The COVID-19 vaccine made by Moderna is the second to get the greenlight from a panel of experts assigned to advise the Food and Drug Administration (FDA).

On Thursday afternoon, a 19-member committee voted unanimously in favor of advising the FDA to recommend booster shots for people who have previously been vaccinated with Moderna’s COVID-19 vaccine. It’s now up to the FDA to make a final decision, which the Centers for Disease Control’s (CDC) Advisory Committee on Immunization Practices will then consider shortly in coming up with the final recommendation for who should get the Moderna booster.

Moderna follows Pfizer-BioNTech, which, in September, received FDA authorization for its booster. The CDC followed up with a booster recommendation for anyone over 65 years old who received their last Pfizer-BioNTech vaccine dose at least six months earlier; and anyone aged 18 to 64 who is more vulnerable to complications of COVID-19 because of underlying health conditions or because they work in a higher risk setting including a hospital, grocery store or school.
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The advisory committee unanimously voted in support of a booster of the Moderna vaccine; however, in a discussion following the vote members raised concerns about whether the data the committee and FDA have been reviewing so far are adequate to justify boosters for the wider population. Several committee members voiced their support of a booster dose for vulnerable populations, including those over age 65 and people with compromised immune systems, who are at greater risk of developing severe COVID-19 and its complications, but noted the dearth of data supporting the benefit among younger people, even those in jobs that might put them at risk of exposure. Still, the members voted to recommend the Moderna shot for a population that mirrors that currently eligible for the Pfizer-BioNTech booster. (The same FDA committee had voted against recommending the Pfizer-BioNTech booster for these groups, but CDC director Dr. Rochelle Walensky decided to include them so those who find themselves in high-risk settings have the option of getting an additional dose.)

Especially lacking, committee members said, were risk-benefit data clearly demonstrating a Moderna booster dose would give younger people who mount adequate immune responses after the initial shots any extra protection. Indeed, FDA scientists presenting to the committee emphasized that the immunity from the original two doses remains strong enough to protect most people from getting sick enough to need hospitalization, or from dying of COVID-19. So many committee members questioned the rationale of extending the booster recommendation to younger people with otherwise healthy immune systems. “Certainly it’s clear there is benefit for older people,” said Dr. Paul Offitt, a committee member and professor of pediatrics at the Children’s Hospital of Philadelphia. “But I’m less impressed with data regarding the younger person. I worry about the broad use of the booster without any clear evidence of benefit would send the wrong message…that would make people feel they are not protected if they don’t get a third dose.”

Offit pointed out that regulators and public health officials haven’t made clear what the goal of the vaccine is. Originally, it appeared that vaccines were recommended to protect people from severe COVID-19 disease, but the push for boosters for everyone seems to shift the target toward preventing infection—which the vaccines are not as effective in doing, and wasn’t the standard by which they were initially tested.

Implicit in the boosters-for-everyone argument is also the idea that boosters may be needed to maintain an adequate immune response longer term, like a gas tank that needs to be topped off regularly. But there isn’t enough information available to know if that makes sense, since vaccine makers have not provided sufficient data on the durability of the immune responses and the protection provided by their shots.

Dr. Patrick Moore, another committee member and a professor at the University of Pittsburgh Cancer Institute, said he voted in favor of the booster “more from a gut feeling rather than based on really, truly serious data. The data itself is not strong, but [is] certainly going in the direction that is supportive of this vote.”

Moderna presented data from a complicated study involving 300 people who were randomly assigned to receive either two regular doses, or two half-doses of vaccine. All of them were then given a booster with the half dose and researchers recorded their antibody levels up to six months later. The company tested how well blood from these people generated antibodies against both the original SARS-CoV-2 virus against which the vaccines were developed, and the Delta variant, and compared these levels to those produced by people enrolled in an earlier study of people who got the two-dose vaccine, but did not receive a booster. The results, Moderna said, showed that the half-dose booster adequately reawakened the immune system to quickly churn out antibodies against SARS-CoV-2, since it was already primed by the previous two doses to do so, and that halving the dose would double the number of people who could get vaccinated with the additional shot while also reducing the incidence of potential side effects. However, the committee noted that the number of people studied—though technically large enough for emergency use authorization consideration—was quite small, and the data from might not be entirely representative of what could happen if hundreds of millions of people got a booster.

To provide context for the potential impact of booster doses, the committee also heard from experts from Israel, which was among the first countries to successfully vaccinate a large proportion of its adult population with the original two doses of Pfizer-BioNTech vaccine, and began seeing breakthrough infections among some of the older people who were vaccinated earliest. Israeli health officials authorized booster doses in August; Dr. Sharon Alroy, director of public health services for the Ministry of Health of Israel, presented data showing that people who received those booster shots experienced a much lower rate of breakthrough infections and were less likely to be hospitalized or die of COVID-19 than those who did not. Alroy-Preis, in fact, credited the booster program, which began with people over age 65 but now includes most people over age 16, with bringing the national incidence of new COVID-19 cases down.

“There is no question in my mind that we were able to break the [rising] curve of cases due to the booster dose,” she said. “We are now coming out of our fourth wave and without the booster, I fear it would have put us in a worse place with a high burden of cases on hospitals of severe, critical patients. We were able to get out of this wave due to the booster dose.” Israel has primarily relied on the Pfizer-BioNTech vaccine, however, so some panel members questioned how applicable Israel’s experience is to the debate over a Moderna booster, although the vaccines from both companies are made the same way, using mRNA technology.

It was clear from the discussions of the booster that big gaps remain in scientists’ understanding of COVID-19 immunity. Experts still don’t have a good understanding of what type of immunity is needed to protect people from getting infected in the first place—What level of antibodies are needed to ward off SARS-CoV-2? How long-lasting are those antibodies? Do the vaccines produce these antibodies in sufficient quantities?—which means they still can’t really say how useful, or needed booster doses will be for most of us.

For now, regulatory officials and public health leaders are working under the assumption that in an emergency pandemic situation, however, you pull all the levers and make available every opportunity for people to be protected, even if the benefit isn’t an overwhelming one.

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