Vaccine advisers at the Centers for Disease Control and Prevention unanimously voted, 15-0, to recommend COVID-19 booster doses for all individuals who received the Johnson & Johnson vaccine, and half of a dose for some individuals who received the Moderna vaccine.

Advisers recommended the half-dose of the Moderna COVID-19 vaccine six months after the second dose for adults ages 65 and up and adults 18 to 64 years old who are at high risk because of underlying medical conditions or their work environments. The advisers also recommended a COVID-19 booster of the Johnson & Johnson vaccine two months after receiving the initial Johnson & Johnson vaccine.

If CDC Director Rochelle Walensky signs off on the recommendations, an estimated total of 99.1 million Americans will qualify for a COVID-19 booster, including more than 39 million eligible Moderna recipients and 13 million Johnson & Johnson recipients. The rest of the eligible population is made up of Americans who received the Pfizer vaccine. Last month, the CDC recommended booster shots for Pfizer vaccine recipients.

The advisory committee’s Thursday afternoon vote follows the Food and Drug Administration’s Wednesday evening decision to authorize the additional shots for these same groups.

Advisers did not formally vote on mixing and matching COVID-19 booster doses but will include recommendations in their clinical considerations. The group agreed that Americans should receive the same booster dose as their primary vaccine regime when possible. But if the same vaccine is not available, they said it is safe to receive a different vaccine for a booster shot.

Despite these new recommendations for an additional shot, the definition of “fully vaccinated” is not changing. For public health purposes, people who completed a primary vaccination series are still considered fully vaccinated against COVID-19 two weeks after their second shot. Americans who receive a booster dose should continue to follow the same guidance fully vaccinated people follow.

Several committee members argued it might not be necessary for people under age 65 to receive an additional dose of the Moderna vaccine due to the original vaccine series’ high efficacy. But since the Biden administration authorized COVID-19 booster shots for some Pfizer recipients under age 65, it could be confusing to authorize booster shots for one mRNA vaccine and not another.

“There are probably many people who are going to get a Moderna booster who don’t need it,” committee member James Loehr said. “However, given the situation that we’ve already approved Pfizer and there are enough people who are looking for a booster, I’m inclined reluctantly to just go ahead and recommend a similar program for the Moderna booster, though I have much stronger feelings for the Johnson booster.”

Committee members also worried that the booster recommendations were not exactly clear or easy to understand, and would require Americans to run their own risk-benefit analysis before receiving an additional shot.

“A really important aspect of all this is being clear and not dancing on a pin so we don’t further confuse the American people,” committee member Beth Bell said, noting there are many conflicting governing bodies that have jurisdiction over COVID-19 vaccine recommendations.

Safety data

Moderna reported a significant spike in COVID-19 breakthrough infections after the delta variant emerged and said this variant along with waning efficacy necessitates a third dose.

As of Thursday’s meeting, more than 1.6 million immunocompromised Americans have already received a third dose of Moderna’s booster shot. But these Americans received a 100-microgram shot, whereas the booster is cleared for 50 micrograms.

Moderna representative Jacqueline Miller said the company went with a 50-microgram dose rather than a 100-microgram shot for its booster because its goal was to use the optimal effective dose and not overdo it. Lower booster doses than those used for the primary series still reactivate immune memory — and help increase worldwide supply.

Moderna booster doses would be shipped out in the same vial as initial doses, but vaccine administrators will need to draw out half-doses. Some CDC advisory committee members had concerns there could be administration errors if pharmacists are required to draw different Moderna doses from the same vaccine vial.

Johnson & Johnson made a similar case for vaccination as Moderna did. After a booster shot, the efficacy of the Johnson & Johnson vaccine is boosted to 94 percent.

Some vaccine advisers had concerns about the smaller sample size of the Johnson & Johnson vaccine trial for a booster shot six months after vaccination, since the company is asking to administer a booster two months out.

Safety data on inflammatory conditions following vaccination, myocarditis and pericarditis, were limited. But the available data show an association with both mRNA vaccines in adolescents and young adults. This risk was greater in men than women.

There is limited information from only a small clinical trial on the risk of myocarditis after a third dose of the Moderna vaccine. But the risk of myocarditis after the booster might be lower than the risk following original vaccination. It’s still unclear if the risk of myocarditis with half a dose of the Moderna vaccine will be the same or lower than a full dose, committee member Keipp Talbot said.

There also are limited trial data from safety monitoring for a booster dose of the Johnson & Johnson vaccine. But the Vaccine Safety Technical Work Group safety assessment concluded that risks after booster doses are unlikely to be greater than after primary vaccination. Tom Shimbukuro, a member of the CDC’s vaccine task force, told the committee there’s limited information about the risk of receiving an mRNA vaccine following the Johnson & Johnson vaccine, but no signals of safety problems so far.

In all age groups, Moderna vaccine effectiveness is higher than Johnson & Johnson’s for adults with chronic health conditions. But Moderna shows a modest trend of decreasing vaccine efficacy and there’s no consistent pattern of vaccine effectiveness waning for Johnson & Johnson recipients.  

The working group found that Moderna’s vaccine effectiveness against infection declines over time, due to both circulation of the delta variant and increasing time since vaccination.

But for Johnson & Johnson, evidence of waning vaccine effectiveness over time is inconsistent across studies and there are less data available so the vaccine effectiveness estimates are less reliable. 

All booster doses are free to Americans during the public health emergency. The advisory committee plans to examine the cost-benefit analysis of booster shots once the public health emergency is over and the shots are no longer free to consumers.