A new study in two states compared the protection of the coronavirus against prior infection and vaccination, concluding that vaccination remains the safest way to prevent COVID-19.

The study, which examined infections last summer and fall in New York and California, found that people who were both vaccinated and who had survived a previous COVID-19 had the greatest protection.

But unvaccinated people who had previously been infected were close behind. By the fall, when the more contagious delta variant had taken over but the booster had not yet become widespread, these people had a lower diagnosis rate than those who had not been infected with the vaccine in the past.

The Centers for Disease Control and Prevention, which released the study on Wednesday, noted several caveats from the study. Some outside experts are cautious about the findings and how to interpret them.

“The most important message is that symptomatic COVID infection does confer some immunity,” said E. John Wherry, an immunologist at the University of Pennsylvania. “But immunity from vaccination is still better than immunity from infection. Much safer.”

Ali Ellebedy, an immunologist at the University of Washington, added that vaccination has been urged even after previous COVID-19 cases because both protections will eventually wane — and there are too many unknowns to rely on past infections alone, Especially an infection from a long time ago. in St. Louis.

“There are so many variables out of your control that you can’t use it to say ‘Oh, I’m infected, and then I’m protected,'” Ellebedy said.

The study does agree with a small group of studies that have found that unvaccinated people have a lower risk of being diagnosed with COVID or getting sick compared to people who have never been infected before.

The findings seem plausible, says Christina Peterson, an epidemiologist at the University of Iowa. Vaccines developed against earlier forms of the coronavirus could become increasingly ineffective against new, mutated versions, she said.

However, she and other experts say there are many other possible factors at play, including whether the effectiveness of the vaccine wears off in many people over time, and the extent to which mask wearing and other behaviors affect all Things that happened.

CDC officials noted that the study was done before the omicron variant took over and before many Americans received booster doses, so it wasn’t clear what effects the boosters might have had. The analysis also did not address the risk of severe illness or death from COVID-19.

The study authors concluded that vaccination “remains the safest strategy to prevent infection” and that “all eligible individuals should be promptly vaccinated against COVID-19.”

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The researchers looked at infections in California and New York, which together account for about 18 percent of the U.S. population. They also looked at COVID-19 hospitalizations in California.

Overall, about 70 percent of adults in each state were vaccinated; another 5 percent were vaccinated and had a history of infection. Less than 20% were unvaccinated; about 5% were unvaccinated but had a history of infection.

The researchers looked at COVID-19 cases from late May to mid-November last year and calculated how often new infections occurred in each group.

Compared to unvaccinated people who have not been diagnosed with COVID-19, the case rate is:

— Those who were vaccinated but not previously infected were 6-fold and 4.5-fold lower in California and New York, respectively.

— 29-fold and 15-fold reductions in the number of people who were infected but never vaccinated in California and New York.

— The number of people infected and vaccinated was 32.5 times lower in California and 20 times lower in New York.