The Centers for Disease Control now says all adults "should" get a COVID-19 booster shot once they become eligible.
The agency changed its previous guidance, which said some adults, who are considered lower risk, "may" get the booster.
People 18 and older can get a Pfizer or Moderna booster shot six months after receiving their second shot. Adults who received the Johnson & Johnson vaccine are eligible for a booster shot after 2 months.
The CDC said Monday that it's strengthening its recommendations due to the emergence of the omicron variant.
"Early data from South Africa suggest increased transmissibility of the omicron variant, and scientists in the United States and around the world are urgently examining vaccine effectiveness related to this variant," CDC Director Dr. Rochelle Walensky said.
Walensky is also encouraging people who are sick to get tested so they can identify where the omicron variant is and whether it's spreading in a community.
Nearly 60% of the U.S. population is fully vaccinated, according to the CDC.
"I strongly encourage the 47 million adults who are not yet vaccinated to get vaccinated as soon as possible and to vaccinate the children and teens in their families as well because strong immunity will likely prevent serious illness," Walensky said.