Many people who’ve been infected with the coronavirus might be able to safely skip the second jab of any two-dose vaccine regimen, a growing number of studies suggest. These results could help to stretch scarce vaccine supplies and are already influencing vaccination policies in some countries. But questions remain about whether the findings hold for all individuals and all vaccines — and therefore how policymakers should respond to the findings.

Studies show that people with previous exposure to SARS-CoV-2 tend to mount powerful immune responses to single shots, and gain little added benefit from another injection^1,2,3. What’s more, for people with immunity gained through infection, one dose typically boosts antibody numbers to levels that are equal to, or often greater than, those found in individuals who have not been infected and have received double doses⁴.

France, Germany and Italy, among other countries, now advise only one dose of vaccine for people with a healthy immune system and a confirmed previous diagnosis. Many scientists who have studied immune responses to vaccination say such policies are a sensible way to make the most of limited supplies in countries that are racing to inoculate their populations.

“To follow the current two-dose vaccination schedule in previously infected individuals, when there are millions of people waiting for their first dose, simply does not make sense,” says Jordi Ochando, an immunologist at the Icahn School of Medicine at Mount Sinai in New York City, who has advised the Spanish government on vaccination guidelines.

Mix-and-match COVID vaccines trigger potent immune response

But scientists still don’t know whether one-jab programmes for the previously infected could leave some individuals with suboptimal protection. Nor is it clear that such programmes would be effective for all types of vaccine.

“If you have been infected before, then probably one dose is sufficient,” says Giuliana Magri, an immunologist at the Hospital del Mar Research Institute in Barcelona, Spain. But putting that knowledge into practice? “It’s complicated,” she concedes.

Just one jab

There’s ample laboratory-based evidence that people who’ve been infected by SARS-CoV-2 benefit from vaccination, prompting the World Health Organization and other public-health agencies to recommend that such individuals still get vaccinated. There’s less clarity, however, on whether they need to roll up their sleeves twice.

A paper published in Nature on 14 June provides some of the most recent evidence that one shot could be all that’s necessary for people who’ve had COVID-19⁵. A team of researchers at the Rockefeller University in New York City and elsewhere studied 26 people who had contracted the virus early in the course of the pandemic. All of them subsequently received at least one dose of either the Pfizer–BioNTech or the Moderna vaccine, both of which are based on messenger RNA.

The researchers analysed participants’ levels of ‘neutralizing’ antibodies, potent immune molecules that can block the virus from entering cells. At the moment, the quantity and vigour of a person’s neutralizing antibodies are the best markers for assessing whether that person is protected from infection and illness — although scientists are still working to confirm that antibody levels can serve as a realistic stand-in for immune protection.