A Food and Drug Administration panel of outside advisers unanimously voted Friday in favor of authorizing booster shots of the Johnson & Johnson COVID-19 vaccine two months after the initial shot has been administered.

However, some members of the Vaccines and Related Biological Products Advisory Committee said they view the additional shot more as Part 2 of the original regimen, which was one shot, than a true booster. 

The panel also discussed the possibility of approving the booster shots at six months, but members expressed concerns about the limited number of participants — just 17 people — observed in the clinical trial at that interval.

The committee's vote is not binding but will factor heavily into the FDA’s final decision. If the agency follows the recommendation, the additional shots would be available to everyone who was initially inoculated with the J&J shot.

On Thursday, the same group of advisers unanimously endorsed booster shots for Moderna’s vaccine for people 65 and older, younger adults with underlying medical conditions that put them at high risk of severe COVID-19 and those with occupational exposure. 

If the FDA indeed authorizes the additional Moderna and J&J shots, all three vaccines available in the U.S. will soon have boosters approved for emergency use. The agency OK’d Pfizer’s booster shots last month for people over 65 and other high-risk adults. Pfizer and Moderna booster shots for immunocompromised people have been approved since August.

A panel convened by the Centers for Disease Control and Prevention is set to meet next week to offer more specifics on administering the additional shots.

Johnson & Johnson presented results from a large study that found giving a second dose just two months after the first bumped protection against symptomatic COVID-19 to 94% from 70% in U.S. recipients.

The J&J vaccine was highly anticipated for its one-and-done formulation when it received emergency use authorization in February, but it wasn’t as effective as the Pfizer and Moderna mRNA vaccines, which clinical trials found to be around 95% effective at preventing symptomatic COVID-19.

“I think this, frankly, was always a two-dose vaccine,” said committee member Dr. Paul Offit of Children’s Hospital of Philadelphia. “I think it's better as a two-dose vaccine. It's hard to recommend this as a single-dose vaccine at this point given those two-months data.”

“I'm inclined to just consider this a two-dose vaccine and how it should probably go forward,” Dr. Michael Kurilla of the National Institutes of Health, also a panel member, agreed.

“This is really, with the second dose, bringing it, I think, on par with those other vaccines in terms of effectiveness,” added Chicago Medical School Dean Dr. Archana Chatterjee, also on the committee.

While the data presented did not seem to raise any red flags about administering Johnson & Johnson booster shots starting at six months, some committee members were reluctant to act on the limited evidence for that interval.

“Although I think it likely to be fine, it's really hard to make a decision for thousands and tens of thousands and millions of people based on 17 people,” Offit said.

Offit, however, acknowledged that it would be difficult logistically to only recommend the additional shots at two months, and not six months.

In its presentation Friday, Johnson & Johnson said its single-dose vaccine maintains its effectiveness of 74% against severe disease and 70% against all symptomatic cases at least six months later. The vaccine’s antibodies peak later than the other vaccines available in the U.S. and is “broadly reactive against multiple strains of variants that we’ve tested,” said Dr. Penny Heaton, global therapeutic area head of vaccines at Johnson & Johnson’s Janssen Pharmaceutical Companies. 

But the company said it is seeking approval for its booster shot to optimize protection. 

“We know that a booster dose will do that,” Heaton said. “It will increase efficacy against severe disease, it will increase efficacy against all symptomatic COVID, and it will increase the breadth of the immune response against various.”

Dr. Johan Van Hoof, managing director for Janssen Vaccines & Prevention B.V., added that J&J wants to “stay ahead of the game” and make sure it’s prepared for potential future variants.

FDA officials said much of the data submitted by Johnson & Johnson arrived too late for scientists to conduct an independent review.

Dr. Doran Fink, deputy director of clinical at the FDA’s vaccines and related products applications division, said the hearing was being rushed due to “public interest and sense of urgency.”

"As a consequence of the review time, we were not able to conduct an independent verification of their data sets or their analyses or conduct our own analyses," Fink said.

Johnson & Johnson representatives said that the safety profile of the booster dose at two and six months was similar to the initial shot. FDA's reviewers, too, said they found no new safety concerns after a second J&J dose, but again, they cautioned about the small number of patients studied and short follow-up period.

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. 

Mixing and matching was being discussed by the FDA advisory panel after Friday’s vote.

During Johnson & Johnson’s presentation, Van Hoof preemptively argued that the National Institutes of Health study only offered “a piece of the puzzle” and that the findings were not conclusive about the benefits and risks of mixing and matching vaccines, also known as heterologous boosting.

Health authorities say all the vaccines used in the U.S. continue to provide strong protection against severe disease or death from COVID-19. 

Aiming for uniform recommendations, Moderna asked the FDA to clear its booster dose at six months, just as Pfizer’s is. But J&J complicated the decision by proposing a second shot over a range of two to six months.

J&J’s rollout earlier this year was hurt by a series of troubles including manufacturing problems and some rare but serious side effects, including a blood-clot disorder and a neurological reaction called Guillain-Barre syndrome. In both cases, regulators decided the shot’s benefits outweighed those risks.