Nearly half of Americans live in areas where they should consider masking indoors to curb a surge in COVID-19 cases, according to data updated Thursday by the Centers for Disease Control and Prevention. That’s up from around a third of Americans last week. 

More than 16% of Americans now live in areas of “high” COVID-19 Community Levels, up from just 9% last week. At this tier, which reflects the worst levels of disease in the agency’s guidance, the CDC says all Americans should don masks in public indoor settings, regardless of vaccination – including in schools. 

An additional 29% now live in areas of “medium” levels of disease, up from 23% last week. At that level, the CDC urges Americans to consider wearing masks and testing before indoor contact with people at high risk of severe disease, like older adults or those with underlying conditions. 

The CDC typically revises its official COVID-19 Community Level metrics once a week on Thursday evenings. However, many local health departments often issue their own warnings   ahead of the updates. 

The boroughs of New York City now rank among the most populous counties in the country at “high” COVID-19 levels. Officials in the city raised their own alert level to “high” on Tuesday, citing a worrying jump in COVID-19 hospitalizations.

“New York City has transitioned to a high COVID alert level, meaning now is the time to double down on protecting ourselves and each other by making choices that can keep our friends, neighbors, relatives, and co-workers from getting sick,” the city’s health commissioner said in a statement.

The city joins much of the region, which has been at “high” levels of COVID-19 for weeks, as counties have recorded spikes in COVID-19 cases and hospitalizations – especially among the oldest, most vulnerable age groups.

Cases in nursing homes have also surged in recent weeks across the Northeast, though recent data published by the CDC suggests the pace of new infections may now be slowing.  

Outside of the Northeast, the CDC says communities from Honolulu to Detroit are currently at “high” levels. Dozens of counties – including Los Angeles, Philadelphia, and Washington, D.C. – are now at “medium.” 

The new levels trigger a range of precautions for buildings in these areas across the federal government. Under the Biden administration’s rules, agencies must require masking in federal facilities and ramp up testing in areas of “high” COVID-19. 

However, few communities now re-entering “high” COVID-19 Community Levels have decided to reimplement mask requirements on residents. 

“For areas currently with high COVID-19 community levels — those in orange — we urge local leaders to encourage use of prevention strategies like masking in public indoor settings and increasing access to testing and to treatment,” CDC Director Dr. Rochelle Walensky told reporters on Wednesday, ahead of the updated figures.

“And in any COVID-19 community levels, individuals may always choose to wear a mask to protect themselves from infection,” Walensky added later. 

Walensky and Dr. Anthony Fauci, the president’s chief medical adviser, said recently they were sometimes opting to wear masks even in areas of “low” COVID-19 levels.

The surge comes as health officials are warning the true tally of COVID-19 cases is increasingly being undercounted, as a growing share of Americans adopt at-home testing. 

Test positivity has soared above 10%, far above the 5% benchmark sometimes used as a sign that many infections are being missed. 

“Home tests are great, by the way. I’ve been a huge fan of home tests for the last two years.  But what that means is we’re clearly undercounting infections,” Dr. Ashish Jha, the White House’s top COVID-19 official, said.