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Vaccine advisers to the US Food and Drug Administration voted unanimously Friday to recommend a booster dose of Johnson & Johnson’s vaccine at least two months after people get the first dose.

The FDA’s Vaccines and Related Biological Products Advisory Committee voted 19-0 to recommend the extra dose for all recipients of the J&J Janssen vaccine, which is authorized for people 18 and older. They asked to simplify the original question being posed by the FDA, which had asked the committee to say whether the data showed that waiting six months or longer after getting the first shot would provide an even stronger immune response.

Then they turned to the question of mix and match boosters – not to decide whether a specific combination might be better, but to weigh in on whether it would be all right to give someone a booster that differs from the brand originally administered.

The FDA will consider the committee’s advice. Then the US Centers for Disease Control and Prevention’s vaccine advisers will be asked to consider it.

Johnson & Johnson says studies have shown boosting at two or six months can bring that effectiveness up to 94%, and it says its effectiveness does not wane over time in the same way that Pfizer’s vaccine does.

But the Johnson & Johnson vaccine has not been shown to be as protective as either the Pfizer or Moderna vaccines, noted VRBPAC chair Dr. Arnold Monto, a professor of public health and epidemiology at the University of Michigan. “So there is some urgency here to do something,” he told the meeting.

And Dr. Amanda Cohn, a member of VRBPAC who is also chief medical officer for the CDC’s National Center for Immunizations and Respiratory Diseases, said various studies suggested real-world efficacy of J&J’s vaccine was anywhere between 50% and 68%.

“Regardless of whether or not there been waning or this is the true effectiveness after a single dose, the effectiveness or protection of a single dose of the J&J vaccine is not equivalent to protection at this time with either two doses of an mRNA vaccine and certainly not in those groups who have now been authorized to receive a booster dose of an mRNA vaccine,” Cohn said.

Members of the committee said while there was not much data to show whether the efficacy of the Janssen vaccine was waning – or was strong to begin with – they agreed people should be given the opportunity for a booster. “I would say I agree a second dose booster is needed to boost immunity back to the 90-plus range,” Dr. Archana Chatterjee, a pediatric infectious diseases expert at Rosalind Franklin University in Chicago, said before the vote.

Several said they thought the J&J vaccine should have been considered a two-dose vaccine from the start. “It was always going to be necessary for a Johnson & Johnson recipient to get a second shot,” said Dr. James Hildreth, president and CEO of Meharry Medical College.

“This does look like a second dose vaccine,” agreed Dr. Michael Nelson of the University of Virginia school of medicine.

The FDA has already given EUA to a booster for Pfizer’s vaccine for people who are six months out from their first two shots who are also either 65 or older or who are at least 18 and have a higher risk of severe disease because of pre-existing conditions or because of work or living conditions.

And Americans are already flocking to get those boosters. Data from the CDC show close to 5% of fully vaccinated people – more than 9 million people – have received booster shots.

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On Thursday, VRBPAC members voted unanimously to recommend booster doses of Moderna’s vaccine to the same groups.

If the FDA gives emergency use authorization to Moderna or Johnson & Johnson boosters, CDC vaccine advisers will meet to discuss which groups to recommend them to. Typically, shots can be administered once the CDC director signs off on the recommendation. CDC’s Advisory Committee on Immunization Practices is scheduled to discuss boosters on October 21.

The FDA asked the committee to discuss data showing that it’s safe to mix the three authorized Covid-19 vaccines – Moderna’s, Pfizer’s, and Johnson & Johnson’s – when giving booster shots. The National Institutes of Health presented early information from an ongoing study showing that it didn’t matter which vaccine people got first and which booster they got – it was safe to mix boosters and it revved up immune response. Mixing boosters also provided a good response to the Delta variant.

Cohn said it would help the CDC in guiding the use of boosters.

“I don’t think there’s any sort of need from a public health perspective to have a preference for mixing or matching,” Cohn told the meeting.

“But I think from a public health perspective there’s a clear need in some situations for individuals to receive a different vaccine. For example, J&J doses – for those 14 million people who have been vaccinated – many of those individuals may not have access to a second dose of J&J. So if there’s not any allowable language in the FDA’s fact sheets or EUA authorization, then those individuals are left behind,” she said.

Plus, she noted, there’s a rare blood clotting side effect known as thrombosis with thrombocytopenia syndrome or TTS that is more common in young women and associated with the J&J vaccine.

“Additionally, the same goes for if an individual is a female who’s 30 years of age who may feel like she’s at risk now for a reaction after she receives a first dose of J&J before the TTS was recognized so that may allow, for example, for that woman to get a different type of vaccine,” Cohn said. “And to the contrary, it allows, for example, in nursing homes where most residents received mRNA vaccines, it would allow a pharmacy to go into a nursing home and only have a single vaccine product to boost individuals who received either Moderna or Pfizer.”