On Wednesday, the Centers for Disease Control and Prevention reported that people who had been infected with the coronavirus before vaccination had better protection against the Delta variant than people who had been vaccinated.

However, infection poses serious health concerns, and vaccination remains the best long-term defense against the virus, according to the researchers.

The data was collected before the widespread use of booster doses and the appearance of the Omicron variant, so the findings may not be applicable to the current outbreak, according to the CDC.

“These findings cannot be generalized to the current Omicron wave,” Benjamin Silk, an epidemiologist at the C.D.C., told reporters on Wednesday. “It’d be like comparing apples and oranges.”

By the end of November, New York and California were responsible for one-sixth of all Covid-related deaths in the United States. Scientists looked at testing, surveillance, and immunization data from the two states to see how well vaccines and previous infections kept people safe.

Throughout the study period, unvaccinated patients were at the highest risk of infection or serious sickness from Covid, according to the researchers. With the emergence of the Delta strain, however, the relative protection provided by vaccination or prior infection shifted.

During the week of May 30, 2021, people who had been vaccinated had the lowest risk of getting coronavirus and going to the hospital. People who hadn’t been vaccinated and had already been diagnosed with Covid had the second-lowest risk of getting the virus and going to the hospital.

Vaccinated individuals with a past diagnosis, on the other hand, fared best against the Delta strain by the week beginning Oct. 3. Unvaccinated people with a Covid background had lower rates of infection and hospitalization than those who were only protected by vaccines.

According to the experts, the findings are similar to patterns shown in other countries’ investigations.

The researchers believe that waning vaccine-derived immunity explains why vaccinated patients were less protected from infection with the Delta variant than those who had previously been diagnosed.

While a recent study of Cleveland Clinic employees found that immunization does not add much benefit to a previous infection for the first few months, it may provide superior long-term protection against symptomatic sickness than immunity from a previous infection.

“The totality of the evidence suggests really that both vaccination and having survived Covid each provide protection against infection and hospitalization,” said deputy director of science at the New York State Department of Health Eli Rosenberg.

However, because having Covid involves high risk, he advises that “becoming vaccinated and staying up to date with boosters really is the only safe choice.”

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