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The US is adding a new COVID vaccine to its pandemic toolbox. On July 13, the Food and Drug Administration gave emergency use authorization to Novavax’s COVID-19 shot, making it the fourth brand of vaccine cleared for use. The two-dose vaccine, which was already available in 170 countries, is only authorized for Americans 18 years and older who need their first immunization series.

Novavax’s COVID-19 vaccine is coming at a time when the US faces a lag in vaccination. As of July, 78.4 percent of the population has received at least one dose, and only 67 percent is fully vaccinated. What’s more, the rapid spread of multiple Omicron variants— BA.5 now makes up nearly 70 percent of coronavirus cases—has accelerated interest in variant-specific vaccines. On June 30, the FDA voted to change the formulation of the next set of COVID-19 boosters to include components of Omicron BA.4/5 for extra protection.

Novavax’s vaccine, though it arrives more than two years into the pandemic, is expected to boost vaccination rates among Americans wary of mRNA technology. In contrast with Pfizer-BioNTech and Moderna, Novavax uses a decades-old vaccine method, widely found in protections against other viruses such as influenza and hepatitis B.

The Novavax COVID-19 vaccine is a protein-based shot. It delivers copies of protein fragments of the virus for the immune system to recognize and destroy. Those protein copies are made in insect cells and packaged into nanoparticles that give off the appearance of a virus. Other ingredients, such as the bark of a South American tree, are included in the nanoparticles to serve as red flags—alerting the immune system to a suspicious character in the body. 

[Related: Omicron variants keep getting better at dodging our immune systems]

Each faux-virus particle is “basically a soap bubble. It’s made of stuff that you find in root beer,” Gregory Glenn, Novavax research chief told NBC Chicago.  “When an immune cell sees that, it becomes quite activated. … We supercharge the immune response.” 

In clinical trials, the Novavax vaccine was 90.4 percent effective in preventing mild, moderate, or severe COVID-19 infections. In people 65 years and older, the vaccine was 78.6 effective. It’s important to note, though, that the data was collected before Delta and Omicron appeared, and the vaccine is based on the genetic sequence from the first coronavirus strain.  

Still, US officials say this immunization will be helpful because it may be an appealing alternative to those who are holding off on getting vaccinated with mRNA vaccines. “Authorizing an additional COVID-19 vaccine expands the available vaccine options for the prevention of COVID-19, including the most severe outcomes that can occur such as hospitalization and death,” FDA commissioner Robert M. Califf said in a statement

The Biden-Harris Administration secured 3.2 million doses of Novavax’s COVID-19 vaccines, and like all other COVID-19 vaccines available in the US, the shots will be free of charge. You won’t see a Novavax vaccine offered at your local pharmacy just yet. The director of the Centers for Disease Control and Prevention will need to officially sign off on a recommendation for the two-dose shots, which is expected to happen after a CDC meeting on July 19.