COVID-19 re-infection can happen sooner than you think, CDC study says

An unvaccinated teenager who tested positive for the COVID-19 Delta variant last year was re-infected with the Omicron variant in as little as 23 days, according to a case study that looked at potential limitations of "infection-induced" immunity from COVID-19.

The study by the U.S. Centers for Disease Control and Prevention (CDC) analyzed 10 people who reported "early re-infections," or those who tested positive for the Omicron variant within 90 days of testing positive for the Delta variant. Delta was the dominant strain in the U.S. until Omicron took over in December 2021.

Among the 10 people studied - five from Vermont, three from Wisconsin, one from Washington state and one from Rhode Island - the shortest time between infections was the teenager in Washington with 23 days. The longest was 87 days.

Eight of the people studied were younger than 18, and only one had received a full series of COVID-19 vaccines. They weren’t eligible for a booster at the time of re-infection. No other patient was up-to-date on recommended COVID vaccines.

The CDC says vaccinations provide additional protection against COVID-19, even if you’ve been infected before.

The agency warns that the findings "might not be generalized to the U.S. population," and the data is limited to the transition period when Omicron surpassed Delta as the dominant U.S. strain.

"Nonetheless, this study highlights potential limits of infection-induced immunity against novel variants," the study concludes. "Although the epidemiology of COVID-19 might change as new variants emerge, vaccination remains the safest strategy."

The study was published about a week before a Florida judge ended the sweeping federal mask mandate on planes, trains and in transit hubs, the last of major pandemic rules at the federal level. The Justice Department said it won’t appeal the judge’s ruling unless the CDC believes the requirement is still necessary.

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Passengers on a Delta Airlines flight clapped as the captain announced that they were no longer required to wear face masks on board on April 18, 2022. Credit: Leticia Elisea via Storyful

As of Tuesday, the CDC had not made a determination.

New reported U.S. cases of COVID-19 are relatively low compared with the past two years, but they have increased lately and are likely an undercount. Hospitalizations are nearly flat and deaths are still declining, according to the Associated Press.

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