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Unvaccinated adults who were previously infected with COVID-19 were twice as likely to be reinfected as those previously infected but also fully vaccinated, researchers found.

A case-control study in Kentucky found a more than two times higher risk of COVID-19 cases among unvaccinated adults with prior infection compared with their fully vaccinated counterparts (OR 2.34, 95% CI 1.58-3.47), reported Alyson Cavanaugh, PhD, of the CDC, and colleagues, writing in an early edition of the

Notably, the data were from May to June 2021, prior to reports of the Delta variant becoming the predominant strain in the U.S.

"This study shows you are twice as likely to get infected again if you are unvaccinated," CDC Director Rochelle Walensky, MD, said in a statement. "Getting the vaccine is the best way to protect yourself and others around you, especially as the more contagious Delta variant spreads around the country."

Cavanaugh's group added that not only does the study suggest that being unvaccinated is associated with a higher likelihood of being reinfected, but "full vaccination is associated with reduced likelihood of reinfection" among those previously infected.

They noted that laboratory evidence suggesting antibody responses following COVID-19 vaccination "provide better neutralization of some circulating variants than does natural infection," but epidemiologic studies among those previously infected have been lacking.

The authors examined a database of Kentucky residents ages 18 or older with positive SARS-CoV-2 infection as confirmed by nucleic amplification testing or antigen testing. They noted that they chose May and June due to vaccine supply and eligibility requirements, adding "this period was more likely to reflect resident choice to be vaccinated, rather than eligibility to receive vaccine."

Case patients were Kentucky residents with a positive SARS-CoV-2 test in 2020 who also had a positive SARS-CoV-2 test in May to June 2021, while controls were those with a positive test in 2020 who were not reinfected during those 2 months in 2021. They were matched 1:2 based on age, sex, and date of positive test result, within 1 week. Vaccination data were from the Kentucky Immunization Registry.

Fully vaccinated individuals received either a single dose of the Johnson & Johnson vaccine or the second dose of the Pfizer-BioNTech or Moderna vaccines at least 14 days prior to reinfection. Partially vaccinated individuals received one dose of mRNA vaccine or received the second dose less than 14 days prior to reinfection.

Overall, there were 246 case patients and 492 controls. About 61% were women, and 83% were previously infected with COVID-19 from October to December 2020. Among case patients, 20.3% were fully vaccinated versus 34% of controls.

While unvaccinated patients with previous infections were twice as likely to be reinfected versus fully vaccinated patients, "partial vaccination was not associated with reinfection," the authors noted (OR 1.56, 95% CI 0.81-3.01).

"If you have had COVID-19 before, please still get vaccinated," Walensky said.

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