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Lower Vaccinated US Counties Saw Greater Incidence Rates During the Delta Surge

A study looked at COVID-19 vaccination rates and new COVID-19 infections across US counties during the Delta surge last summer. Many of the highly infected counties were considered rural, highlighting some of the vaccination challenges surrounding these areas.


During the Delta surge last summer, US counties with vaccinations lower than 30% saw a large increase in the number of COVID-19 cases. The study period was July 1 to August 31, 2021.

The results were published in JAMA Network Open.

“COVID-19 infections per 100 000 people increased from 190 infections (95% CI, 188-193 infections) during July 1 to 15 to 1272 infections (95% CI, 1263-1280 infections) during August 16 to 31,” the investigators reported. “COVID-19 infections per 100 000 people in areas with vaccination rates higher than 50% increased from 71 infections (95% CI, 70-72 infections) during July 1 to 15 to 531 infections (95% CI, 529-532 infections) during August 16 to 31, and areas with lower vaccination rates (ie, <30%) had 2.4 more new COVID-19 infections per 100 000 people compared with areas with high vaccination rates (ie, >50%) during August 16 to 31.”

A large number of the counties (82.2%) with the low vaccination rates were considered rural (369 of 449 areas). Conversely, rural counties that were consider to have high vaccination rates made up 34.8% of areas (131 of 376).

This study highlights some of the underlying challenges of vaccination in rural communities.

“Rural areas in the US face many challenges in responding to the pandemic, including lower health care resources, compared with urban communities,” they wrote. “These areas have been characterized by vaccination hesitancy, limited vaccine availability, and hospital staff shortages that can be associated with the successful distribution of vaccines and hence the vaccination campaign’s overall outcome.”

The investigators used the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline, and the US county data was collected from Johns Hopkins University.

The investigators aggregated the vaccination rates into 4 cohorts, which included: less than 30%; 30% to less than 40%: 40% to 50%: and greater than 50%.

According to the investigators, the epidemic grew in many southern states including Arkansas, Oklahoma, Mississippi, Tennessee, South Carolina, and Florida, and also grew in the western US.

“Areas with low vaccination experienced a more intense surge of new cases during the third wave of the pandemic in the US, primarily driven by the Delta variant,” the investigators reported. “The results presented here illustrate the association of the spatial heterogeneity of vaccination coverage with the overall COVID-19 epidemic in the country.”