U.S. health advisers endorsed a booster of Johnson & Johnson’s (J&J) Covid-19 vaccine Friday, citing concern that Americans who got the single-dose shot aren’t as protected as those given two-dose brands.

J&J told the Food and Drug Administration (FDA) that an extra dose adds important protection as early as two months after initial vaccination — but that it might work better if people wait until six months later. Unable to settle the best timing, the FDA’s advisory panel voted unanimously that the booster should be offered at least two months after people got their earlier shot.

“I think this frankly was always a two-dose vaccine,” said FDA adviser Dr. Paul Offit of Children’s Hospital of Philadelphia. “It would be hard to recommend this as a single-dose vaccine at this point.”

The FDA isn’t bound by the recommendation as it makes a final decision — and adding to the debate’s complexity, new research also suggests that J&J recipients might have a stronger immune response if their booster dose is from a competing brand.

Preliminary results from an ongoing study of different ways to “mix and match” different shots showed that a booster of any sort revved up people’s levels of virus-fighting antibodies — at least for a few weeks. And the most dramatic jump came from giving a Pfizer or Moderna shot after the single-dose J&J vaccination.

FDA’s advisers didn’t vote on whether that should be recommended but told the government to allow flexibility with boosters, saying there were no safety red flags even if it’s not yet clear just how much difference, if any, mixing and matching may make in long-term protection.

“In the real world all these kind of combinations are already happening so I think it’s a matter of some urgency for the FDA to help sort out what is admittedly a complicated and challenging scenario,” said Dr. Ofer Levy of Boston Children’s Hospital.

The government says all three U.S. vaccines continue to offer strong protection against hospitalization and death from Covid-19, and that the priority is getting first shots to the 66 million eligible but unvaccinated Americans who are most at risk. But with the spread of the extra-contagious delta variant and signs of waning immunity against milder infections, the nation is moving toward a broader booster campaign.