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Dynamics of the Delta and Omicron variants of SARS-CoV-2 in the United States: the battle of supremacy in the presence of vaccination, mask usage and antiviral treatment

Multiple safe and effective vaccines and antiviral drugs have been approved or authorized for use against the COVID-19 pandemic in the United States. The effectiveness of these and other intervention measures is threatened by the emergence of numerous SARS-CoV-2 variants of concern. We present a model for studying the transmission dynamics of two of these variants (Delta and Omicron) in the presence of vaccination, treatment of individuals with clinical symptoms of the disease and the use of face masks in the community. The model was fitted using daily case data for the COVID-19 pandemic in the United States corresponding to the period starting from when Omicron first emerged (end of November 2021) to date. It is shown, based on simulating the model with the current COVID-19 data, that the reproduction number of the Delta variant (denoted by Rvd) is much smaller than one (Rvd = 0.28), while that of Omicron (denoted by Rvo) is approximately equal to unity (Rvo = 0.96). This shows that the Delta variant has essentially died out, and that Omicron is currently the predominant variant of concern in the United States. Furthermore, if the current baseline levels of the control measures being implemented in the United States are maintained, the Omicron variant will also be on a rapid decline (towards elimination). The analysis and simulations of the calibrated model show that vaccine-derived immunity can be achieved in the United States if at least 68% of the population is fully-vaccinated with either the Pfizer or Moderna vaccine. It is shown that the COVID-19 pandemic can be eliminated in the United States by June of 2022 if the current baseline level of the proportion of individuals that is fully-vaccinated is increased by about 20%. The threshold vaccination coverage needed to achieve the vaccine-derived herd immunity decreases if the vaccination program is combined with a face mask use strategy, particularly one that emphasizes the use of moderate to high quality masks (e.g., surgical or N95 masks). Greater reduction in disease burden (in comparison to the baseline scenario) are recorded if the very high quality N95 masks are prioritized in the community, followed by the moderately-effective surgical masks and then the lowly-effective cloth masks. We also showed that having high percentage of the populace wearing the moderately-effective surgical mask is more beneficial to the community than having low percentage of the populace wearing the highly-effective N95 masks (this result does not hold for the case when cloth masks compliance is compared with that of N95 masks). However, if a certain (fixed) percentage is to give up masking, our study showed that it is more beneficial if they give up wearing surgical masks and not N95 masks (in other words, in a head-to-head comparison, N95 is always superior than surgical mask). This study showed that waning natural and vaccine-derived immunity (if considered individually) offer marginal impact on disease burden, except for the case when they wane at a much faster rate (e.g., within three months), in comparison to the baseline (estimated to be within 9 months to a year). Greater reduction or increase in disease burden is recorded if both the vaccine-derived and natural immunity wane at the same time (rather than the case when we considered only one of them varying, while the other is at baseline). For instance, if both vaccine-derived and natural immunity wane within three months, a 14% increase in the peak daily cases will be recorded, in comparison to the baseline. For this case, where immunity wanes within three months, our study predicts another (but milder) Omicron wave in the United States that peaks around July 2022 (with the peak 72% lower than the original Omicron peak). Under this (fast waning) scenario, our study suggests that a fourth dose of the two mRNA vaccines would need to be approved in the United States to aid and accelerate the prospect of SARS-CoV-2 elimination in 2022. It is shown that while the treatment of symptomatic individuals has marginal effect in reducing daily cases of SARS-CoV-2, in comparison to the baseline, it has significant impact in reducing daily hospitalizations. It is further shown that, while treatment significantly reduces hospitalization, the prospects of COVID-19 elimination in the United States is more significantly enhanced if investments in control resources are focused on mask usage and vaccination rather than on treatment.