Yet given resistance to vaccinations, requiring a booster dose could be a thorny decision for policy makers and health authorities.

A booster shot is already becoming a fact of life at some places, however. More than 75 universities have required boosters for students returning to campus in the winter, according to data compiled by the Chronicle of Higher Education. New Mexico mandated the extra dose for some state employees.

Molson Coors Beverage Co. recently decided to implement a booster requirement for US employees after instituting a vaccine mandate in August.

“Omicron has made the need for three shots more of an acute critical matter because vaccine efficacy and effectiveness drops further,” said Eric Topol, director of the Scripps Research Translational Institute in La Jolla, Calif.

Centers for Disease Control and Prevention Director Rochelle Walensky said Wednesday that the agency is looking at changing the definition of what it means to be fully vaccinated. Yet people familiar with the deliberations said the agency was unlikely to make a change soon.

The Biden administration wants to encourage boosters, but changing the definition of what it means to be fully vaccinated would be complicated because it would affect vaccine mandates that are being challenged in courts, one of the people said.

Adding to the practical challenges of taking official action is that children under 16 years aren’t currently eligible for boosters, the people said.

The administration is also discussing whether to stop using the phrase “fully vaccinated’’ and replace it with “up-to-date,” which may make it easier for people to understand that Covid-19 shots are a regular part of life, people familiar with the matter said.

The CDC uses the terminology when referring to other kinds of regular vaccinations that aren’t related to Covid-19.

Until now, health authorities have defined fully vaccinated as getting two doses of the vaccine from either Moderna Inc. or Pfizer Inc. and partner BioNTech SE, or one dose of Johnson & Johnson’s shot.

People who got the Pfizer-BioNTech and Moderna vaccines are eligible for boosters at least six months past their second shot, while J&J vaccine recipients are eligible for boosters two months after their primary dose.

Yet some experts said the need for an additional dose crystallized in August, when some vaccinated people began getting breakthrough Delta infections resulting in moderate to severe illness.

“As we get into the issue with Omicron, it’s going to become equally apparent that three doses are going to be very important,” said Michael Osterholm, an epidemiologist at the University of Minnesota.

Pfizer-BioNTech and Moderna have released lab studies in recent days indicating a third dose of their messenger RNA vaccines is needed to maintain antibody levels for neutralizing the virus.

The Pfizer-BioNTech and AstraZeneca PLC vaccines were estimated to be no more than 20% effective against symptomatic Omicron infection, while a booster shot was projected to raise efficacy to 55% to 80%, according to Imperial College London researchers.

Gigi Gronvall, a professor at the Johns Hopkins Bloomberg School of Public Health, said a third dose improves the quality and quantity of peoples’ antibodies, helping to decrease the incidence of severe disease.

“The vaccines are not a bug zapper that prevents the virus from getting into you. You will get exposed, the virus will start making copies of itself, and it’s a matter of, is your immune system prepared to shut that down?” she said.

Among the universities instituting booster mandates for students returning to campus are American University, Cornell University and the University of Michigan.

American University President Sylvia Burwell said in a note to students and faculty that the university is requiring boosters starting Feb. 10 because of factors including increasing Covid-19 transmission in Washington, D.C., large numbers of people six months past their second shot, holiday travel and the emergence of the Omicron variant.

“As the weather gets colder and people gather indoors, getting a Covid-19 vaccine or booster can help protect you and those around you,” she said.

New York’s Metropolitan Opera said it is requiring all staff, performers and audience members to get a booster to attend a performance, starting Jan. 17. The L.A. Music Center, including venues such as Walt Disney Concert Hall and the Ahmanson Theatre, said it would require boosters for audience members and staff who are eligible.

Danny Meyer, head of Union Square Hospitality Group, said on CNBC Wednesday that his restaurants such as Gramercy Tavern and Union Square Cafe would require customers to show proof of a booster to dine indoors, and employees and new hires must also receive boosters.

A survey of 200 major employers conducted earlier this month by Gartner found 8% of the businesses are redefining what it means to be fully vaccinated and are requiring workers to get booster shots.

About 50% of the surveyed employers said they have or plan to institute vaccine requirements.

It could be tougher for the federal government to require a booster, given the opposition to the existing vaccine mandate, analysts said.

The Supreme Court has scheduled a special hearing next month to assess the legality of a vaccine or testing mandate aimed at large employers and a vaccination requirement for certain healthcare workers.

If the U.S. changes the definition of fully vaccinated to include three shots, the Biden administration’s vaccine or testing requirement would likely require millions more people to get tested every week because only a minority of vaccinated Americans have gotten the extra dose, said Larry Levitt, executive vice president at the Kaiser Family Foundation.

Such a move would also be challenging because most people aren’t eligible for booster doses until six months after their second shot.

“A requirement in workplaces or entertainment settings for a booster would certainly boost boosters, but it would take time and there would be chaos in the interim,” Mr. Levitt said.

President Biden and other senior administration officials have been urging people to get the extra dose as the best defense against Omicron.

“Boosters provide the strongest protection,” Mr. Biden said on Tuesday, as he announced new measures to confront Omicron such as adding thousands of new vaccination sites to make boosters more accessible. “Unfortunately, we still have tens of millions eligible for booster shots who have not yet gotten them.”

In the U.S., 62% of the population is fully vaccinated, but less than one-fifth have gotten a booster shot, according to the CDC.

A survey of 1,065 adults published this week by the Kaiser Family Foundation found that half of the vaccinated adults who haven’t yet received a booster dose said they were more likely to get one given the recent Omicron news.