Unvaccinated people who previously had Covid-19 are significantly more likely to test positive for the disease again than fully vaccinated people who have never had Covid-19, according to a recent CDC study published in the Morbidity and Mortality Weekly Report.

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Study details

For the study, CDC examined data on 7,348 people hospitalized with a Covid-like illness at 187 hospitals in nine states from Jan. 1 to Sept. 2. All patients were ages 18 and older and had a Covid-19 test between 14 days before hospital admission and 72 hours after.

The researchers found that unvaccinated people with a prior Covid-19 infection were more than five times as likely to test positive for Covid-19 than those who had been fully vaccinated and never had the disease.

In total, 8.7% of unvaccinated adults in the study who had a prior Covid-19 infection tested positive for the disease again while just 5.1% of the fully vaccinated participants who never had a prior infection did.

Discussion

The study authors noted there were some limitations to the research. For example, the study's findings might not translate to patients who were not hospitalized and had different levels of access to medical care. It's also possible that some of the patients in the vaccinated group had a prior Covid-19 infection and didn't know it.

The researchers added that other studies have found conflicting results on the protective power of prior infection. For instance, a preprint study from Israel found that prior infection was more protective against the delta coronavirus variant than vaccination.

CDC Director Rochelle Walensky, in a statement accompanying the study, said, "We now have additional evidence that reaffirms the importance of Covid-19 vaccines, even if you have had prior infection."

She added, "This study adds more to the body of knowledge demonstrating the protection of vaccines against severe disease from Covid-19." (Mueller, New York Times, 10/29; Walker, MedPage Today, 10/29; Sullivan, The Hill, 10/29)