The US government’s leading infectious diseases official, Anthony Fauci, on Sunday stepped up calls for Americans to get a Covid-19 booster shot, as the US is approaching 800,000 lives lost to coronavirus since the start of the pandemic.

Fauci warned that the Omicron variant appeared to be able to “evade” the protection of two initial doses of the mRNA-type Covid vaccines – Pfizer/BioNTech’s and Moderna’s – as well as post-infection therapies such as monoclonal antibodies and convalescent plasma.

Omicron is spurring new fears as US infections begin to surge again, with infections currently still led by the highly-transmissable Delta variant that has dominated since the summer.

Fauci said an extra vaccine shot provides “optimal” protection against Omicron, even though the government’s official designation of “fully vaccinated” remained at two doses of Pfizer or Moderna, or one of Johnson & Johnson’s vaccine, which was developed by another method.

“Preliminary data show that when you get a booster, for example a third shot of an mRNA, it raises the level of protection high enough that it then does do well against the Omicron,” Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the president, said on ABC.

It was, he said: “Another reason to encourage people who are not vaccinated to get vaccinated, but particularly those who are vaccinated to get boosted because that diminution in protection seems to go way back up again. If you want to be optimally protected, you really should get a booster.”

Although vaccination rates, particularly boosters, have picked up significantly in recent weeks, about 40% of eligible adults in the US are still not fully protected, and the take-up rate for children ages five to 11, who are newly eligible, remains below 20%, according to the Kaiser Family Foundation.

“Follow the CDC [Centers for Disease Control and Prevention] guidelines, that when you are in an indoor congregate setting and you don’t know the vaccination status of the people around you, wear a mask. Masking is not going to be forever but it can get us out of the very difficult situation we’re in now,” Fauci said.

Experts are still discovering the characteristics of Omicron. CDC director Rochelle Walensky last week said that very preliminary data so far showed Omicron was comparatively mild.

Bill de Blasio, the mayor of New York City, meanwhile, defended his new vaccine mandate for private employees.

“Omicron is here, it’s all over the country, this variant moves fast,” he said on CNN’s State of the Union show.

“We have to move faster. What I hear from our business community, their greatest fear is shutdowns, going back to where we were in 2020, to restrictions, to people losing their livelihood.

“The greatest threat to employment is that Omicron and the cold winter months are going to supercharge Covid and take us backwards. Since I put mandates in place starting in August, we have seen over a million more doses, 71% of our people fully vaccinated. A lot of those people made the decision because the mandate was there,” he said.

A federal judge is set to rule on Tuesday on a challenge to De Blasio’s vaccine mandate for the city’s 160,000 public sector employees, including police, firefighters and sanitation workers.