WASHINGTON — Influential government advisers are deciding Thursday how best to expand the nation’s COVID-19 booster campaign, including whether and when it’s OK to “mix and match” brands for the extra dose.

The advisers to the Centers for Disease Control and Prevention are slated to discuss who should get extra doses of the Moderna or Johnson & Johnson vaccines — and the bigger question of getting a different brand for the booster than people’s original vaccination.

The Food and Drug Administration authorized both steps Wednesday, as part of a federal push to broaden booster access for the U.S. public. But the CDC, guided by its advisory panel, provides the final blessing.

About two-thirds of Americans eligible for COVID-19 shots are fully vaccinated, and several million have gotten additional doses of Pfizer’s vaccine after the FDA and CDC gave that go-ahead last month. While health authorities hope boosters will shore up waning immunity against milder coronavirus infections, all the vaccines still offer strong protection against hospitalizations and death — and getting the unvaccinated their first shots remain a priority.

“For most individuals, in most circumstances, death from COVID-19 is vaccine-preventable,” said CDC adviser Dr. Matthew Daley of Kaiser Permanente Colorado.

Thursday’s meeting promises tough decisions, just like the panel faced with Pfizer’s boosters. Ultimately those boosters were recommended for everyone 65 and older, nursing home residents, and younger adults at increased risk of infection because of health problems, jobs or living conditions. That includes health care workers, teachers and people in jails or homeless shelters.

The FDA cleared Moderna’s half-dose booster for those same groups, also six months after the last shot.

But for J&J’s single-shot vaccine, the FDA said all U.S. recipients need wait only two months following their initial vaccination. The agency said any adult who got the J&J shot should be eligible for a booster — a decision driven by research showing a single J&J dose was significantly less effective than a two-dose Pfizer or Moderna.

As for the mix-and-match question, the FDA opened the way for anyone eligible for a booster to get any of the country’s three authorized brands for that extra dose. The FDA stressed the practicality of being able to get whatever booster a particular pharmacy or clinic is offering, particularly in nursing homes and other institutional settings where residents have received different shots over time.