The Food and Drug Administration authorized the Novavax COVID-19 vaccine for people 18 and older, the fourth shot cleared in the United States. It’s been a long road for this vaccine, and the company faced multiple setbacks over the past few years.

It’s a different type of vaccine than the ones already on the market in the US — it’s a protein-based vaccine that delivers pieces of the virus spike protein directly to the body. Clinical trials found that the shot was 90 percent effective at preventing COVID-19. The trials, though, were conducted in early 2021 — before the delta and then omicron variants of the virus took over, so those numbers would likely be lower now.

Early on in the pandemic, experts thought that the Novavax vaccine was one of the best bets against the coronavirus. The US government invested $1.6 billion in Novavax’s efforts, more than any other company at the time. But the company had never brought a vaccine to market. It had issues with its manufacturing, and it didn’t start testing its vaccine until well after US frontrunners Moderna and Pfizer / BioNTech. Even after the clinical trial results showed that the shot worked well, the manufacturing issues delayed any submissions to the FDA.

FDA leadership hopes that having an alternative to the mRNA vaccines could be a way to encourage people who are still unvaccinated to get a shot. “Having a protein-based alternative may be more comfortable for some in terms of their acceptance of vaccine,” said Peter Marks, the FDA’s director of the Center for Biologics Evaluation and Research, during a meeting of the agency’s independent expert committee.