The U.S. Food and Drug Administration gave the go ahead Friday for all adults to get booster shots made by either Pfizer-BioNtech or Moderna.

The authorization paves the way for millions of Americans to get additional COVID-19 protection six months after their second dose.

This should eliminate confusion about who can get a booster shot, the director of the FDA's Center for Biologics Evaluation and Research, said in a statement.

Over 31 million Americans have already received a booster shot, but current Centers for Disease and Control Prevention guidelines recommend extra doses only for some population groups -mainly the elderly and immunocompromised.

The CDC is expected to recommend booster shots for all adults.

The FDA said its decision was supported by data showing that a booster of the vaccine increased the immune response in studies of both Moderna and the Pfizer/BioNTech vaccine.

Pfizer has reported data from a large clinical study that showed a booster dose of its vaccine was 95.6% effective against the coronavirus when compared with a vaccinated group that did not get the third shot.

The FDA authorization comes as new COVID cases start to rise again in the United State– after falling for nearly two months and as millions prepare to travel for the Thanksgiving holiday.

Nearly 60% of adult Americans - some 195 million people - are considered fully vaccinated, having received two doses of either the Pfizer/BioNTech or Moderna vaccines, or one shot of the Johnson & Johnson vaccine.

Shares of Moderna rose roughly 5 percent in early Friday trading.

Pfizer was up as well.