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The Food and Drug Administration on Thursday expanded the emergency use authorization for Pfizer and BioNTech’s Covid-19 booster shot to cover 16- and 17-year-olds, making it the only booster shot currently available in the United States for teenagers in this age group.

Within hours, Rochelle Walensky, director of the Centers for Disease Control and Prevention, signed off on the expanded authorization, clearing the way for 16- and 17-year olds to book booster shot appointments.

“Today, CDC is strengthening its booster recommendations and encouraging everyone 16 and older to receive a booster shot,” Walensky said in a statement. “We know that Covid-19 vaccines are safe and effective, and I strongly encourage adolescents ages 16 and 17 to get their booster if they are at least 6 months post their initial Pfizer vaccination series.”

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In a later statement, the CDC clarified that the agency is saying that 16- and 17-year-olds “may” get a booster shot. For everyone 18 and older, the recommendation is stronger; CDC says they “should” be boosted. While the difference may seem like a matter of semantics, it has implications for vaccine mandates. Should schools require students to be vaccinated against Covid, for instance, they likely could not require three shots based on the current recommendation.

The move to make the shots available to 16- and 17-year-olds comes at a time when the Biden administration is trying to increase uptake of booster shots to build up immunity to battle a surge in Delta variant cases and a feared winter wave of infections potentially caused by the new and highly transmissible Omicron variant.

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“As people gather indoors with family and friends for the holidays, we can’t let up on all the preventive public health measures that we have been taking during the pandemic. With both the Delta and Omicron variants continuing to spread, vaccination remains the best protection against COVID-19,” Acting FDA Commissioner Janet Woodcock said in a statement.

The FDA made the decision without consulting its independent vaccine advisers, the Vaccines and Related Biological Products Advisory Committee, or VRBPAC. In its statement, the agency said the committee had already had extensive deliberations about boosters and “the FDA concluded that the request does not raise questions that would benefit from additional discussion by committee members.”

Likewise, Walensky sidestepped CDC’s independent vaccine experts, the Advisory Committee on Immunization Practices, or ACIP — an unusual move that is already drawing criticism.

Both advisory committees would likely have asked pointed questions about the risks associated with giving booster shoots to 16- and 17-year-olds, given the fact that the messenger RNA vaccines — like the Pfizer product and Moderna’s vaccine — have been linked to elevated rates of myocarditis and pericarditis, an inflammation of heart muscle or tissue around the heart, respectively.

“The FDA has determined that the benefits of a single booster dose of the Pfizer-BioNTech Covid-19 vaccine or Comirnaty outweigh the risks of myocarditis and pericarditis in individuals 16 and 17 years of age to provide continued protection against Covid-19 and the associated serious consequences that can occur including hospitalization and death,” the FDA statement said.

A number of public health experts expressed concern about the way the authorization was approved, saying that by sidelining the advisory committees, the administration has given vaccine opponents powerful ammunition with which to sow doubt about boosters for this age group.

“I think short-circuiting the process may seem like the expedient protective action to take. But there are long-term damages that come with that — and potentially short-term damages, if people believe that the process was the result of politics and not science,” said Jennifer Nuzzo, an infectious diseases epidemiologist at Johns Hopkins University’s Outbreak Observatory.

“I just hate that there’s a talking point now that this was rushed through, that this wasn’t carefully considered by the top scientific minds of the country with particular expertise in these areas,” she added. “The anti-vax community — and it’s really expansive about Covid vaccines — will take any daylight you give and just seize on it. And this to me seemed like something in particular that would benefit from open deliberation.”

“It is not unreasonable for parents to be worried about this,” Nuzzo continued. “They need to be reassured and they need to know that every data point was scrutinized with the utmost attention to safety.”

Walid Gellad, a professor of medicine at the University of Pittsburgh, also expressed concern, questioning if the evidence of benefit from an additional dose for this age group justifies the policy.

“There is this relentless push that’s happening for boosters. Boosters, boosters, boosters, boosters for everybody. And no discussion of the unintended consequences. And the risk of myocarditis is just one of them,” Gellad said.

Vaccine doses used to boost low-risk individuals in wealthy countries are delaying the vaccination of people in parts of the world where few people have received even one dose of vaccine, he said.

“Obviously there are international equity issues. And not just equity,” Gellad said. “If we all of a sudden decide that everyone in the Northern Hemisphere is going to get a third dose, we are going to have another variant six months from now and be in the exact same spot.”

This is the third time the CDC director has either overruled the ACIP or avoided asking the group’s advice on a Covid booster-related issue. In September, when it narrowly voted against offering boosters to healthy people aged 18 to 64 at high risk of contracting Covid because of where they worked or lived, Walensky overruled the group. In mid-November the group voted to recommend that people 18 to 49 who have no underlying health conditions “may” — rather than “should” — get a booster. Walensky signed off on that recommendation, but the agency quietly changed it 10 days later to strengthen the recommendation to say this group “should” get boosted.

The big question about boosting 16- and 17-year-olds relates to the risk of myocarditis and pericarditis, and whether the elevated rates of these conditions that were seen after the second dose of the Pfizer and Moderna vaccines will be seen after the third.

Reports of myocarditis and pericarditis after the administration of mRNA vaccines have mainly been seen in males, with rates highest among those in their teens and early 20s. In a presentation at a Nov. 2 meeting of the ACIP, it was reported that the highest rate of reported cases of myocarditis and pericarditis after mRNA vaccinations was among males 16- and 17-years-olds, with the conditions reported at a rate of 69 cases per 1 million doses of mRNA vaccines given.

These conditions can be caused by a number of triggers, including some infections. Classic cases of myocarditis and pericarditis peak in the mid to late teens, when they appear at a rate of about 18 cases per 1 million, according to the CDC. Most of the myocarditis and pericarditis cases linked to the vaccination are less severe and quicker to resolve themselves than standard cases of pericarditis and myocarditis.

The incidence of myocarditis and pericarditis after Covid vaccination is higher after the second dose. It is currently not known how frequently they might occur after a third shot, though evidence from Israel, which was in the vanguard of booster administration, suggests the rate may be lower than after second dose of the vaccine, but higher than after the first.

The Israeli data, presented at a VRBPAC meeting in mid-October, showed that among boys aged 16 to 19, there were 5.2 cases of myocarditis or pericarditis for every 100,000 booster doses of Pfizer vaccine administered. The rate for this age group was 16.1 cases per 100,000 doses after the second dose, and 1.2 cases per 100,000 after the first dose.

As with older individuals, the booster shot for 16- and 17-year-olds should be given at least six months after the second shot of the primary vaccination series. Many states opened up Covid vaccine access to 16- and 17-year-olds in late April, meaning some teens in this age group are already eligible to get a booster shot.

“While new variants, including Omicron, emerge across the globe, we believe that the best way to minimize the spread of Covid-19 and any future variants is getting all eligible people fully vaccinated with the first two-dose series and a booster dose as recommended,” Pfizer CEO Albert Bourla said in a statement.

In the statement, Pfizer and BioNTech said they plan to submit a supplemental biologics license application for approval of a booster dose of their Covid-19 vaccine in individuals 16 years of age and older. The partnership was the first to earn full approval of the primary series of their vaccine; to date the vaccine, sold under the brand name Comirnaty, is the only Covid vaccine in use in the United States to be fully licensed.

Correction: An earlier version of this story misspelled Walid Gellad’s surname.

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