Advisory committee members say clinical data proves the second shot should be considered part of the primary series, while Janssen wants it called a single-shot vaccine with a booster.

Despite lack of enthusiasm with immunobridging study data, the panel endorsed emergency use in certain at-risk populations, in part because it would be hard to turn down the Moderna booster when an identical EUA had been issued for Pfizer/BioNTech's mRNA vaccine.

Agency assessors couldn’t independently verify several Janssen analyses of booster shot data and raised concerns about small samples sizes and an unvalidated immunogenicity assay.

Immune responses to a single booster dose of mRNA-1273 would be considered successfully bridged to the two-dose primary series if each of two immunobridging criteria were met, but the mRNA vaccine missed the prespecified success criteria for seroresponse rate, the FDA said.