The impact of vaccination on COVID-19 outbreaks in the United States

Moghadas, Seyed M.; Vilches, Thomas N.; Zhang, Kevin; Wells, Chad R.; Shoukat, Affan; Singer, Burton H.; Meyers, Lauren Ancel; Neuzil, Kathleen M.; Langley, Joanne M.; Fitzpatrick, Meagan C.; Galvani, Alison P.

BACKGROUND: Global vaccine development efforts have been accelerated in response to the devastating COVID-19 pandemic. We evaluated the impact of a 2-dose COVID-19 vaccination campaign on reducing incidence, hospitalizations, and deaths in the United States (US). METHODS: We developed an agent-based model of SARS-CoV-2 transmission and parameterized it with US demographics and age-specific COVID-19 outcomes. Healthcare workers and high-risk individuals were prioritized for vaccination, while children under 18 years of age were not vaccinated. We considered a vaccine efficacy of 90% against infection following 2 doses administered 28 days apart achieving 40% vaccine coverage of the overall population. We specified 10% pre-existing population immunity for the base-case scenario and calibrated to an effective reproduction number of 1.5, accounting for current COVID-19 interventions in the US. RESULTS: Vaccination reduced the overall attack rate to 1.6% (95% CI: 1.3% – 1.8%) from 7.1% (95% CI: 6.3% – 7.9%) across the same period without vaccination. The highest relative reduction (83–90%) was observed among individuals aged 65 and older. Vaccination markedly reduced adverse outcomes, with non-ICU, ICU hospitalizations, and deaths decreasing by 85.2% (95% CI: 82.3% – 87.6%), 85.3% (95% CI: 82.3% – 87.8%), and 87.8% (95% CI: 85.1% – 90.1%), respectively. CONCLUSIONS: Our results indicate that vaccination can have a substantial impact on reducing disease transmission and adverse clinical outcomes. However, with uptake of 40% or less in the population, vaccination is unlikely to completely eliminate the need for non-pharmaceutical interventions.