Even as booster shots of COVID-19 vaccines become widely available in the U.S., some vaccine experts are not convinced that the shots are broadly necessary.
While studies indicate that older adults have a higher risk of severe breakthrough Covid cases, limited data are available on the need for boosters among younger age groups.
In addition, pressure from the federal government has made it difficult for some vaccine experts to make objective decisions about boosters, several experts said in interviews with the New York Times.
Over 13 million Americans have received an additional vaccine dose as of October 25, including about 18 percent of seniors.
Booster shots are now widely available for Americans, but some experts aren't sure they are widely needed. Pictured: A nurse receives a Pfizer booster in Miami, Florida, October 2021
Over 13 million Americans have received a booster dose so far, according to the CDC. States in darker blue on this map have administered boosters to larger shares of their vaccinated populations, with Alaska at the highest share (11%)
Additional Covid vaccine doses were first approved for Americans with weakened immune systems in mid-August.
Then, in late September, the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) authorized booster shots for seniors and other people considered high-risk who had received the Pfizer vaccine.
The agencies followed this with additional authorizations last week, enabling Americans who received Moderna and Johnson & Johnson's vaccines to receive additional doses.
As of October 25, about 13.3 million Americans have received a booster shot.
This includes 8.2 million seniors, representing about 18 percent of Americans over age 65.
Despite this broad adoption, some vaccine experts are skeptical about whether the additional vaccine doses are actually necessary - particularly for younger age groups.
Experts who serve on FDA and CDC advisory committees conveyed their concerns over limited data and political pressures in interviews with the Times.
'These are not evidence-based recommendations,' Dr Sarah Long, a pediatric infectious disease expert who services on the CDC's vaccine advisory committee, told the Times.
The current federal authorizations for boosters allow seniors, adults who have medical conditions that put them at high risk for severe Covid, and those who live and work in high-risk settings (such as hospitals and schools) to receive Pfizer and Moderna boosters.
In addition, all adults who initially received the one-shot Johnson & Johnson vaccine are eligible for a J&J booster.
'I don't think that we have evidence that everybody in those groups needs a booster today,' Dr Matthew Daley, also a member of the CDC's vaccine advisory committee, told the Times.
The evidence in favor of booster shots for seniors is clearer than the evidence for other groups. Pictured: Booster vaccination at a Washington, D.C. Safeway, October 2021
When it comes to seniors, the evidence in favor of booster shots is fairly clear.
The majority of breakthrough Covid cases requiring a patient to be hospitalized have occurred in seniors, CDC data have shown.
Other studies of breakthrough Covid patterns have found that older adults and those with chronic conditions are more susceptible to severe cases.
Seniors tend to have less protective immune systems than younger adults, one expert told the American Association of Retired Persons.
When it comes to other groups of adults, however, the data are not so clear-cut.
The CDC has very limited data on breakthrough Covid infections that do not end in severe disease, as the agency has focused on tracking only those cases that lead to hospitalization or death since the spring.
However, CDC studies have found that the vaccines remain highly effective against hospitalization for younger Americans - including those working in hospitals, long-term care facilities, and other healthcare settings.
Other studies have similarly found that the Covid vaccines continue to offer strong protection for younger adults.
In one analysis, New York Department of Health officials found that vaccines remained over 90 percent effective against hospitalization for adults ages 18 to 64 during the state's Delta surge in summer 2021.
At the same time, the study attributed a decline in vaccine effectiveness against infection to the highly contagious variant and changes in human behavior - not to waning antibodies.
While vaccine companies often cite data on antibody levels as prospective evidence that their products become less protective over time, scientists do not have a clear understanding of how these measurements correspond to protection against Covid.
Different aspects of the immune system - such as memory B and T cells - likely also play a role in long-lasting protection, but are harder to measure.
Meanwhile, the vaccine companies offered limited information on their booster shots' safety and effectiveness. One expert told the Times that Moderna and J&J's data was 'of very low quality.'
Despite the lack of evidence that younger adults need booster shots, the Biden administration announced in August that all Americans would soon be eligible for the additional doses.
Experts who spoke to the Times said that the administration's public stance added pressure to their decision-making.
'The door just got bigger and bigger and bigger, it got wider and wider with each step,' Dr Paul Offit, an expert on vaccines for children and member of FDA's advisory committee, told the Times.
'The companies got what they wanted, the administration got what they wanted.'
In fact, after the CDC's advisors voted against recommending Pfizer booster shots for adults who live and work in higher-risk environments, the agency's director overruled that vote.
After this overruling, votes on Moderna and J&J vaccines were unanimous - even though many concerns remained and not all votes were based solely on evidence.
'We don't know if boosters are necessary,' Dr Cody Meissner, a member of the FDA committee, told the Times.
But, he said, 'if you're going to do it for one group, I think fairness kind of dictates you have to do it for all the groups.'