Pfizer is testing out a a third dose of its COVID-19 vaccine to see if helps protect against new coronavirus variants.
The third dose is exactly the same as the previous 2.
Participants will receive a third dose 6 to 12 months after their earlier two doses.
Then scientists will look at if they develop antibodies that protect against new virus strains.
Pfizer is also talking to regulators about the possibility of testing a modified version of its vaccine against those variants.