Pfizer CEO Albert Burla said Thursday that a third dose of the vaccine might have to be taken 12 months after the two doses.

CNBC quoted Burla as saying, "It is also possible that people will need to be vaccinated against the virus annually."

Fears persist about insufficient doses of the Coronavirus vaccine in the United States and Europe after the United States stopped vaccinating with the Johnson & Johnson vaccine because of blood clotting concerns. 

Scientists generally believe that the benefits of vaccines are greater than any harm caused.