So what about a person's level of protection for the opposite scenario — when a fully vaccinated person later has a breakthrough infection? "You would presumably have superior immunity at this point," says Dr. Gregory Poland, director of the Mayo Clinic's Vaccine Research Group.
Most likely, this brand of hybrid immunity would rival what's seen in people who recovered from COVID-19 and then got vaccinated, but there could be subtle differences in the details of the immune response, says Wherry. "But these things that might be different may not matter for any given person to ward off the next infection."
All of this needs to be studied to arrive at a more conclusive answer. Until then, the takeaway is the same regardless of how you arrived at hybrid immunity: There's no great urgency to get a booster for people who are otherwise healthy and low-risk. But if you're higher risk due to underlying conditions, or live or work closely with someone who is, it could be a good added layer of protection for you and those around you.
"The potential benefits of it providing greater immunity are there and the risks are small," says Wesemann. "I would probably recommend it for a group of individuals that I thought were at high risk."