regulators expanded eligibility for booster shots of COVID-19 vaccines to all adults on Friday, allowing millions more Americans to get additional protection against the virus amid a recent rise in infections.

Food and Drug Administration broadened its authorization of booster doses to all adults who had received their second shot of either the Pfizer (PFE.N)/BioNTech or Moderna Inc (MRNA.O) vaccine at least six months prior.

"Booster shots have demonstrated the ability to safely increase people’s protection against infection and severe outcomes and are an important public health tool to strengthen our defenses against the virus as we enter the winter holidays," Walensky said in a statement.

A patient receives their coronavirus disease (COVID-19) vaccine booster during a Pfizer-BioNTech vaccination clinic in Southfield, Michigan, U.S., September 29, 2021.

Eric Topol, director of the Scripps Research Translational Institute in La Jolla, California, said the guidance did not go far enough to encourage booster doses.

More than 32 million Americans have received boosters, which had been authorized for the immunocompromised, those aged 65 and above, and for individuals with health issues that put them at high risk of severe disease, including obesity, or who are regularly exposed to the virus through work or living conditions