Federal health researchers and vaccinemaker Novavax announced Monday that they will begin a Phase 3 trial for Novavax’s COVID-19 vaccine candidate in the United States, marking the fifth such shot in the country to reach its key final stage of testing.

“We’ve come this far, this fast, but we need to get to the finish line,” Dr. Francis Collins, director of the National Institutes of Health, said in a statement. “That will require multiple vaccines using different approaches to ensure everyone is protected safely and effectively from this deadly disease.”

Novavax plans to enroll some 30,000 people across 115 sites in the United States and Mexico to test the shot, known as NVX-CoV2373. The company is also conducting a Phase 3 trial of the vaccine in the United Kingdom, where enrollment was completed last month.

Similar to other COVID-19 vaccines, Novavax’s two-shot regimen is designed to boost the body’s immune response against the distinctive coronavirus spike protein. While the mRNA vaccines rolled out by Pfizer and Moderna rely on the body’s cells producing antigens to hone the body’s defenses, Novavax manufactures its own antigens mimicking the virus’ spike protein. The antigen “can neither replicate, nor can it cause COVID-19,” Novavax stated.

The Maryland-based company has announced promising data from early trials of the vaccine indicating the shot is safe and effective, provoking only mild reactions typical of other vaccines and “robust antibody responses” against the coronavirus.

If it proves effective in the Phase 3 trials, Novavax’s vaccine would have one advantage over the versions currently being produced by Pfizer-BioNTech and Moderna, which must be kept frozen — in Pfizer’s case, at ultra-cold temperatures. Novavax’s vials could be distributed across standard supply chains at refrigerator temperatures, without the need for specialized freezers.

In addition to Pfizer and Moderna, whose vaccines began rolling out in the U.S. in recent weeks,two other vaccine developers have Phase 3 trials underway in the United States: Johnson & Johnson’s Janssen and AstraZeneca. In October, Novavax said its Phase 3 trial faced delays over issues with large-scale manufacturing of its doses. Unlike pharmaceutical behemoths like Pfizer, Novavax is relying on contractors to produce its doses.

Novavax is also among the companies to receive an infusion of funds from the Trump administration to spur vaccine development, awarded $1.6 billion by Operation Warp Speed over the summer.

But because other vaccines are already starting to become available to the public, Novavax may face an additional obstacle as it moves into the final phase of testing.

Pfizer and Moderna had credited widespread enthusiasm for taking part in their trials — and the rapid spread of COVID-19 in the United States — in speeding their Phase 3 testing to completion. But in its announcement on Monday, Novavax acknowledged some Americans could be hesitant to risk receiving a placebo in Novavax trial versus waiting to secure their own dose of the Pfizer-BioNTech or Moderna vaccines now being distributed across the United States. 

“We recognize that volunteers considering our trial may have questions about the potential impact on their ability to receive an authorized vaccine when it becomes available to them,” Dr. Gregory Glenn, the firm’s president of research and development, said in a news release.

“We wish to reassure participants that we are working to ensure that their involvement in our trial does not negatively impact their ability to be vaccinated at the appropriate time.”