If you and your family members are fully vaccinated against Covid-19, it's OK for you to ditch the masks this holiday season when you're around each other, Dr. Anthony Fauci told CNN Sunday.

"That's what I'm going to do with my family," Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN's Dana Bash on "State of the Union."

However, the nation's top infectious disease expert also noted if you are traveling or are unaware of thevaccination status of the people around you, then you should wear a mask in those situations.

"Get vaccinated and you can enjoy the holidays very easily. And if you're not, please be careful," Fauci said.

"Get tested if you need to get tested when you're getting together, but that's not a substitute for getting vaccinated. Get yourself vaccinated and you can continue to enjoy interactions with your family and others."

Fauci's comments come as the US faces the second holiday season of the pandemic, but the first with safe and effective vaccines now available to people ages 5 and older. Still, a significant part of the eligible population remains unvaccinated.

According to data published Friday by the US Centers for Disease Control and Prevention, nearly 196 million people, or 59% of the total US population is fully vaccinated. But about 26.6% of the eligible population, or 83 million people, have yet to receive a first dose.

The vast majority of Covid-19 deaths so far this year have been among unvaccinated people, Dr. Francis Collins, director of the National Institutes of Health, said Sunday.

"More than a thousand people dying every day -- the vast majority of them unvaccinated. Those are preventable deaths, probably at least 100,000 of the deaths that have happened this year didn't need to," Collins said.

The seven-day average of vaccinations has increased -- about 36% compared to last week -- due in large part to vaccinations among newly eligible children.

Definition of 'fully vaccinated' remains the same

Another factor here is vaccine booster doses: As of Friday, the CDC and the US Food and Drug Administration have approved boosters for every adult who received Moderna or Pfizer/BioNTech's Covid-19 vaccine six months ago or longer.

The agencies had already said everyone who got Johnson & Johnson's Janssen vaccine two months ago or more should get a booster.

For now, Fauci said the US definition of "fully vaccinated" remains the same -- two doses given within a certain timeframe -- but those guidelines could potentially change with more data.

"Two shots for a time frame means you are fully vaccinated, but the thing you want to get the people and the viewers to understand, it isn't the effectiveness of the vaccine -- it's quite effective -- it's how long it lasts," he said.

"We're going to take a look right now at what the durability is of the booster. We're going to follow people who get boosted," Fauci told Bash. "It will be guided by the science -- and people should not be put off by the fact that as time goes by and we learn more and more about the protection that we might modify the guidelines."

Fauci underscored the importance of vaccines and boosters going into the next few months, as he noted an uptick in cases is "not unexpected" in the winter.

Those who are eligible for vaccines but remain unvaccinated pose a risk not just to themselves and other people who are unvaccinated, he said, "but it also spills over into the vaccinated people, because no vaccine is 100% effective."

"We have a lot of virus circulating around. We know that there are breakthrough infections and that's how you get the uptick in cases," Fauci said. "The bottom line, common denominator of all of this data, is we should get vaccinated if you're not vaccinated, and boosted if you have been vaccinated."