COVID-19 Incidence and Hospitalization During the Delta Surge Were Inversely Related to Vaccination Coverage Among the Most Populous U.S. Counties

Abstract

We tested whether COVID-19 incidence and hospitalization rates were inversely related to vaccination coverage among the 112 most populous counties in the United States, each with a population exceeding 600,000. We measured vaccination coverage as the percent of the total population fully vaccinated as of July 15, 2021, with the exception of 11 Texas counties, where the cutoff date was July 14, 2021. We measured COVID-19 incidence as the number of confirmed cases per 100,000 population during the 14-day period ending August 12, 2021. We measured hospitalization rates as the number of confirmed COVID-19 admissions per 100,000 population during the same 14-day period. COVID-19 incidence was significantly higher among counties in the lower half of the distribution of vaccination coverage (incidence 543.8 per 100,000 among 56 counties with mean coverage 42.61%) than among counties in the lower half of the distribution of coverage (incidence 280.7 per 100,000 among 56 counties with mean coverage 57.37%, p < 0.0001). Hospital admissions were also significantly higher among counties in the lower half of the distribution (55.37 per 100,000) than in the upper half of the distribution (20.48 per 100,000, p < 0.0001). In log-linear regression models, a 10-percentage-point increase in vaccination coverage was associated with a 28.3% decrease in COVID-19 incidence (95% confidence interval, 16.8 – 39.7%), a 44.9 percent increase in the rate of COVID-19 hospitalization (95% CI, 28.8 – 61.0%), and a 16.6% decrease in COVID-19 hospitalizations per 100 cases (95% CI, 8.4 – 24.8%). Higher vaccination coverage is associated not only with significantly lower COVID-19 incidence, but also significantly less severe cases of the disease.