Scientists at the US Food and Drug Administration (FDA) said on Tuesday that Moderna had not met all of the agency's criteria to support use of booster doses of its COVID-19 vaccine, possibly because the efficacy of the shot's first two doses has remained strong.
FDA staff said in documents quoted by Reuters that data for Moderna's vaccine showed that a booster does increase protective antibodies, but the difference in antibody levels before and after the shot was not wide enough, particularly in those whose levels had remained high.
The documents were released ahead of a meeting later this week of the FDA's outside expert advisers to discuss booster doses of the vaccine.
In addition, a panel of advisers to the US Centers for Disease Control and Prevention (CDC) will meet next week to discuss specific recommendations on who can receive the boosters, if the FDA authorizes them.
Moderna is seeking authorization for a 50-microgram booster dose, half the strength of the original vaccine given in two shots about four weeks apart.
The company has asked regulators to clear a third round of shots for adults aged 65 and over, as well as for high-risk individuals.
Last month, the FDA authorized a booster dose of the Pfizer and BioNTech vaccine for older adults and some Americans at high-risk of illness.
The FDA's advisers will also consider booster doses for Johnson & Johnson's single-dose vaccine on Friday.