Novavax Inc has begun a large late-stage study of its experimental COVID-19 vaccine in the United States, the drug developer said on Monday, after delaying the trial twice due to issues in scaling up the manufacturing process.

It will enroll up to 30,000 volunteers across about 115 sites in the United States and Mexico, with two-thirds of them receiving the shot 21 days apart and the rest getting placebo, the company said.