Vaccinated people who previously recovered from Covid-19 had more protection against reinfection and hospitalization during the delta surge than those who were only vaccinated or were unvaccinated, according to a recent study from CDC. But the authors caution the findings cannot be applied to the omicron variant.

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Study details

For the study, researchers analyzed people in New York and California from May 30, 2021, to Nov. 20, 2021 who were:

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1. Unvaccinated and did not have a prior Covid-19 infection
2. Unvaccinated and previously had Covid-19
3. Vaccinated and did not have a prior Covid-19 infection
4. Vaccinated and previously had Covid-19

The researchers found that, in early October, vaccinated people with no prior Covid-19 infection saw 6.2-fold lower infection rates in California and 4.5-fold lower infection rates in New York than unvaccinated people without prior Covid-19 infection.

Meanwhile, unvaccinated people who previously had Covid-19 saw 29-fold lower infection rates in California and 14.7-fold lower infection rates in New York than unvaccinated people who hadn't had Covid-19. And vaccinated people who previously had Covid-19 saw 32.5-fold lower infection rates in California and 19.8-fold infection rates in New York than unvaccinated people with no prior infection.

For hospitalizations, the researchers only looked at California, as there was no hospitalization data available from New York. They found that, compared to unvaccinated people with no prior Covid-19 infection, vaccinated people with no prior infection had 19.8-fold lower hospitalization rates while unvaccinated people with prior Covid-19 infection saw 55.3-fold lower rates and vaccinated people with prior infection saw 57.5-fold lower rates.

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Eli Rosenberg, deputy director for science at the New York State Department of Health and an author on the study, said "the totality of the evidence suggests … that both vaccination and having survived Covid each prove protection against subsequent infection and hospitalization."

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However, "only one of those is the safe choice that we would recommend," he added. "And that's vaccination."

"The bottom line message is that from symptomatic Covid infection you do generate some immunity," said E. John Wherry, an immunologist at the University of Pennsylvania. "But it's still much safer to get your immunity from vaccination than from infection."

Vaccination following prior infection from Covid-19 has long been recommended because both natural and vaccine-induced immunity eventually wane. In addition, much still isn't known about natural immunity, particularly from an infection from a long time ago, said Ali Ellebedy, an immunologist at Washington University in St. Louis.

"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,'" Ellebedy said.

In its report, CDC noted that most people had not yet received booster shots during the analyzed time period. The agency also said the results of the study could not be applied to the omicron variant, but added it would release more studies focused on omicron in the coming days.

"These findings cannot be generalized to the current omicron wave," said Benjamin Silk, an epidemiologist at CDC. "It'd be like comparing apples and oranges." (Banco, Politico, 1/19; Joseph, STAT News, 1/19; Mandavilli, New York Times, 1/19; Walsh, Forbes, 1/19; Stobbe, Associated Press, 1/19; Walker, MedPage Today, 1/19)