Covid booster shots are “optimal care” as the deadly virus continues to mutate and spread, but the U.S. government is staying firm for the time being on the definition of fully vaccinated, top U.S. infectious disease expert Dr. Anthony Fauci said Sunday.

Currently, two doses of the Pfizer–BioNTech or the Moderna vaccines or one dose of the Johnson & Johnson vaccine provide full vaccination. Health officials will continue to evaluate whether that definition needs to change, Fauci said on ABC’s “This Week With George Stephanopoulos.”

“I think if you look at the data, the more and more it becomes clear that if you want to be optimally protected you really should get a booster,” Fauci said. “It’s the optimal care.”

The push for booster shots comes in tandem with the rise in cases of the omicron coronavirus variant. Twenty-five states so far have detected the variant, Rochelle Walensky, director of the Centers for Disease Control and Prevention, said in a Friday briefing. Officials expect that number to continue to rise.

It’s still too early to tell what the long-term impacts of the virus are, but early data suggests those who contract the omicron variant experience mild illness. Vulnerable patients who are older, unvaccinated or have underlying conditions have a much higher risk of developing severe disease, Maria Van Kerkhove, the World Health Organization’s technical lead on Covid-19, said in a press briefing.

As of Sunday, the U.S. was approaching 800,000 coronavirus-related deaths.

The new variant has pushed some government officials to reinstate health restrictions to slow the spread. On Monday, New York will implement its statewide mask mandate, which requires protective face coverings to be worn in public places and businesses, unless proof of vaccination is required.

“This variant moves fast, we have to move faster,” New York City Mayor Bill de Blasio said Sunday on CNN’s “State of the Union.”