Unvaccinated Adults Who Had Virus Face Risk of Reinfection, C.D.C. Says

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Unvaccinated people who have had Covid-19 may be more than twice as likely to get infected again than those who tested positive and bolstered their natural immunity with a vaccine, according to a small study that assessed the likelihood of reinfection.

The study, published on Friday by the Centers for Disease Control and Prevention, examined the risk of reinfection during May and June among hundreds of Kentucky residents who tested positive for the virus in 2020.

Those who did not get vaccinated this year faced a risk of reinfection that was 2.34 higher than those who did get their shots. Released on Friday, the study suggests that for those who had overcome an infection, the addition of a vaccine offered better protection than the natural immunity generated by their original bout with the virus alone.

Even though the study examined only a small number of people in Kentucky, it would seem to counter the argument by one of its home-state U.S. senators, Rand Paul, who has repeatedly asserted that vaccination is unnecessary for people like him who had the virus and developed immunity.

Dr. Rochelle P. Walensky, the director of the C.D.C., said the data reinforced the importance of vaccination, even for those who had already had the virus.

“If you have had Covid-19 before, please still get vaccinated,” Dr. Walensky said on Friday. “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.”

The study’s authors cautioned that much is still not known on how long natural immunity to the virus lasts and that genomic sequencing was not conducted to confirm that the reinfections were not simply flare-ups of the remains of the subjects’ initial infection.

The C.D.C. and the Biden administration have been aggressively campaigning to increase vaccinations in recent weeks as cases and hospitalizations have soared in the last month, largely because of the Delta variant, and especially in regions of the country where immunization rates are low.

In the last week, the number of new virus cases reported each day has averaged 100,200, as of Thursday, the first time the daily average surpassed 100,000 since mid-February, according to a New York Times database. On Friday, the country logged 106,723 new daily cases.

Another study published on Friday reported that vaccinations dramatically reduced Covid hospitalizations among the elderly in February, March and April. The study examined data on 7,280 patients from a Covid hospitalization surveillance network, using state records to look at their vaccination status. The vast majority of the hospital patients had not been vaccinated or were only partially vaccinated; only 5 percent were fully vaccinated.

Although vaccination did not entirely eliminate infections, the risk of being hospitalized was significantly lower for those who were fully vaccinated. Among those 65 to 74, the Pfizer-BioNTech and Moderna vaccines reduced the risk of Covid-related hospitalization by 96 percent and the Johnson & Johnson one-dose vaccine reduced hospitalizations by 84 percent. In the age group of 75 and older, the Pfizer vaccination reduced hospitalizations by 91 percent; the Moderna vaccine by 96 percent; and the Johnson & Johnson vaccine by 85 percent.