We tested whether COVID-19 incidence and hospitalization rates during the Delta surge in July-August 2021 were inversely related to vaccination coverage among the 112 most populous counties in the United States, each with a population exceeding 600,000, together comprising 44% of the total U.S. population.
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Both COVID-19 incidence and hospitalization rates were significantly higher among counties in the lower half of the distribution of vaccination coverage than among counties in the upper half of the distribution of coverage.
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In log-linear regression models, a 10-percentage-point increase in vaccination coverage was associated with a 28.3% decrease in COVID-19 incidence, a 44.9 percent decrease in the rate of COVID-19 hospitalization, and a 16.6% decrease in COVID-19 hospitalizations per 100 cases.
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Inclusion of demographic factors and county-specific diabetes prevalence as additional variables in our regression models did not weaken the observed inverse relations between vaccination coverage and COVID-19 incidence and hospitalization.
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Higher vaccination coverage is associated not only with significantly lower COVID-19 incidence, but also significantly less severe cases of the disease.
Abstract
Objective
We tested whether COVID-19 incidence and hospitalization rates during the Delta surge were inversely related to vaccination coverage among the 112 most populous counties in the United States, comprising 44 percent of the country's total population.
Methods
We measured vaccination coverage as the percent of the county population fully vaccinated as of July 15, 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 and hospitalization rates as the number of confirmed COVID-19 admissions per 100,000 population during the same 14-day period.
Results
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 decrease 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%). Inclusion of demographic covariables, as well as county-specific diabetes prevalence, did not weaken the observed inverse relationship with vaccination coverage. A significant inverse relationship between vaccination coverage and COVID-19 deaths per 100,000 during August 20 – September 16 was also observed. The cumulative incidence of COVID-19 through June 30, 2021, a potential indicator of acquired immunity due to past infection, had no significant relation to subsequent case incidence or hospitalization rates in August.
Conclusion
Higher vaccination coverage was associated not only with significantly lower COVID-19 incidence during the Delta surge, but also significantly less severe cases of the disease.
Public Interest Summary
We tested whether COVID-19 incidence and hospitalization rates during the Delta variant-related surge were inversely related to vaccination coverage among the 112 most populous counties in the United States, together comprising 44 percent of the country's total population. A 10-percentage-point increase in vaccination coverage was associated with a 28.3% decrease in COVID-19 incidence, a 44.9 percent decrease in the rate of COVID-19 hospitalization, and a 16.6% decrease in COVID-19 hospitalizations per 100 cases. Inclusion of demographic covariables, as well as county-specific diabetes prevalence, did not weaken the observed inverse relationship with vaccination coverage. A significant inverse relationship between vaccination coverage and COVID-19 deaths per 100,000 during August 20 – September 16 was also observed. Higher vaccination coverage was associated not only with significantly lower COVID-19 incidence during the Delta surge, but also significantly less severe cases of the disease.
Keywords
COVID-19
SARS-CoV-2
Vaccination
Delta variant
Cited by (0)
The sources of data for this study are publicly accessible via the Internet links cited in the reference section. We have posted our data analyses at the Open Science Framework (OSF) in a project entitled 112-County COVID-19 Incidence-Vaccination Study (https://osf.io/wtb6j/).
