Jan. 20, 2022 -- A new study from California and New York found that people who were both vaccinated and had a prior COVID-19 infection had the most protection against the coronavirus last summer and fall.

Unvaccinated people with a past infection came in second. By October, when the more contagious Delta variant had become dominant and booster shots weren’t available to most people, the unvaccinated group had a lower case rate than vaccinated people without a previous infection.

But the CDC, which released the study on Wednesday, noted several caveats to the research and concluded that getting vaccinated is still the safest way to prevent COVID-19. Public health experts are also urging caution around the study results due to the potential risks for hospitalization, long COVID, and death if infected.

“The bottom line message is that from symptomatic COVID infection, you do generate some immunity,” E. John Wherry, PhD, an immunologist at the University of Pennsylvania, told The Associated Press.

“But it’s still much safer to get your immunity from vaccination than from infection,” he said.

Vaccination continues to be recommended after prior COVID-19 infection because both types of protection eventually wane, and there are too many unknowns to rely on protection from a past infection alone -- particularly one that occurred earlier in the pandemic, public health experts said.

“There are so many variables you cannot control that you just cannot use it as a way to say, ‘Oh, I’m infected then I am protected,’” Ali Ellebedy, PhD, an immunologist at Washington University in St. Louis, told the AP.

By Nov. 30, about 130,000 COVID-19 deaths -- or 1 in 6 of all U.S. deaths -- had occurred in California and New York. The research team examined data from the two states between May to November to understand how protection differed among unvaccinated people without a prior infection, vaccinated people without a prior infection, unvaccinated people with a prior infection, and vaccinated people with a prior infection.

During that time, COVID-19 rates in both states were highest among unvaccinated people without a prior COVID-19 diagnosis. By the end of May, COVID-19 cases were lower among vaccinated people without a prior infection than unvaccinated people with or without a prior infection. Hospitalization rates were similar.

But when the Delta variant became dominant, the patterns changed. By the beginning of October, COVID-19 cases were substantially lower among both vaccinated and unvaccinated people who had a prior infection. The same was true for hospitalizations.

“Importantly, infection-derived protection was higher after the Delta variant became predominant, a time when vaccine-induced immunity for many persons declined because of immune evasion and immunologic waning,” the researchers wrote.

The study has a few caveats. The COVID-19 cases included in the study were based on lab-confirmed tests reported by California and New York, which means that self-tests weren’t included and some COVID-19 cases likely weren’t counted. The “staunchly unvaccinated” aren’t likely to get tested, Wherry told the AP.

“It may be that we’re not picking up as many reinfections in the unvaccinated group,” he said.

What’s more, the study took place before the Omicron variant was detected and before many U.S. residents received booster doses. The analysis also didn’t include information about the severity of past infections that might lead to protection or negative long-term outcomes.

In both states, about 70% of adults were vaccinated and didn’t have a previous infection. About 18% of people were unvaccinated without a previous infection, about 5% were vaccinated and had a previous infection, and about 5% were unvaccinated and had a previous infection.

“Vaccination remains the safest strategy for averting future SARS-CoV-2 infections, hospitalizations, long-term sequelae and death,” the researchers concluded. “Primary vaccination, additional doses and booster doses are recommended for all eligible persons.”

Show Sources

CDC: “COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis -- California and New York, May-November 2021.”

The Associated Press: “Prior infection, vaccines provide best protection from COVID.”

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