Vaccines can dramatically cut the risk of coronavirus infection—even among people who've already had Covid-19, according to a small, real-world study in CDC's Morbidity and Mortality Weekly Report.

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Unvaccinated 2x as likely to get reinfected, study finds

For the study, researchers assessed 246 Kentucky residents who contracted a confirmed case of Covid-19 in 2020 and who in May or June of this year tested positive again. They were compared to a control group of 492 people who also had a confirmed Covid-19 case in 2020 but who had not tested positive through June 2021.

Overall, the researchers found that those in the study who were unvaccinated were 2.34 times as likely to be reinfected with Covid-19 than those who had been vaccinated. According to CDC, natural immunity after recovering from Covid-19 "is suspected to persist for ≥90 days in most persons," although it is "not well understood."

According to The Hill, the study provides some real-world data on a topic that previously had been studied primarily in laboratory settings.

The researchers noted their study had some limitations, including its small overall size and the fact that they did not confirm the reinfection cases through genome sequencing, which is the "gold standard," the Wall Street Journal reports. However, the researchers noted that while a repeated positive test can in some instances result from prolonged shedding of the virus, the long time period between the initial infection in 2020 and the repeat positive test results in 2021 suggest reinfection is the most likely explanation.

The researchers also noted that the reinfection numbers "might be overestimated," since vaccinated people may be less likely than unvaccinated people to get tested again.

Findings released as nation surpasses 100K daily Covid-19 cases

CDC released the findings as the nation's seven-day average of new daily Covid-19 infections hit 100,200—the first time the United States has surpassed 100,000 since mid-February, the New York Times reports. On Friday, according to the Times, the country reported 106,723 new cases of Covid-19.

The researchers noted that the emergence of new coronavirus variants may affect how long the estimated 90-day natural immunity could last. They explained that their study took place when the alpha variant was the most prevalent variant—as opposed to now, when the highly contagious delta variant is the nation's dominant strain.

"These findings suggest that among persons with previous SARS-CoV-2 infection, full vaccination provides additional protection against reinfection," the researchers wrote in the report. "To reduce their risk of infection, all eligible persons should be offered vaccination, even if they have been previously infected with SARS-CoV-2."

Separately, CDC Director Rochelle Walensky on Friday said, "If you have had Covid-19 before, please still get vaccinated. 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." (Coleman, The Hill, 8/6; Toy, Wall Street Journal, 8/6; Gonzalez, Axios, 8/6; Rabin, New York Times, 8/6)