Estimating COVID-19 Infections, Hospitalizations and Deaths After U.S. Vaccination Campaigns

The findings of this simulation model suggest that the U.S. COVID-19 vaccination program was associated with an estimated reduction in total hospitalizations and deaths by nearly half during the first six months of 2021.

Vilches, et al. (2021) report that the COVID-19 pandemic has caused more than 745 000 deaths in the US. However, the toll might have been higher without the rapid development and delivery of effective vaccines. As of Oct. 28, 2021, 69% of 258 million U.S. adults had been fully vaccinated.

Quantifying the population impact of COVID-19 vaccination can inform future vaccination strategies. Randomized clinical trials have established individual-level efficacy of authorized vaccines against the original strain, which exceeds 90% in preventing symptomatic and severe disease. However, the population-level effectiveness of the vaccination campaign in the U.S,. in terms of association with reduced infections, hospitalizations, and deaths, is not as well documented, and the researchers evaluated this using a simulation model.

The researchers expanded their previous agent-based model to include transmission dynamics of the Alpha (B.1.1.7), Gamma (P.1), and Delta (B.1.617.2) variants in addition to the original strain. The model was parameterized with the U.S. demographics and age-specific risks of severe COVID-19 outcomes. A 2-dose vaccination strategy was implemented based on the daily vaccines administered in different age groups. Vaccine efficacies against infection, symptomatic disease and severe disease after each dose and for each variant were derived from published estimates. The model was calibrated and fitted to reported national level incidence from Oct. 1, 2020, to June 30, 2021.

The researchers simulated pandemic trajectory under 2 counterfactuals: a no vaccination scenario and a program that achieved only half the daily vaccination rate of actual rollout. For each scenario, cumulative infections, hospitalizations, and deaths were compared with the simulated trends under the US vaccination program.

Credible intervals (CrIs) were generated from simulation outputs using the bias-corrected and accelerated bootstrap method (with 500 replications) in June 2021. The model was implemented in Julia Language Programming, version 1.6 (Julia), and outputs were analyzed in MATLAB, version 2017a (MathWorks). No significance tests were performed for this simulation study.

Compared with the no vaccination scenario, the actual vaccination campaign saved an estimated 240 797 (95% CrI, 200 665-281 230) lives and prevented an estimated 1,133,617 hospitalizations from Dec. 12, 2020, to June 30, 2021. The number of cases averted during the same period was projected to exceed 14 million. Vaccination prevented a wave of COVID-19 cases driven by the Alpha variant that would have occurred in April 2021 without vaccination, with a projected peak of 4,409 deaths and 17,979 hospitalizations. Under the second counterfactual with daily vaccination rates at half the reported pace, the researchers projected that the U.S. would have still endured an additional 77,283 deaths and 336,000 hospitalizations.

Reference: Vilches TN, Moghadas SM, et al. Estimating COVID-19 Infections, Hospitalizations, and Deaths Following the US Vaccination Campaigns During the Pandemic. JAMA Netw Open. 2022;5(1):e2142725. doi:10.1001/jamanetworkopen.2021.42725