People vaccinated against Covid-19 in high-risk areas of the United States should resume wearing masks indoors, the country’s top health official said Tuesday, reversing previous guidance and emphasizing the country’s struggle to control the Delta variant.

President Joe Biden stated that the announcement demonstrated that America needs to “do better” in terms of vaccinations, adding that a vaccine mandate for the country’s more than two million federal workers is now “under consideration.”

Rochelle Walensky, director of the Centers for Disease Control and Prevention, cited new data indicating that rare breakthrough cases involving Delta have an increased risk of onward transmission”. In areas with significant and high transmission, the CDC recommends that fully vaccinated people wear masks in public indoor settings,” she explained.

Last week, the CDC defended its surprise decision in May that vaccinated people did not need to wear masks indoors in most cases.

In another setback, the White House ordered all of its employees to mask up again on Tuesday due to local transmission rates in Washington.

According to the most recent CDC data, 63 percent of the country’s more than 3,200 counties have significant or high transmission. Substantial is defined as 50 to 100 daily cases per 100,000 people over a seven-day period, while high is defined as more than 100 daily cases per 100,000 people over a seven-day period.

Biden said on Thursday that he would lay out new steps to overcome the country’s strong start to its vaccination program.

When asked about a possible vaccine mandate for US federal workers in a separate address to the US intelligence community, Biden replied, “That’s under consideration right now.”

The Veterans Affairs Department announced on Monday that it would require its frontline health workers, approximately 115,000 people, to get the shot, making it the first federal agency to do so. Walensky emphasized that so-called “breakthrough” cases among vaccinated people are extremely rare; shots reduce the risk of symptomatic disease by a factor of seven, and hospitalizations and deaths by a factor of twenty.

However, according to new CDC research, when a vaccinated person becomes infected, their viral load is comparable to that of an unvaccinated person.

“This leads us to believe that breakthrough infections, as rare as they are, have the potential to spread at the same rate as an unvaccinated person,” she said. The CDC will recommend universal masking for teachers, staff, students, and visitors, regardless of vaccination status, when schools reopen in the fall to combat the spread of the Delta variant, she added.

The Delta variant, which accounts for approximately 90% of cases, is now increasing the number of infections in the United States.

The most recent seven-day average of daily cases is over 56,000, which is comparable to levels last seen in April.

In the United States, more than 610,000 people have died as a result of Covid-19.

Although 49% of the US population is fully vaccinated, the vaccination rate is heavily skewed towards politically liberal areas of the country. Walenksy emphasized that masks were only a “temporary” solution, adding, “What we really need to do to drive down these transmissions in high transmission areas is to get more and more people vaccinated.”

“This is a solution to a problem that exists because we have high levels of people who are not vaccinated,” said Eric Cioe-Pena, director of global health at Northwell Health in New York.

“The vaccination rate is the main predictor of how much Covid will surge in an area,” he added, so areas that are highly vaccinated will be exempt from the new mask guidance. According to a recent paper published in the journal Virological, the amount of virus found in the first tests of patients with the Delta variant was 1,000 times greater than patients in the virus’s first wave in 2020, significantly increasing its contagiousness.

Due to the Delta variant, Israel reinstated mask mandates just 10 days after lifting them, and local US jurisdictions, including Los Angeles County, have taken similar steps.