Will Vaccine-derived Protective Immunity Curtail COVID-19 Variants in the US?

Mancuso, M.; Eikenberry, S.; Gumel, A.

Multiple effective vaccines are currently being deployed to combat the COVID-19 pandemic (caused by SARS-COV-2), and are viewed as the major factor in marked reductions in disease burden in regions around the world with moderate to high coverage of these vaccines. The effectiveness of COVID-19 vaccination programs is, however, significantly threatened by the emergence of new SARS-COV-2 variants that, in addition to being more transmissible and potentially more virulent than the wild (resident) strain, may at least partially evade existing vaccines. A new two-strain (one resident, the other wild) and two-group (vaccinated or otherwise) mechanistic mathematical model is designed and used to assess the impact of the vaccine-induced cross-protective efficacy on the spread the COVID-19 pandemic in the United States. Analysis of the model, which is fitted using COVID-19 mortality data for the US, shows that vaccine-induced herd immunity can be achieved if 61% of the American population is fully vaccinated with the Pfizer or Moderna vaccines. Parameter sensitivity analysis suggests three main factors that significantly affect the COVID-19 burden in the US, namely (a) daily vaccination rate, (b) the level of cross-protection the vaccines offer against the variant, and (c) the relative infectiousness of the dominant variant relative to the wild strain. This study further suggests that a new variant can cause a significant disease surge in the US if (i) the vaccine coverage against the wild strain is low (roughly < 50%), (ii) the variant is much more transmissible (e.g., twice more transmissible) than the wild-type strain, or (iii) the level of cross-protection offered by the vaccine is relatively low (e.g., less than 70%). A new variant will not cause such surge in the US if it is only moderately more transmissible (e.g., 1:56 more transmissible) than the wild strain, at least 66% of the population of the US is fully vaccinated, and the three vaccines being deployed in the US (Pfizer, Moderna, and Johnson & Johnson) offer a moderate level of cross-protection against the variant.