One study will look at what kind of protection is given when people receive a third shot about six to 12 months after the initial two-dose regimen. In addition, the companies said they were speaking to regulators about testing an adapted version of the vaccine that would protect against the variant from South Africa, known as B.1.351.

Moderna, which developed a vaccine using the same technology as the Pfizer-BioNTech product, said on Wednesday that it had shipped doses of a newly adjusted vaccine to the National Institutes of Health for testing. The adapted vaccine also addresses the B.1.351 variant, which seems to dampen the effectiveness of the existing vaccines.

Dr. Phil R. Dormitzer, Pfizer’s vice president and chief scientific officer of viral vaccines, said in an interview that the companies believed that a third shot, even in the original formulation, could strengthen the body’s immune system in fending off virus mutations.