factors, the different adjusted factors in different studies may also contribute to the heterogeneity. Secondly, publication bias may exist in both ambient temperature and relative humidity. Researchers were more trend to publish statistically significant findings, which may lead to the ignorance of negative results. Thirdly, all studies for meta-analysis were from East Asian (Chinese mainland, China Taiwan and Japan), with limited adjustment for confounders, which may limit the extension to other places. Fourthly, most of the included studies tend to report the largest effect estimates, and the effect estimates of different lag days is not fully available for analysis, we ignored the lag effect and chose the largest effect estimates for meta-analysis, which also contributed to heterogeneity. Fifthly, although our study was performed with strict inclusion and exclusion criteria, a total of 14 studies were excluded for no relevant data, which may also affect our results. Sixthly, in China, the mumps vaccine has been used since 2008 and its efficacy is a function of age, coverage, and timing of the inoculation. All Chinese studies included were conducted after 2008. Thus, even by controlling for confounding, it is very difficult to disentangle the contribution of meteorological factors from vaccination efficacy on mumps incidence. However, as the widely use of mumps vaccine, our results may be more suitable for current preventive measures. Finally, different confounding factors were adjusted in different studies, which may also lead to heterogeneity and bias of risk. Nevertheless, our study is still the most comprehensive about the association between incidence of mumps and ambient temperature/relative humidity.

Conclusion
Overall, the pooled results of our meta-analysis provide systematic evidence that ambient temperature may increase the incidence of mumps, especially in east Asia regions. As limitation of the number of studies for relative humidity and the significant effect of relative humidity on other air-borne diseases, the effect of relative humidity on mumps needs further exploring. Further studies are still needed to clarify the relationship between ambient temperature and mumps in areas outside of east Asia regions, and many other meteorological factors. These results of ambient temperature are important for establishing preventive measures on mumps, especially in temperate regions.

Data availability
Only aggregated summaries of the data are provided in this manuscript. However, all data generated in this study can be made publicly available on request. Please contact the corresponding author for any kind of data request. www.nature.com/scientificreports/