Estimating the number of symptomatic SARS-CoV-2 infections among vaccinated individuals in the United States-January-April, 2021

Kugeler, K. J.; Williamson, J.; Curns, A.; Healy, J. M.; Nolen, L. D.; Clark, T. A.; Martin, S. W.; Fischer, M.

As of March 2021, three COVID-19 vaccines have been authorized by the U.S. Food and Drug Administration (FDA) for use in the United States. Each has substantial efficacy in preventing COVID-19. However, as efficacy from trials was <100% for all three vaccines, disease in vaccinated people is expected to occur. We created a spreadsheet-based tool to estimate the number of symptomatic vaccine breakthrough infections based on published vaccine efficacy (VE) data, percent of the population that has been fully vaccinated, and average number of COVID-19 cases reported per day. We estimate that approximately 51,000 symptomatic vaccine breakthrough infections (95% CI: ~48,000-55,000 cases) occurred in the United States during January-April 2021 among >77 million fully vaccinated people, reflecting <0.5% of COVID-19 cases that occurred during that time. With ongoing SARS-CoV-2 transmission and increasing numbers of people vaccinated in the United States, vaccine breakthrough infections will continue to accumulate before population immunity is sufficient to interrupt transmission. Understanding expectations regarding number of vaccine breakthrough infections enables accurate public health messaging to help ensure that the occurrence of such cases does not negatively affect vaccine perceptions, confidence, and uptake.