Modeling strategies for the allocation of SARS-CoV-2 vaccines in the United States

Abstract

The Advisory Committee on Immunization Practices (ACIP) recommended phased allocation of SARS-CoV-2 vaccines in December 2020. To support the development of this guidance, we used a mathematical model of SARS-CoV-2 transmission to evaluate the relative impact of three vaccine allocation strategies on infections, hospitalizations, and deaths. All three strategies initially prioritized healthcare personnel (HCP) for vaccination. Strategies of subsequently prioritizing adults aged ≥65 years, or a combination of essential workers and adults aged ≥75 years, prevented the most deaths. Meanwhile, prioritizing adults with high-risk medical conditions immediately after HCP prevented the most infections. All three strategies prevented a similar fraction of hospitalizations. While no model is capable of fully capturing the complex social dynamics which shape epidemics, exercises such as this one can be a useful way for policy makers to formalize their assumptions and explore the key features of a problem before making decisions.

Keywords: COVID-19; Mathematical Modeling; SARS-CoV-2; Vaccines.

Fig. 1

Timing of Modeled Vaccine Rollout. The width and position (relative to the x-axis) of the purple, green, and yellow boxes indicate the period during which vaccination phases 1A, 1B, and 1C occur, respectively. For reference, the y-axis shows the modeled daily incidence of SARS-CoV-2 infection without vaccination when $R_t$ on January 1^st, 2021 is set to 1.0 (black), 1.1 (blue), and 0.9 (pink). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Population-Wide SARS-CoV-2 Outcomes Prevented by Vaccination in the Six Months Following Introduction. Bars indicate the modeled % of infections, hospitalizations, and deaths prevented by vaccination across the entire population in the 6 months after vaccination begins. This averted burden is shown for scenarios in which Phase 1B of vaccination (following healthcare workers) includes adults aged 65+ years old (green), adults with high-risk medical conditions (orange), and a combination of essential workers and adults aged 75+ years old (purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Relative Number of SARS-CoV-2 Outcomes Prevented Across Vaccination Strategies and Scenarios. Each set of three points connected by black lines (solid for the primary analysis, dashed for sensitivity analyses) represents a unique combination of values for the time-varying reproduction number (Rt) in January 2021, the level of essential worker distancing, and the relative risk of severe disease for those with underlying medical conditions (see methods for the specific values considered). Within each unique parameter set, points represent the number of SARS-CoV-2 infections, hospitalizations, or deaths prevented by a given vaccination strategy, standardized as a percent of the outcomes averted by vaccination by the strategy of targeting adults aged ≥65 years in Phase 1B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)