Prior Covid Infection Offered More Protection Against Delta Than Vaccines — But Both Together Did Best

Topline

When the coronavirus’ delta variant surged last year, vaccinated people with prior infections were less likely to catch the virus than those who only had immunity from either an earlier case or vaccination, according to a study released Wednesday by the Centers for Disease Control and Prevention — another datapoint in the confusing and often-mixed research into the power of natural immunity.

Key Facts

The CDC’s study looked at data from California and New York between May and November 2021, before and after the virus’ delta variant became dominant in July.

In late May and early June, before the delta variant took root nationwide, vaccinated people with no previous coronavirus infection had lower case rates than unvaccinated but previously infected residents in both New York and California, according to the study.

However, by early October, New Yorkers and Californians who were both vaccinated and previously infected had lower case rates than previously infected but unvaccinated people, and both groups fared better than vaccinated but uninfected residents.

In the first full week of October, vaccinated New Yorkers with a prior Covid-19 case were 19.8 times less likely to catch the virus than their unvaccinated and uninfected peers, whereas people who were unvaccinated but previously infected were 14.7 times less likely, and vaccinated but uninfected New Yorkers were just 4.5 times less likely.

California recorded a similar trend in October, and the CDC also found Californians who were both vaccinated and previously infected were less likely to be hospitalized for Covid-19 than those who’d solely faced a prior infection or received a vaccine.

The study’s authors said “vaccination remains the safest and primary strategy to prevent [coronavirus] infections, associated complications, and onward transmission.”

What We Don’t Know

The CDC’s study ended weeks before the coronavirus’ omicron variant became the dominant strain in the United States last month, so the impact of omicron on vaccinated and previously infected people is still unclear. Also, the study didn’t weigh Covid-19 booster shots, which many experts believe could prevent vaccine effectiveness from waning. 

Key Background

Just over two-thirds of eligible Americans are vaccinated against Covid-19, leaving tens of millions of people without the immunity offered by vaccines. Evidence on whether some of those people are protected partially by prior Covid-19 infections — often called “natural immunity” — has been mixed. Two CDC studies from August and October found vaccines offered more protection than natural immunity, but a separate study from the U.K.’s Office of National Statistics last fall found vaccines and natural immunity were roughly equal. An August paper from Israel roughly lines up with Wednesday’s CDC study: It found vaccinated people were more likely to catch the delta variant than those with prior infections, but Israelis who took one dose of Pfizer’s vaccine after being infected had extra protection against delta. Some researchers have suggested this combination of vaccination and previous infection could offer what the Nature academic journal called “super-immunity” last year. Meanwhile, the CDC has argued vaccines remain highly effective for both groups, and has urged people not to forgo the coronavirus vaccines just because they’ve already survived Covid-19.

Tangent

Natural immunity has turned into a political lightning rod in some circles, with conservatives like Florida Gov. Ron DeSantis (R) arguing the Biden Administration’s push to mandate vaccines in some workplaces doesn’t give credit to people with immunity from prior infections.

Further Reading

Vaccines Offer More Protection Against Covid Than Prior Infection, CDC Study Suggests (Forbes)

Full coverage and live updates on the Coronavirus