Novavax (NVAX) was asked by the US Food and Drug Administration to run an additional randomized clinical trial on its COVID-19 vaccine, The Wall Street Journal reported Friday, citing people familiar with the matter.

That may cost the company tens of millions of dollars, vaccine experts said, according to the report.

The vaccine had showed 90% efficacy in a 30,000-person, placebo-controlled trial, the report said.

The FDA request allows the company to negotiate for a smaller, less expensive study, the report said, citing a person familiar with the matter.

The Department of Health and Human Services and Novavax didn't immediately respond to requests for comment from MT Newswires.

Novavax shares fell 1.4% in after-hours trading after dropping 5.7% in the regular session.

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