Critical assessment of the impact of vaccines and waning/boosting of immunity on the burden of COVID-19 in the U.S

Abstract

The 2019 coronavirus (COVID-19) pandemic continues to have a devastating impact on health systems and economies across the globe, with the United States (U.S.) among the worse impacted nations. Implementing public health measures in tandem with effective vaccination strategies is instrumental in halting the transmission of the virus and curtailing the burden of the pandemic. Currently, the U.S. Food and Drug Administration has authorized the use of the PfizerBioNTech, Moderna, and the Johnson & Johnson vaccines to prevent COVID-19 in the U.S. However, these vaccines have varying efficacies (≈ 95% for the Pfizer-BioNTech and Moderna vaccines and ≈ 70% for the Johnson & Johnson vaccine) and waning effects against major COVID-19 strains, hence, understanding their impact on the incidence of COVID-19 in the U.S. is critical. Here, we formulate and use mathematical models 1) to investigate the impact of each vaccine type and booster doses (single/double) on the incidence of COVID-19 in the U.S., and 2) to predict future trends of the disease in the U.S., if existing control measures are reinforced or relaxed. The models are fitted to part of the new daily confirmed case data from the U.S., and validated using the remaining part of the daily data, as well as the full cumulative case data. The fitting and numerical simulations of the models show a 44% (71%) reduction in the reproduction number (number of new daily confirmed cases) at the peak during the wave in which vaccination peaked compared to the preceding wave. Additionally, the estimated disease transmission rate is ≈ 3 times higher for the Omicron variant. Simulations of the model show that in the absence of booster shots, the time to elimination of community transmission in the U.S. would have increased by at least two months compared to the baseline case. However, had more people (i.e., 70% of the fully vaccinated population) been boosted by mid-August 2021, ≈ 78% of the daily incidence could have been prevented as at the time the first case of Omicron was reported in the U.S. Our findings suggest that booster shots with the Pfizer-BioNTech or Moderna vaccines conferred superior protection than those with the Johnson & Johnson vaccine. Furthermore, the simulations show that the baseline value of the new daily cases at the peak of the Omicron variant in January 2022 would have dropped significantly (by ≈ 20%) if a fourth dose of the Pfizer-BioNTech or Moderna vaccine was administered at the start of the Omicron wave. Specifically, three million cumulative cases in the U.S. could have been averted between late November 2021 and March 2022. The study proves that early administration of vaccines and booster doses could have significantly reduced the surge in cases and the observed peak size. In particular, we showed that, while late boosting will result in an increase in the number of cases (compared to the baseline value), early boosting will lead to a decrease in the number of cases. Additionally, we showed that a second booster dose using the Pfizer-BioNTech or Moderna vaccine is important in curtailing the burden of the pandemic in the U.S. Particularly if this second dose is administered soon after the first dose. Furthermore, the study shows that early relaxation of existing control measures can lead to a more devastating wave, especially if both vaccination and transmission rate reducing measures such as mask-use are relaxed simultaneous. Keywords: COVID-19 pandemic; Vaccine efficacy; Booster doses; Delta and Omicron variants; Waning vaccine-derived and natural immunity; Infectious disease models.