For most of last year, many of us called for the Centers for Disease Control and Prevention to release its data on reinfection rates, but the agency refused. Finally last week, the CDC released data from New York and California, which demonstrated natural immunity was 2.8 times as effective in preventing hospitalization and 3.3 to 4.7 times as effective in preventing Covid infection compared with vaccination.

My Johns Hopkins colleagues and I conducted the study. We found that among 295 unvaccinated people who previously had Covid, antibodies were present in 99% of them up to nearly two years after infection. We also found that natural immunity developed from prior variants reduced the risk of infection with the Omicron variant. Meanwhile, the effectiveness of the two-dose Moderna vaccine against infection (not severe disease) declines to 61% against Delta and 16% against Omicron at six months, according to a recent Kaiser Southern California study. In general, Pfizer’s Covid vaccines have been less effective than Moderna’s.

The CDC study and ours confirm what more than 100 other studies on natural immunity have found: The immune system works. The largest of these studies, from Israel, found that natural immunity was 27 times as effective as vaccinated immunity in preventing symptomatic illness.