Late-stage trial data suggest that a new COVID-flu vaccine offers good protection against both infections, but experts expect the shot's approval may be delayed.

This vaccine did previously have emergency approval by the FDA for use but can now be marketed in the U.S. for use, the letter said.

Novavax shares surged more than 17% before the bell on Monday, following the long-delayed approval of its COVID-19 vaccine from the U.S. health regulator, albeit with new conditions.