The variant is believed to be causing the recent surge in coronavirus cases.
Research finds that getting a third dose of the vaccine at least six months after the second dose produced neutralizing antibodies against the beta and delta variants.
Pfizer also announced early this month that trials are scheduled to start for the booster in August.
Earlier this week, the CDC recommended that even vaccinated people wear masks indoors in parts of the U.S. where the coronavirus is surging due to the delta variant.
The country is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations.
Most new infections in the U.S. continue to be among unvaccinated people. But “breakthrough” infections, which generally cause milder illness, can occur in vaccinated people. When earlier strains of the virus predominated, individuals who were infected, but also vaccinated, were found to have low levels of virus, and were deemed more unlikely to spread COVID-19 to others, CDC Director Dr. Rochelle Walensky said.