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The US Centers for Disease Control and Prevention has recommended that everyone get a COVID-19 vaccine, even if they’ve had the virus before. Yet many skeptics have held off getting the shots, believing that immunity generated by their previous infection will protect them if they should encounter the virus again. 

A new study published today in the CDC’s Morbidity and Mortality Weekly Report pokes holes in this notion. It shows people who have recovered from COVID-19 but haven’t been vaccinated have more than double the risk of testing positive for the virus again, compared with someone who was vaccinated after an initial infection.

The study looked at 738 Kentucky residents who had an initial bout of COVID-19 in 2020. About 250 of them tested positive for COVID-19 a second time between May and July of 2021, when the Delta variant became dominant in the US. 

The study matched each person who’d been reinfected with two people of the same sex and roughly the same age who had caught their initial COVID infection within the same week. The researchers then cross-matched those cases with data from Kentucky’s Immunization Registry.

They found that those who were unvaccinated had more than double the risk of being reinfected during the Delta wave. Partial vaccination appeared to have no significant impact on the risk of reinfection.

Among those who were reinfected, 20% were fully vaccinated, while 34% of those who did not get reinfected were fully vaccinated.

The study is observational, meaning it can’t show cause and effect; and the researchers had no information on the severity of the infections. Alyson Cavanaugh, PhD, a member of the CDC’s Epidemic Intelligence Service who led the study, says it is possible that some of the people who tested positive a second time had asymptomatic infections that were picked up through routine screening.

Still, the study backs up previous research and suggests that vaccination offers important additional protection.

“Our laboratory studies have shown that there’s an added benefit of vaccine for people who’ve had previous COVID-19.  This is a real-world, epidemiologic study that found that among people who’d previously already had COVID-19, those who were vaccinated had lower odds of being reinfected,” Cavanaugh said.

“If you have had COVID-19 before, please still get vaccinated,” said CDC Director Rochelle Walensky, MD, in a written media statement. “This study shows you are twice as likely to get infected again if you are unvaccinated. 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.”

In a White House COVID-19 Response Team briefing in May, Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Disease, explained why vaccines create stronger immunity than infection. He highlighted new research showing that two doses of an mRNA vaccine produce levels of neutralizing antibodies that are up to 10 times higher than the levels found in the blood of people who’ve recovered from COVID-19. Vaccines also enhance B cells and T cells in people who’ve recovered from COVID-19, which broadens the spectrum of protection and helps to fend off variants.

The study has some important limitations, which the authors acknowledge. The first is that second infections weren’t confirmed with genetic sequencing, so the researchers couldn’t definitively tell if a person tested positive a second time because they caught a new virus, or if they were somehow still shedding virus from their first infection. Given that the tests were at least 5 months apart, though, the researchers think reinfection is the most likely explanation.

Another bias in the study could have something to do with vaccination. Vaccinated people may have been less likely to be tested for COVID-19 after their vaccines, so the association or reinfection with a lack of vaccination may be overestimated. 

Also, people who were vaccinated at federal sites or in another state were not logged in the state’s immunization registry, which may have skewed the data.

MMWR Morb Mortal Wkly Rep. Published online August 6, 2021. Full text

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