The director of the Centers for Disease Control and Protection spoke on Wednesday about evidence that vaccinated people can spread the Covid-19 Delta variant to others, after the nation’s top health agency expanded on its new guidance that fully vaccinated Americans wear masks indoors in certain places.

Rochelle Walensky said “new science” observed in recent days demonstrated that new variants of the coronavirus were transmissible by people who have been fully vaccinated in some cases.

“With prior variants, when people had these rare breakthrough infections, we didn’t see the capacity of them to spread the virus to others,” Walensky told CNN.

“But with the Delta variant we now see in our outbreak investigations that have been occurring over the last couple of weeks, in those outbreak investigations we have been seeing that if you happen to have one of those breakthrough infections that you can actually now pass it to somebody else.”

The CDC revised its mask guidance on Tuesday to recommend fully vaccinated Americans wear masks in “public indoor settings” with “substantial and high transmission”, a shift from its earlier guidance issued on 13 May, which said vaccinated individuals did not need to wear masks in most indoor settings.

The move came as Joe Biden said requiring all federal workers to get a coronavirus vaccine is “under consideration” as the Delta variant surges in the US. Some local and state leaders, including New York’s mayor, Bill de Blasio, and the California governor, Gavin Newsom, have already announced such mandates for their government employees.

Walensky also spoke on Wednesday about the threat of Covid-19 to children. “If you look at the mortality rate of Covid, just this past year for children, it’s more than twice the mortality rate that we see in influenza in a given year,” she said.

On Tuesday the CDC changed its advice and now recommends that fully vaccinated people living with vulnerable household members, such as those who are immunocompromised and children, wear masks in indoor public spaces. In addition, the agency recommended everyone in K-12 schools wear masks, “including teachers, staff, students and visitors, regardless of vaccination status”, Walensky said in a press briefing on Tuesday.

“In recent days I have seen new scientific data from recent outbreak investigations showing that the Delta variant behaves uniquely differently from past strains of the virus that cause Covid-19,” Walensky said on Tuesday, referring to scientists’ discovery of the Delta strain shedding as actively in breakthrough infections as it does in unvaccinated individuals, despite the rarity of breakthrough cases.

For months Covid cases, deaths and hospitalizations were falling steadily, but the highly infectious Delta variant of the coronavirus has fueled steep rises in case numbers, particularly among unvaccinated Americans and amid struggles with disinformation and resistance, particularly on the political right.

“Nobody wants to go backward but you have to deal with the facts on the ground, and the facts on the ground are that it’s a pretty scary time and there are a lot of vulnerable people,” Robert Wachter, chairman of the department of medicine at the University of California, San Francisco, told the Washington Post.

“I think the biggest thing we got wrong was not anticipating that 30% of the country would choose not to be vaccinated.”

In recent weeks, a growing number of cities and towns have restored indoor masking rules. St Louis, Savannah, Georgia, and Provincetown, Massachusetts, are among the places that reimposed mask mandates this month.

At a White House briefing last week, the surgeon general, Vivek Murthy, said 97% of hospital admissions and 99.5% of Covid deaths were occurring among unvaccinated people.

More than 162.7 million Americans are vaccinated – or 49% of the population, according to the CDC.

California and New York City announced on Monday that they would require all government employees to get the coronavirus vaccine or face weekly Covid-19 testing, and the Department of Veterans Affairs became the first major federal agency to require healthcare workers to receive the shot.

In New York City, Mayor Bill de Blasio announced that all municipal workers – including teachers and police officers – will be required to get vaccinated by mid-September or face weekly Covid-19 testing, making the city one of the largest employers in the US to take such action.

“Let’s be clear about why this is so important: this is about our recovery,” De Blasio said.

California said it will similarly require proof of vaccination or weekly testing for all state workers and healthcare employees starting next month.

Associated Press contributed to this report