Coronavirus vaccines were just rolling out in December when more than 1,000 staffers at Cedars-Sinai Medical Center in Los Angeles volunteered for a sweeping study. The goal: pinpoint how immune reactions to the jab might vary.

By last month, a clear pattern in the data "popped out at us,” said research leader Susan Cheng. Those who had recovered from COVID-19 responded to their first shot so robustly that the results rivaled never-infected colleagues who had received both shots. The implication was clear. If you’ve had COVID-19, you may only need one of the two doses recommended by Pfizer and Moderna.

"We did not expect that this was going to jump out like a smoking gun,” said Cheng, who co-authored the Nature Medicine write-up. In fact, if you already had the virus, your immune response after one vaccine is likely to be even better than a never-infected person’s after two, according to Italian research just out in the New England Journal of Medicine.