WASHINGTON — The Food and Drug Administration is wrestling with whether and when to offer another dose of the single-shot Johnson & Johnson COVID-19 vaccine, while a new study out Wednesday raises the prospect that using a different brand as the booster might work better.

In an online review posted Wednesday, FDA scientists didn’t reach a firm conclusion about whether there’s enough evidence for J&J boosters, citing shortcomings with the company’s data and little information on protection against the extra-contagious delta variant of the coronavirus.

The review comes ahead of meetings today and Friday when an FDA advisory panel will recommend whether to back booster doses of both the J&J and Moderna vaccines. That’s one step in the government’s vaccine review process: Next week, the FDA will make a final decision on authorizing those boosters and then the Centers for Disease Control and Prevention will debate who actually should get them.

Adding to the complexity is whether it’s OK to use a booster that’s a different brand than someone’s initial shots. Preliminary results of a U.S. government study suggest that mixing and matching boosters will work at least as well — and maybe far better for J&J recipients. Those people had a stronger immune response if they got either a Moderna or Pfizer shot as their booster than if they received another dose of the J&J vaccine, according to results posted online Wednesday. Mix-and-match is also up for discussion by the FDA panel this week.