Peter Marks, director of FDA's Center for Biologics Evaluation and Research, said the US could see a second or even a third wave of novel coronavirus outbreak infections as social distancing is relaxed, and the spread could continue until a vaccine is developed.

A resurgence is likely to hit areas that have not been severely affected during the first wave, particularly the "interior of the county where there are a lot of susceptible people," according to Mayo Clinic professor Gregory Poland.