Dr Anthony Fauci, the Director of the National Institute of Allergy and Infectious Disease, said he believes that the definition of "fully vaccinated" will soon include having a third Covid booster shot.
The doctor made the comments during an interview on CNN.
"It's going to be a matter of when, not if" getting a booster shot will be considered being "fully vaccinated," Dr Fauci said.
He stressed that the comment was his personal view, and was not indicative of any current policy discussions in the Biden administration to make coronavirus vaccine boosters mandatory for full vaccination status.
Dr Fauci's comments come as US health workers continue to grapple with a spike in cases and hospitalisation caused by the delta variant and researchers continue to investigate the newly emerged omicron variant.
According to Dr Fauci, the omicron variant - first identified by researchers in South Africa - is not likely to be as severe as the delta variant. However, he also noted that omicron is highly transmissible.
At least 17 US states have already recorded patients infected with the omicron variant, including fully vaccinated individuals. Symptoms have reportedly been mild in individuals who have been fully vaccinated.
Some of the earliest results from research into the omicron variant has suggested that a booster shot of the Pfizer coronavirus vaccine is effective at countering the virus.
According to the World Health Organisation, the existing coronavirus vaccine should still work at mitigating infection by the omicron variant, but noted that a third dose would improve protection.
"We have highly effective vaccines that have proved effective against all the variants so far, in terms of severe disease and hospitalisation, and there's no reason to expect that it wouldn't be so" for omicron, WHO emergency director Dr Mike Ryan told the AFP.
However, a small study out of South Africa found that the omicron variant may reduce the antibody protection given by the Pfizer vaccine.
The Africa Health Research Institute and a team of researchers tested blood samples from 12 people who had coronavirus vaccines to determine the shots' efficiency at countering the omicron variant.
The researchers found a 41-fold drop in the ability for antibodies from vaccines to stop the variant as compared to the original coronavirus. The team recommended booster shots to help bolster people's protection against the virus.
Dr Ryan echoed Dr Fauci's conclusion that the omicron variant was less likely to make people severely ill than other variants of the virus.