A change to the definition of “fully vaccinated” for COVID-19 to include booster doses is inevitable, according to Dr. Anthony Fauci, the government’s top infectious disease expert.

“My own personal opinion: It’s going to be a matter of when, not if,” Fauci said on CNN Wednesday.


Federal data show that nearly 72% of U.S. adults are fully vaccinated for COVID-19, meaning two weeks have passed since they received either the single-dose Johnson & Johnson shot or the second dose of the Pfizer or Moderna two-shot vaccine. The Centers for Disease Control and Prevention authorized extra doses of vaccine for all adults once six months have passed since their last shot in response to growing evidence that immunity to the disease afforded by vaccines wanes over time.

The Biden administration has fallen under pressure to update the standard definition for “fully vaccinated” to assuage general confusion, and the CDC has flipped back and forth on whether to do so as more people choose to get boosters. CDC Director Rochelle Walensky said in an October press briefing that the agency “may need to update our definition of 'fully vaccinated' in the future." She backpedaled the next month to say the agency was “not examining” a change to the standard definition.


Fauci’s comments came just hours after Pfizer-BioNTech presented early evidence that a booster dose increased the neutralizing antibody titers by twenty-fivefold compared to the initial two doses of the vaccine. The data, which have not been peer-reviewed yet, showed that a booster of the vaccine could stop severe infection from the new omicron variant.

Nearly 49 million U.S. adults have received booster doses, according to federal data.