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As the world learns more about the Omicron coronavirus variant and Delta continues to cause Covid-19 cases to rise around much of the United States, the need for booster shots becomes clearer than ever – even beyond the growing data about waning vaccine immunity.

On Wednesday, Pfizer and BioNTech announced that preliminary lab studies show a third dose of the Pfizer/BioNTech vaccine can improve protection against Omicron.

The finding was not surprising to Dr. Leana Wen, emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health.

“The data from Pfizer out this morning are not surprising. Actually, this is what we have been anticipating all along, that there’s good news and bad news – the bad news being that there’s some degree of immune escape, that two doses may not work as well as against previous variants. But I actually think that’s really good news that the third dose does appear to give that really significant additional boosting effect,” Wen, a CNN medical analyst, told CNN’s Ana Cabrera on Wednesday.

“So, it adds more reason for everybody to get a booster, certainly who’s eligible,” Wen said. “But also I do hope that our federal health officials will be quick to reevaluate the definition of what it means to be fully vaccinated, especially in light of what we’re learning about Omicron.”

The US Centers for Disease Control and Prevention recommends booster doses for all adults who received their second dose of Moderna or Pfizer/BioNTech vaccine at least six months ago and those who received a Johnson & Johnson shot at least two months ago. Eligible adults can get a booster of any of the three authorized vaccines.

On Thursday, the US Food and Drug Administration authorized the Pfizer/BioNTech Covid-19 vaccine booster dose for younger ages: people ages 16 and 17. It’s the first Covid-19 vaccine booster authorized for this age group in the United States.

Vaccine makers have known for months now that the immunity vaccines elicit against Covid-19 can wane over time, leading to the need for a booster dose. The boosters appear to restore immunity to where it was initially.

Two separate studies from israel published Wednesday showed booster doses of Pfizer/BioNTech’s coronavirus vaccine reduced infections tenfold and reduced Covid-19 deaths by 90%.

The two studies, published in The New England Journal of Medicine, look at the effects of Israel’s campaign to offer boosters to everyone 12 and older with the spread of the Delta variant in the summer. While deaths and severe cases were low among fully vaccinated people, booster shots lowered them dramatically more.

It’s more evidence that boosters not only restore waning immunity, but improve protection against emerging coronavirus variants.

The evidence has led some to suggest that the definition of being fully vaccinated should shift from completing two doses of the Pfizer/BioNTech vaccine to having completed three.

What counts as fully vaccinated now?

The primary series of receiving the Pfizer/BioNTech coronavirus vaccine should be considered a three-dose series rather than a two-dose series plus booster, Uğur Şahin, CEO of German biotechnology company BioNTech, said during a news conference Wednesday.

“Particularly with the data now coming for the Omicron variant, it is very clear our vaccine – for the Omicron variant – should be a three-dose vaccine,” Şahin said.

Pfizer and BioNTech are working to develop a variant-specific vaccine for Omicron and say it will be available by March, if needed. But Şahin said even though an Omicron-specific vaccine is being developed, people who are eligible to receive booster shots now should not wait.

Since the benefit of a third coronavirus vaccine dose has become more clear in the scientific data, some public health experts have asked whether the US definition of what it means to be fully vaccinated should change – while others call it a “large leap” to change the definition.

Currently, the CDC considers people to be fully vaccinated against Covid-19 two weeks after their second dose of the Pfizer/BioNTech or Moderna vaccines or two weeks after a single shot of the Johnson & Johnson vaccine.

CNN has contacted the CDC for comment on updating that definition.

“I think that going from the hard science – which is so encouraging, I’m so glad to hear that information from Pfizer – to public policy and redefining what’s vaccinated, that’s a large leap. Let’s not make things more complicated than they are at the present time,” Dr. William Schaffner, chair of the department of preventative medicine at Vanderbilt University School of Medicine and liaison to the CDC’s Advisory Committee on Immunization Practices, told CNN’s Victor Blackwell and Alisyn Camerota on Wednesday.

“Changing the requirements of what fully vaccinated means, I think, has all kinds of downstream effects for all kinds of institutions in the country,” he said. “I don’t think we’re quite there yet.”

But it’s a matter now “of when, not if” the definition of fully vaccinated will change to include three doses, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Wednesday.

“It’s a technical, almost semantic definition, and it is the definition for requirements if someone says, ‘Are you fully vaccinated?’ to be able to attend class in a university or a college or be able to work in a workplace,” Fauci told CNN’s Kate Bolduan.

He added that he doesn’t see the definition changing “tomorrow or next week.”

But in terms of the protection that a booster dose provides, “I don’t think anybody would argue that optimal protection is going to be with a third shot. Whether or not it officially gets changed in the definition, I think that’s going to be considered literally on a daily basis. That’s always on the table,” Fauci said. “It’s going to be a matter of when, not if.”

Currently, only about a quarter – 25.9% – of fully vaccinated adults in the United States have received a booster dose of coronavirus vaccine, according to data from the CDC. That percentage is an uptick from the share of vaccinated adults who were boosted a month ago.

The pace of vaccinations is rising quickly after a dip over the Thanksgiving holiday. An average of more than 950,000 booster doses have been reported administered each day over the past week, accounting for more than half of all vaccine doses administered, according to CDC data.

But there is still room for improvement on vaccine uptake. Based on CDC guidance, more than 144 million adults should get a booster. So far, only about 48 million adults have received one.

While boosters roll out in the United States, in other parts of the world the focus remains on getting first doses of vaccines into arms – so much so that the World Health Organization has warned against putting too much emphasis on booster shots.

A global call to vaccinate the unvaccinated

Initial rounds of vaccination against Covid-19 are still important to fight the spread of the virus, WHO Chief Scientist Dr. Soumya Swaminathan said in a news briefing Wednesday.

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“I think the message is loud and clear, that it’s the primary course of vaccination that is going to protect against severe disease and death. That has to be our goal,” Swaminathan said in a media briefing. She added that WHO has recommended additional vaccine doses for people who are immunocompromised.

But among the general public, “unfortunately, even in countries that have adequate supplies or more than adequate supplies, there is still a substantial proportion of people who have not been vaccinated – 30%, 40%, 50% complete the course of vaccination,” Swaminathan said.

“So, the boosters, unfortunately, are probably not the solution to this,” she said. “At this point, the benefits that we will get from reaching those people who have not received primary courses of vaccination are going to be higher than giving additional doses to those who’ve already completed a primary course.”

CNN’s Deidre McPhillips, Amanda Sealy, Jamie Gumbrecht, Naomi Thomas and Virginia Langmaid contributed to this report.