FDA and CDC on Thursday opened up booster doses of Pfizer-BioNTech's Covid-19 vaccine to 16- and 17-year-olds—and as additional doses become more widely available, some public health experts say the definition of "fully vaccinated" should be updated to require boosters.

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Boosters authorized for 16- and 17-year-olds

Federal regulators on Thursday expanded authorization of Pfizer-BioNTech Covid-19 booster doses to include 16- and 17-year-olds who received their second dose at least six months earlier—a move that opens up additional shots to several million teenagers, the New York Times reports.

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"Since we first authorized the vaccine, new evidence indicates that vaccine effectiveness against Covid-19 is waning after the second dose of the vaccine for all adults and for those in the 16- and 17-year-old age group," said Peter Marks, head of FDA's vaccine division. He added that boosters "will help provide continued protection against Covid-19 in this and older age groups."

To make its decision, FDA reviewed immune response data from around 200 adults that showed antibody levels increased a month after receiving a booster dose. According to the Times, the agency said this data also demonstrated the effectiveness of a booster shot for adolescents.

In addition, FDA said that as Covid-19 cases continue to rise in the United States, the benefits of booster doses for adolescents outweighed the potential risk of rare side effects in the age group, such as myocarditis.

CDC Director Rochelle Walensky soon after signed off on FDA's decision and encouraged adolescents to receive a booster dose. "Although we don't have all the answers on the omicron variant, initial data suggests that Covid-19 boosters help broaden and strengthen the protection against omicron and other variants," she said. "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."

According to federal data, two-thirds of 16- and 17-year-olds, or around 5.5 million adolescents, have received at least one dose of Pfizer-BioNTech's vaccine, and more than 4.7 million have received two doses. Of the latter group, around 3 million received their second shot at least six months ago and will be eligible for a third dose this month.

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Will the definition of 'fully vaccinated' change to include boosters?

As new evidence emerges suggesting an additional dose may be needed to protect against the omicron variant, several public health experts say that the definition of "fully vaccinated" should eventually change to include a third dose.

"Certainly, when you want to talk about what optimal protection is, I don't think anybody would argue that optimal protection is going to be with a third shot," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. He added that he believes that "it is going to be a matter of when, not if" the definition of fully vaccinated changes.

Similarly, Peter Hotez, dean for the National School of Tropical Medicine at the Baylor College of Medicine, said people who have had two doses of a Covid-19 vaccine will need a third one. "We should think of the Covid-19 vaccine as a three dose vaccine," he said.

In addition, Leana Wen, a visiting professor of health policy and management at the George Washington Milken Institute of Public Health, said she hoped federal health officials will "be quick to reevaluate the definition of what it means to be fully vaccinated, especially in light of what we're learning about omicron."

However, William Schaffner, chair of the department of preventive medicine at Vanderbilt University School of Medicine and liaison to CDC's Advisory Committee on Immunization Practices, said it would be a "large leap" to change the definition.

"Changing the requirements of what fully vaccinated means, I think, has all kinds of downstream effects for all kinds of institutions in the country," Schaffner said. "I don't think we're quite there yet." (LaFraniere/Weiland, New York Times, 12/10; Hopkins/Armour, Wall Street Journal, 12/9; Neergaard/Stobbe, Associated Press, 12/9; Weixel, The Hill, 12/9; Weixel, The Hill, 12/8; Schuster-Bruce, Business Insider, 12/9)