New York CNN Business  — 

Get ready for booster mandates.

Millions of US workers are already required to show proof of a Covid vaccine to their employer. Soon many could be forced to show proof that they also got a booster shot.

With the rise of the Omicron variant of Covid-19, public health officials are stressing that a booster shot is more important than ever. Many who got their initial vaccine dose or doses more than six months ago could have limited protection against the easily transmissible Omicron. A booster dose is believed to provide important protections against it.

“Omicron has turned this undoubtedly into a three-dose vaccine, if it wasn’t heading that way already,” Andy Slavitt, a former senior pandemic adviser to President Joe Biden, told CNN last week. “Two doses just aren’t going to be sufficient any longer. If everybody is boosted, that’s your best shot at having everyone back [in the office].”

Among the states with a booster mandate already in place is New Mexico, which just required that health care workers who were already required to be vaccinated must get a booster shot by Jan. 17. State workers and public school teachers, who must be vaccinated or submit to weekly testing, will also need to get booster shots to be considered fully vaccinated.

“New Mexico is doing well with boosters, better than the national average, but we still must do better,” said Dr. David Scrase, the state’s acting health secretary.

Many colleges and universities are also requiring booster shots for students, faculty and staff, including Yale, Princeton, Columbia, Stanford, Harvard, Dartmouth, MIT, Amherst, Williams, Emory and Northwestern, mostly when students are due to return to campus following the holiday break.

New York’s Metropolitan Opera just announced that all performers, staff and audience members will have to provide proof of a booster shot in order to attend a performance. New York City, which is requiring all businesses to mandate a vaccine for employees, is considering a booster mandate for city workers. The Pentagon has also confirmed it is considering a booster mandate for members of the US military.

A survey of 200 major employers conducted by consulting firm Gartner last week found 8% of employers are changing their definition of what constitutes “fully vaccinated” and requiring workers to get booster shots. That’s a roughly one of every six of the 46% of US employers that either have a vaccine mandate in place or plan to institute one.

The percentage requiring boosters is expected to increase as Omicron cases spread and companies try to figure out how to reopen offices that were closed or staffed with limited workers for most of the pandemic, said Brian Kropp, chief of research at Gartner’s HR practice.

“This is the first time we’ve asked about booster mandates. I’m pretty sure if we asked a month ago it would have been 0%,” he said. “By the time we get to the middle of January, it’ll be a lot higher, probably 15-20% is my best guess at this point.”

Why employers could start mandating boosters

There are numerous reasons employers want to impose a vaccine mandate on workers, including reducing absenteeism and health care costs. The Omicron surge has forced some businesses to close due to a lack of healthy workers. And Delta Air Lines has disclosed it spent an average of $50,000 on medical care for each employee hospitalized with Covid. According to a December survey by Gallup, 36% of those surveyed said their employer is requiring them to get a vaccination.

That percentage also could soon rise. By February there could be federal rules mandating that every business with 100 or more employees require either vaccination or weekly testing of workers. Employers could avoid the cost and complexity of tracking the testing results by simply requiring vaccination.

Still, when it comes to requiring boosters and not just vaccines, Kropp said companies are reluctant to get out in front of the Centers for Disease Control as to what constitutes “fully vaccinated.”

“Employers who are facing opposition from some of their employees to vaccine mandates want to be able to say, ‘We’re following what CDC says,’” Kropp said. If the CDC changes what it considers fully vaccinated to require booster shots, he added, the percentage of employers requiring boosters could shoot up to 60% or more.

‘Fully vaccinated’

Changing the official definition of what it means to be “fully vaccinated” to include only those who have also received a booster shot is “on the table and open for discussion,” according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview on CNBC on Friday.

“There’s no doubt that optimum vaccination is with a booster,” he said.

But as of now the CDC says that someone who has gotten the two doses of the Pfizer or Moderna vaccines or a single dose of the Johnson & Johnson vaccine is considered to be fully vaccinated, even without a booster shot. The agency is strongly recommending booster shots for anyone who is six months removed from their initial vaccination.

So far CDC data said that 139 million Americans ages 18 to 64 are fully vaccinated, and of that group, only 34.5 million, or 25%, have gotten a booster shot. So there are a lot of workers out there who are willing to get vaccinated who have not gotten the booster yet.

Most people who today are considered fully vaccinated are planning to get a booster shot, according to surveys by KFF, the health care think tank.

“Our poll shows there is a still some lack of awareness about boosters, unsure if it’s recommended. Many didn’t think they were eligible, or they didn’t think they needed it,” said Ashley Kirzinger, KFF’s associate director for public opinion and research. But she said news about the threat posed by Omicron is raising interest in boosters. A majority of the vaccinated who have yet to get the booster say Omnicron makes them more likely to get an additional shot, according to a survey KFF completed Monday.

Mandates could also be very effective getting those who have yet to get a booster to do so, Kirzinger said.

“For the unvaccinated, 48% say nothing will convince them to get vaccinated,” she said. “But for those who are already vaccinated, they wouldn’t have a strong negative view of vaccines. So a booster mandate could get much better compliance.”

– CNN’s Matt Egan contributed to this report.