The FDA's expert panel just unanimously backed Moderna's booster shot for high-risk groups, paving way for the agency's OK

• On Thursday the FDA's expert panel voted in favor of booster shots of Moderna's COVID-19 vaccine.
• Moderna is seeking an OK for all older adults (65+) and younger adults at high risk of severe COVID-19.

Moderna just got a step closer to rolling out a COVID-19 booster shot to the general public.

A panel of independent experts convened by the Food and Drug Administration voted unanimously Thursday in favor of a Moderna booster shot for certain people who got their shot at least six months ago. The group supported offering an extra dose to adults 65 years and older, as well as younger adults who are at high risk of severe COVID-19.

The recommendation, backed by a 19-0 vote by the committee, clears the way for the FDA to authorize a booster of Moderna's vaccine for more people. The FDA typically follows the recommendations of its advisory panel, but it is not required to do so.

For many people in the US, the booster-shot campaign has already begun. In August, the agency OK'd a third dose of Moderna's shot for people who are moderately or severely immunocompromised. The FDA also authorized a third dose of Pfizer's coronavirus vaccine in September for all older adults and younger adults at high risk of severe disease.

So far, about 8.6 million people in the US have received a booster shot, according to the Centers for Disease Control and Prevention.

Moderna's booster shot will be a smaller dose

The Massachusetts biotech is seeking to use a booster shot that is half the strength of the first two shots. Moderna said this lower dose still provided a robust immune response, as seen by higher levels of virus-fighting proteins called antibodies, and carried a similar side-effect profile as the second dose.

Study results with Moderna's booster are limited, as the study is still ongoing and accruing data. Moderna's application includes trial data from 149 volunteers who got the lower-dose booster after being fully vaccinated. That trial failed to meet one of the two success criteria the FDA said it's looking for when reviewing booster shots, the agency's review said. The study succeeded in showing a booster raised antibody levels as high as the second dose, but the trial failed to meet the FDA's bar of showing the extra dose raised antibody levels at least fourfold from pre-booster levels.

In a sign of the urgency fueling the booster debate, panelist Dr. Patrick Moore said he voted in favor of authorization from "more gut feeling than based on really, truly serious data."

"The data itself is not strong, but it is certainly going in a direction that is supportive of this vote," said Moore, a microbiology and molecular genetics professor at the University of Pittsburgh.

The safety profile of Moderna's booster shot was generally similar to the second dose. People under 65 years old reported more frequent side effects than older volunteers.

The most common side effects after a booster shot included injection site pain, fatigue, headache and muscle pain.

A Johnson & Johnson booster shot could be next

The FDA is also reviewing Johnson & Johnson's application to give an extra dose for its single-shot vaccine. The expert panel will meet again Friday to discuss and vote on J&J's booster shot.

Boosters shots have turned into a contentious topic, particularly on when exactly they are needed. It's unclear how long protection lasts from initial vaccination, and it's also unknown how long a booster shot would protect people from COVID-19.

Scientists are also divided over if booster shots should only be used if protection against severe illness is declining. Others say there's benefit in preventing milder COVID-19 cases among the vaccinated, and it's better to proactively respond to any signs of waning protection.

While the FDA's expert panel is a critical hurdle to clear, the review process is still ongoing. The CDC will have its own advisory committee meet on October 20 and 21 to vote on recommendations for using Moderna and J&J booster shots, if the FDA OKs the shots by then. The final step in the review will be the CDC's director finalizing its agency's recommendations on how to use the booster shots.