The FDA said Novavax's COVID-19 vaccine is effective, but the regulator raised concerns about potential heart risks associated with the shot in briefing documents released June 3, according to The Wall Street Journal.
Data from a clinical trial involving about 30,000 adults found the vaccine was 90.4 percent effective at preventing COVID-19. However, the study was conducted before the emergence of the highly transmissible delta and omicron variants, the FDA said.
Six participants also developed myocarditis or pericarditis — both types of heart inflammation — after vaccination.
"These events raise the concern for a causal association with this vaccine, similar to the association documented with mRNA COVID-19 vaccines," the FDA said.
Novavax's shares fell by more than 20 percent after the FDA released its report.
"We believe there is insufficient evidence to establish a causal relationship," the company said June 3 in response to the FDA's report. "We have learned that we can expect to see natural background events of myocarditis in any sufficiently large database."
An FDA panel of outside experts is slated to meet June 7 to discuss whether to recommend the vaccine for emergency use authorization.