U.S. health advisers are tackling who should get boosters of Johnson & Johnson’s single-shot COVID-19 vaccine and when — and whether using a competing brand for the second dose might provide better protection.

Adding another twist, the experts also will discuss preliminary data from a government “mix-and-match” study that suggested J&J recipients may have a far stronger immune response if they get either a Moderna or Pfizer booster rather than a second J&J dose.

On Thursday, U.S. health advisers said that some Americans who received Moderna’s COVID-19 vaccine should get a half-dose booster to bolster protection against the virus.

The panel of advisers to the Food and Drug Administration voted unanimously to recommend a booster shot for seniors, adults with other health problems, jobs or living situations that put them at increased risk for COVID-19.