In a document published Tuesday, scientists said data show that Moderna's two-shot vaccine provides enough immunity from the virus and that they are aware of studies the agency has not reviewed that may show a third shot is necessary, but did not take a stance on approval of booster shots.
In September, Moderna submitted data to the FDA's Vaccines and Related Biological Products Advisory Committee in support of a booster shot. The biotech company argued that the vaccine weakens over time, and a third shot was needed to protect against the virus, particularly among older adults. Moderna said it also protected against the Delta variant.
FDA scientists submitted the document to the committee, which will consider the request Thursday. The scientists acknowledged some studies have pointed to declining effectiveness of Moderna's vaccine.
"However, overall, data indicate that currently U.S.-licensed or authorized COVID-19 vaccines still afford protection against severe COVID-19 disease and death in the United States," they wrote. "There are many potentially relevant studies, but FDA has not independently reviewed or verified the underlying data or their conclusions."
The committee said that reported breakthrough cases of COVID-19 among vaccinated individuals have been uncommon and "have not raised a concern about vaccine-enhanced disease."
In September, the FDA granted emergency clearance for a third booster dose of the Pfizer-BioNTech two-shot vaccine for older people, as well as those at high risk of a severe COVID-19 case or who face frequent exposure to the virus at work.
The FDA already, in August, approved boosters of both the Pfizer-BioNTech and Moderna shots for solid organ transplant patients and people with severely compromised immune systems.
Earlier this month, a European Union drug regulator signed off on boosters of Moderna's vaccine, as well as Pfizer's two-dose shot, for immunocompromised people. European regulators based the recommendation on the additional shot for people who had organ transplants or weakened immune systems.
Johnson & Johnson has also asked for approval of a booster shot for its COVID-19 vaccine for adults age 18 and older.
According to data from the Centers for Disease Control and Prevention, 65% of Americans have had at least one dose of a COVID-19 vaccine. Just 4% of Americans have received a booster shot.