A panel of outside experts on Friday advised the Food and Drug Administration to authorize a booster dose of the Johnson & Johnson coronavirus vaccine for people 18 and older, with a recommendation it be given at least two months after the first shot.

The unanimous recommendation on the Johnson & Johnson booster will be taken up by the FDA, which is expected to make a decision within days. The move will chart a path forward for the 14 million people in the United States who have received the vaccine, many of whom have felt left behind as widely used shots employing a different technology garner greater attention from researchers and the public.

The committee’s decision marked another turning point in the story of coronavirus shots, arriving 10 months after regulators authorized the first vaccine, a development that helped alter the course of the pandemic, which has claimed more than 722,000 lives in the United States.

But even as the committee sent one signal on a booster shot for Johnson & Johnson recipients, it heard about data from a small National Institutes of Health study that could muddy that recommendation.

The NIH study evaluated what happened when people got a booster dose different from the vaccine they originally received. That research found that people who initially received a Johnson & Johnson shot would experience a bigger boost in disease-fighting antibody levels in the short term if they received a booster shot of the messenger RNA Pfizer-BioNTech or Moderna vaccines. But limitations in the way the study was designed and in the follow-up made it difficult to draw conclusions.

“We’re in a situation where we’ve just approved a booster for J&J, and we have data that suggests that the mRNA vaccine boost, at least according to antibody responses … looks better,” said Steven Pergam, an associate professor in the vaccine and infectious-disease division at the Fred Hutchinson Cancer Research Center. “I think this is a challenge for people out in public to sort this out and to make decisions about what they’re going to do.”

Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, was noncommittal but said it was possible the agency might issue an emergency use authorization at some later date to allow boosters from different companies.

That approach could simplify the logistics of providing boosters.

But University of Maryland School of Medicine professor Kirsten E. Lyke, who presented the data, cautioned against using the findings to select one combination instead of another because of limitations to the research.

“We’ve designed this trial to inform public health decisions, but it’s not powered or designed to compare between groups,” Lyke said.

The Johnson & Johnson vaccine, unlike the the two-dose messenger RNA shots, was conceived as a one-dose vaccine in hopes it would allow the vaccination of twice as many people and help bring the pandemic under control.

Most experts said from the start that the Johnson & Johnson vaccine would ultimately require a second dose, based on knowledge of human immunity and knowing that many vaccines require a second dose to rev up the immune response. But the rollout of the vaccine was snared in setbacks, including production problems and a pause to investigate a rare clotting syndrome after vaccination.

Because many more people have received messenger RNA vaccines, there have been far more studies of the real-world performance of those shots, and the companies that make them — Pfizer-BioNTech and Moderna — have pushed more aggressively toward boosters. Nearly 70 million people in the United States are fully vaccinated with Moderna, and 104 million with Pfizer-BioNTech.

The vote in favor of the Johnson & Johnson booster came despite experts’ hesitation about the “scant” amount of data, particularly on a second shot delivered at six months — a timeline expected to more closely reflect the real-world use of the vaccine in the United States.

A Centers for Disease Control and Prevention analysis showed that rates of covid-19 and deaths were significantly lower for people receiving any of the three vaccines available in the United States compared with rates among unvaccinated people. But the analysis found that while the Johnson & Johnson vaccine offered a strong shield of protection, it was somewhat weaker than the other products.

Another recent study found that all three vaccines were protective against hospitalization, but that Johnson & Johnson offered the weakest protection — 71 percent, compared with 93 percent protection from Moderna and 88 percent protective from Pfizer-BioNTech.

“We have shown how the [Johnson & Johnson] covid-19 vaccine could help the U.S. further protect individuals from covid-19, by optimizing immune responses, increasing protection from symptomatic infection, preparing for future variants of concern and potentially helping to reduce transmission,” said Johan Van Hoof, global therapeutic area head of vaccines for Janssen, a division of Johnson & Johnson.

Friday’s recommendation from the advisory committee differs from action the panel took on other boosters, when the committee recommended limiting an extra shot to people at risk because of age or other factors. Johnson & Johnson sought a broader authorization, and several committee members said they had long assumed a second shot would be necessary.

“Prior to, I don’t know, November or December of 2019, the human species was all immunologically naive to this virus. Any single-shot vaccine was likely to induce a primary response and the second shot would be necessary,” said James E.K. Hildreth, president of Meharry Medical College. “It was always going to be necessary for J&J recipients to get a second shot.”

The Johnson & Johnson vaccine has been associated with rare severe adverse events, including a clotting syndrome that can be fatal and is more common in women 18 to 49 years old. It has also been associated with Guillain-Barre syndrome, a rare neurological disorder.

On Thursday, the same panel advised the FDA to authorize a booster shot of the Moderna vaccine for a select population — people 65 and older and adults 18 and over at high risk of severe illness or complications of covid-19 because of underlying risk factors or exposure at their jobs. Regulators last month authorized a booster shot of the Pfizer-BioNTech vaccine for similar groups.

During its two-day review, the committee wrestled with the purpose of boosters, because much of the rationale for boosting has been based on the drop in the level of antibodies over time — even though it is clear that other parts of the immune system contribute to protection.

“It’s important to ask, when the concern is for waning immunity, what exactly are we trying to target by trying to increase the flexibility and availability of vaccines for the population?” asked Michael Kurilla, director of the division of clinical innovation at the National Center for Advancing Translational Sciences at NIH. “If we’re trying to drive to zero covid, that’s not going to work. So, I think we need to be a little more careful and deliberate about what impact are we actually trying to create here.”

An advisory committee to the CDC is scheduled to meet Wednesday and Thursday to make recommendations on how booster doses should be used.