Dr. Rochelle 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. “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 authors cautioned that much is still not known on how long natural immunity to the virus lasts and that genomic sequencing to confirm reinfections among those in the study was not conducted.
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.