Novavax's COVID vaccine was shown to be 90% effective at preventing new cases in a clinical trial but carried the possible risk of causing heart inflammation, particularly in young males, Food and Drug Administration staff said in a review posted online Friday.

Why it matters: The company is seeking FDA emergency use authorization for the shot, positioning it both as a booster for the vaccinated and an alternative first-line defense for people who have refused mRNA shots.

Driving the news: The staff evaluation comes days before an FDA advisory panel will decide whether to green-light the shot for use in adults 18 and over.

• The review noted instances in which people who took the vaccine had an increased potential risk of the heart conditions myocarditis and pericarditis, especially within seven days of the second shot.
• Four of the events occurred in young men, a subject population known to be at higher risk for mRNA COVID-19 vaccine-associated myocarditis.
• The study was conducted before the emergence of the Delta and Omicron variants, raising questions about the vaccine's overall effectiveness.
• FDA staff said the shot is likely to provide some meaningful protection against severe disease from Omicron but that its utility as a booster needs to be evaluated further.
• There is not enough data to evaluate the safety of the vaccine in children, pregnant women and immunocompromised people, FDA staff said.

Catch up quick: Novavax was one of America's first big bets under Operation Warp Speed, winning $1.6 billion in federal backing in 2020.

• The vaccine uses a protein from the target virus combined with what's called an adjuvant which enhances the body's immune response. It's easier to store than Moderna and Pfizer's vaccines.
• The company ran into early production problems and struggled to meet FDA standards. But it has won approval from regulators overseas, along with the World Health Organization and the European Commission for primary vaccination of adults 18 and older.