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Past Covid infection protected against delta, but vaccines safest to prevent dire illness, CDC says

It's unclear, however, whether the findings apply to infections caused by the omicron variant of the virus.
A healthcare worker administers a Pfizer-BioNTech Covid-19 vaccination in San Francisco on Jan. 10, 2022.
A health care worker administers a Pfizer-BioNTech Covid-19 vaccine shot in San Francisco on Jan. 10.David Paul Morris / Bloomberg via Getty Images file

New research from the Centers for Disease Control and Prevention and health officials in California and New York indicates that a prior case of Covid-19 protected people from infection better than vaccinations did during the delta wave last summer and fall.

The findings, published Wednesday in the CDC's Morbidity and Mortality Weekly Report, appear to contradict public health messaging that pushes vaccinations.

But experts maintain that the shots remain the safest way to protect against the most dire consequences of Covid. Even during the height of the delta wave last summer, virtually all hospitalized Covid patients were unvaccinated.

"Vaccines continue to reduce a person's risk of contracting the virus that causes Covid-19 and are highly effective at preventing severe illness," Benjamin Silk, one of the study's authors and an infectious diseases epidemiologist at the CDC, said during a call with reporters Wednesday.

The new research is based on analyses of Covid infections among more than 1.1 million adults in California and New York from May to November last year.

That time frame is limited to a period when the highly transmissible delta variant was picking up steam, and immunity among those first vaccinated in the early days of last year was beginning to diminish. Booster shots were not yet widely available.

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As such, the findings cannot be applied to the current omicron variant surge, which accounts for more than 99 percent of new Covid cases in the United States.

"Omicron has changed things completely," said Dr. Paul Sax, an infectious diseases expert at Brigham and Women's Hospital and Harvard Medical School. The omicron variant has led to "much higher rates of reinfections and a much higher rate of breakthrough infections," he said.

The CDC is expected to release research on how vaccines and boosters fare against omicron later this week.

The study released Wednesday, from researchers at the CDC, the California Department of Public Health, the New York State Department of Health and the New York City Department of Health and Mental Hygiene, found that people who had been infected with the coronavirus were better protected against reinfections during the delta wave than those who'd never been infected but had been vaccinated.

And the most robust protection against the delta variant was seen in people who were both vaccinated and had previously been diagnosed with the virus — sometimes referred to as "hybrid immunity."

But this level of protection may change as the virus evolves.

"Although the epidemiology of Covid-19 might change with the emergence of new variants, vaccination remains the safest strategy to prevent SARS-CoV-2 infections and associated complications," the study authors wrote.

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Case and hospitalization rates were highest among people without any immunity: those who had never been diagnosed with Covid and were never vaccinated.

And Covid-19 infection comes with dire risks. By the end of the study period, more than 130,000 people in California and New York had died of the disease.

Still, that the findings suggest strong immunity after infection may be important for public health policies moving forward, said Dr. Jeffrey Klausner, a professor of medicine and public health at the Keck School of Medicine of the University of Southern California.

"No one who cares about public health is ever going to say that it's better to get infected than get vaccinated when we have a safe vaccine," Klausner said. "But in terms of policy, this supports all the clinical research and other data that suggest that immunity after infection is real, is durable."

Because of that, he said, "policy in the United States should be updated, like in many European countries, to allow for people to go to work, to go to school, if they have evidence of recovery of infection without requiring vaccination."

Anyone ages 12 and older is eligible for a Covid booster shot five months after their last dose of either the Pfizer-BioNTech or the Moderna vaccine. A booster for those who originally received the Johnson & Johnson shot is given after two months. 

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