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Who’s “fully vaccinated” against Covid-19 — and who’s not — is starting to get a lot more complicated. The Centers for Disease Control and Prevention says you are only granted the status two weeks after you get a single-dose vaccine or the second dose in a two-dose series. But with the advent of boosters, certain colleges, the NBA, and the state of New Mexico are saying you’re only there with three shots.

“For the time being, the official definition of ‘fully vaccinated’ is two,” although that determination could change as we learn more about the Omicron variant, Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said at a White House briefing last week.

This isn’t just a discussion of semantics: The CDC’s definition matters for how states refine vaccine mandates and requirements for travel, dining, and other indoor activities. Even the phrase “fully vaccinated” can shape the public’s imagination in ways that can affect their response to public health interventions, and signal an end in sight to the pandemic.

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Experts who spoke with STAT raised questions about a potential switch, noting that it could have negative implications for additional boosters and international vaccine equity. Here are the biggest questions they have about changing the term “fully vaccinated” to mean three vaccine doses.

What if a ‘three dose’ definition undermines booster shots?

Changing “fully vaccinated” to mean three doses might undermine the way boosters were designed to work, if such a change meant that a booster would be accelerated, according to Keri Althoff, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

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A booster dose might seem similar to an extra dose, but one of its main distinguishing features is time — the space in between the last dose of the primary series and the booster is key to mounting an effective immune response, Althoff said.

“The goal [with a booster] isn’t to supplement your initial response. The goal there is to boost an older response,” she said.

She said the term “up-to-date” could replace “fully vaccinated,” particularly as more research comes out about long-term protection from the Covid vaccines. But a person receiving a complete primary series — two doses of an mRNA vaccine, or one Johnson & Johnson shot — should still be considered “fully vaccinated.”

However, Eric Topol, the founder and director of Scripps Research Translational Institute, said he believes a person’s booster status, not just their initial dose or doses, should factor into whether they are considered fully vaccinated.

“It was very clear that no one expected two shots to last a lifetime,” Topol said. “I consider now that the third dose — so-called booster — is an essential part of vaccination against Covid. In order to be considered ‘fully vaccinated,’ it takes three doses for all vaccines,” excluding the Johnson & Johnson vaccine, he told STAT.

Getting Americans vaccinated with boosters is critical to a nationwide plan to slow Covid’s spread, just as getting shots into the arms of unvaccinated people is, Topol said. We now know that immunity wanes after six months, so both tasks are time-sensitive.

“In the U.S., we have more people waning every day than we have getting a new vaccine, so we’re actually losing ground.”

What if Omicron changes what ‘fully vaccinated’ means?

Since it’s not clear yet how well current vaccines will protect us against the Omicron variant, it’s far too early to talk about changing the term “fully vaccinated,” according to Ali Ellebedy, an associate professor of pathology and immunology at Washington University School of Medicine in St. Louis.

“We are fully vaccinated against the original SARS-CoV-2, but the original SARS-CoV-2 is not circulating anymore,” he said. “​​Having that new variant is changing the rules.”

In fact, Ellebedy said, we might need to throw out our current definition of “fully vaccinated” entirely, at least until we learn how Omicron affects the protections the vaccines offer. On Tuesday, data from a small, preliminary study was posted online, showing a significant drop in how well vaccine-elicited antibodies target the Omicron variant of the coronavirus.

Why change it now, if we’re going to change it again?

Walid Gellad, a professor of medicine at the University of Pittsburgh, argued that any declaration that we know what “fully vaccinated” looks like is inappropriately confident.

“The bottom line for me is about the level of certainty we have on what is the right number of shots? Will there be more shots needed? Is it the right dosage status?” he said. “It’s all just premature to talk about what is the definition of fully vaccinated.”

He added that rather than make statements to later revise them, public officials should acknowledge uncertainty as the details of an ideal vaccination schedule are still being worked out.

Could a three-dose definition widen vaccine inequities?

The U.S. isn’t the only country grappling with this definition. Israel, Austria, and the European Union have introduced policies that all but mandate booster shots to maintain one’s fully vaccinated status. But a similar policy in the U.S. could make travel harder for citizens of other countries who may not have access to boosters, Gellad said.

“What happens if we start saying you need to be fully vaccinated with three shots to, for example, fly to the U.S.?” he said. “What does that mean for people internationally? What does that mean economically?”

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