The Food and Drug Administration will allow certain people with weakened immune systems to receive a third dose of either Pfizer's or Moderna's coronavirus vaccine, announcing late Thursday an expanded authorization for the two shots.
The move, which comes as the highly contagious delta variant has driven COVID-19 case counts in the U.S. to their highest levels since January, is aimed at better protecting people who experts view as most at risk of breakthrough infections, such as those who've received an organ transplant.
Both Pfizer and Moderna have pressed the Biden administration to administer booster shots to guard against the risk of waning immunity as well as the greater threat posed by delta. After turning back the companies earlier this summer, U.S. government officials now reportedly view booster shots as likely to be necessary for a number of groups. The FDA's update Thursday applies to fewer than 3% of U.S. adults, however.
"After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna Vaccines," said Acting FDA Commissioner Janet Woodcock in an Aug. 13 statement. "Other individuals who are fully vaccinated are adequately protected and do not need an additional dose of COVID-19 vaccine at this time."
The expanded authorization does not apply to Johnson & Johnson's vaccine, however.
The case for additional doses gained support in late July, when a CDC advisory panel weighed emerging data on third doses for immunocompromised people and encouraged federal officials to take action for this group.
The same panel, called the Advisory Committee on Immunization Practices, will meet again on Friday, when they're expected to recommend the extra dose for immunocompromised people. Their sign-off would clear the way for doctors to start administering doses to their patients, or for those individuals to seek out an additional shot.
In expanding Pfizer's and Moderna's vaccine authorizations, the FDA specifically identified people who have received solid organ transplants, or those with an equivalent level of immunocompromise. According to ACIP, people who have cancer, HIV, or are taking immune-suppressing drugs are also considered immunocompromised.
Supporting the FDA's decision are data from a range of studies that have suggested vaccinated people with suppressed immune systems are more likely to have breakthrough infections and have less protection from vaccination.
In one study published on the preprint server medRxiv in early July, for example, 20 of 45 vaccinated people who got sick with COVID-19 were immunocompromised. Another showed that in the first few weeks following the second dose of Pfizer's vaccine, protection was noticeably less among immunocompromised people than in healthy individuals.
Immunocompromised people are also more likely to get severely ill from COVID-19, more likely to transmit the virus to household contacts and have lower antibody levels to fight off variants, according to the CDC.
The expanded authorization, while narrowly focused, is a boost for both Pfizer and Moderna, executives of which have been pushing for government action on booster shots throughout the summer. On July 12, Pfizer representatives met privately with U.S. government health officials to discuss authorization of an additional shot. The parties did not reach an agreement, however, and officials were said then to be waiting for additional data.
The pharmaceutical company pressed its case again on July 28, when a preprint paper indicated the vaccine's efficacy against symptomatic disease gradually declined after vaccination.
In clearing the way for certain people to receive additional doses, the U.S. trails several other countries, most notably Israel, which had moved to offer booster shots to older adults soon after Pfizer's paper. France and the U.K. have also begun or plan to administer boosters.
In August, Moderna's president said booster shots would "likely to be necessary this fall," as the company reported antibody levels decrease over time. The company has emphasized that vaccination remains highly protective for at least six months, although the supporting data for that claim was collected earlier this year when the delta variant was less prevalent. Results showed efficacy was 92.4% four to six months after the second dose.
While the FDA indicated booster shots are not needed yet for other fully vaccinated people with properly functioning immune systems, that could eventually change, according to Anthony Fauci, the longtime director National Institute of Allergy and Infectious Diseases.
"We do not believe that others — elderly or nonelderly — who are not immunocompromised, need a vaccine right at this moment," Fauci said during Wednesday's White House briefing.
"If the data shows us that in fact we do need to do that, we'll be very ready to do it and do it expeditiously," he added.
Should the FDA authorize booster doses more broadly down the line, the Biden administration could find itself under heavier criticism for not sharing more vaccine doses with the many countries that still do not have enough supplies to immunize their citizens for the first time.
While the U.S. has paid billions of dollars to Pfizer and Moderna to lock up sufficient vaccine supplies, initiatives like the World Health Organization's COVAX framework are struggling to meet their goals of providing vaccines to poorer countries.
In addition to boosters, the FDA is weighing whether to grant Pfizer's vaccine — and soon thereafter Moderna's — a full approval, which experts view as important to encourage vaccinations among Americans who have still not received a shot.
Pfizer and Moderna are also testing their vaccines in 5- to 11-year-olds, a particularly important group as school years begin to get underway.