CDC report finds prior infection provided strong protection against COVID-19 in 2021, but experts warn it's 'playing with dynamite' to get sick on purpose

• A new report from the CDC shows how vaccine-induced protection against COVID-19 changed during the Delta wave.
• Prior infections and vaccinations both provided good immunity against Delta, but infections had a slight edge.

A large new study from the Centers for Disease Control and Prevention that charted COVID-19 infections among New Yorkers and Californians from May to November of 2021 suggests that while vaccination is the safest way to develop immunity against the coronavirus, prior infections also worked very well at stopping the Delta variant — even better than vaccination alone.

"Before the Delta variant, COVID-19 vaccination resulted in better protection against a subsequent infection than surviving a previous infection," CDC epidemiologist Benjamin Silk said on a call with reporters shortly before the new MMWR report was released on Wednesday.

But once Delta became predominant in the spring and summer of 2021, the balance shifted, Silk said: "Surviving a previous infection now provided greater protection against a subsequent infection than vaccination."

You can see this clearly during the week of October 3, when Delta dominated. COVID-19 case rates then were worst among people with no prior infections and no vaccines. Compared to them:

• Vaccinated people had between 5- and 6-fold lower rates of laboratory-confirmed infections
• Previously infected people had between 15- and 29-fold lower rates of laboratory-confirmed infections
• And people with hybrid immunity (both previously infected and vaccinated) fared best of all, with a roughly 20- to 33-fold lower rate of infection.

During that same week, similar trends held true for hospitalizations in California. Compared to unvaccinated people with no previous COVID-19 diagnoses, hospitalization rates were about:

• 20-fold lower among vaccinated people without a previous COVID-19 diagnosis
• 55-fold lower among unvaccinated people with a previous COVID-19 diagnosis, and
• 58-fold lower among vaccinated people with a previous COVID-19 diagnosis

Getting infected confers protection, but also 'carries with it significant risks'

"Both vaccination and having survived COVID each provide protection against subsequent reinfection," study author Eli Rosenberg, deputy director for science at the New York State Department of Health, said during the call, summing up the new findings.

But that doesn't mean that getting infected is a winning pandemic strategy.

"Having COVID the first time carries with it significant risks," Rosenberg said.

The study authors cautioned that vaccination is still the only safe way to develop immunity against COVID-19, since infection carries many short- and long-term risks, including long COVID and MIS-C, which are impossible to predict.

"You'd be crazy to try to get infected with this," Dr. Robert Murphy, executive director of the Havey Institute for Global Health at Northwestern University Feinberg School of Medicine, recently told CNN. "It's like playing with dynamite."

Additionally, the study examined data from a time before most Americans had booster shots, which early studies suggest can improve the body's overall immune response to Omicron. Extrapolating this Delta data to Omicron, which is a different coronavirus variant with many new mutations, would be like comparing apples to oranges, Silk said.

Even though Omicron is on the decline now across much of the US, experts say there's a good possibility we could see another variant that acts differently than either Delta or Omicron have so far, and it's unclear what role vaccine- or infection-induced immunity will play if and when that happens.

"SARS-CoV2 continues to challenge us," Dr. Erica Pan, a state epidemiologist for the California Department of Public Health said on the call, referring to the virus that causes COVID-19. "I think we still have so much to learn."