Hospitals are beginning to revive mask mandates as the highly contagious COVID-19 variant known as JN.1 overspreads the United States and health systems grapple with a spike in other seasonal respiratory viruses.

Driving the news: There were almost 35,000 COVID hospitalizations across the country for the week ending Dec. 30, a 20.4% increase over the prior week, according to Centers for Disease Control and Prevention figures updated Friday.

• Confirmed flu cases are surging, up 17.5% over the previous week as of Dec. 30 as 38 states report high levels of respiratory illnesses.
• Health care facilities in New York, California, Illinois and Massachusetts have made masks mandatory among patients and providers, Reuters reported.
• Mass General Brigham, the biggest health system in Massachusetts, this week began requiring staff to mask during direct interactions with patients and "strongly encouraged" patients and visitors to do the same.
• NYC Health + Hospitals in New York City imposed a mandate at its 11 public hospitals, as well as clinics and nursing homes, partly to protect staff as cases rise, ABC 7 reported.
• All Los Angeles County health care facilities similarly reinstated mandates in response to the rising number of COVID infections and hospitalizations, KTLA reported.

The mandates hark back to the early days of the pandemic but are reviving a public health precaution that turned hugely controversial, spurring multiple legal challenges and a patchwork of state requirements.

• Even after CDC and state mask orders were dropped, masking continued to divide the medical community while some evidence emerged that it had moderate or minimal benefit in the absence of other precautions.

Zoom out: The expert consensus still backs the use of high-quality masks when case counts soar.

• "What we don't want is staffing shortages," New York City health commissioner Ashwin Vasan told ABC 7. "When we saw the omicron wave in 2022, the biggest issues were not only people getting sick, but that we had a lot of frontline health workers, they were out with COVID."
• Some 2 million Americans are getting infected with COVID daily, according to Eric Topol, executive vice president of Scripps Research, who pointed to wastewater surveillance that's become a key way to track virus spread given how few people are testing and how few of those results are reported to health authorities.
• That makes the current wave associated with JN.1 the second biggest of the pandemic after omicron emerged, Topol says, but the U.S. hasn't seen nearly the same surge in hospitalizations.

Only about 19% of U.S. adults have received the latest version of the COVID vaccine, which is thought to offer more protection against JN.1

• Just over 1 in 3 nursing home residents — a particularly vulnerable group — are up to date with COVID vaccinations, while fewer than 1 in 10 people who staff those facilities have received the latest shot, according to CDC data updated Thursday.

The bottom line: A rise in respiratory illness was expected during the holiday season, as post-pandemic travel hit record levels and people gathered indoors.

• Health systems so far aren't being crushed to the same degree they were in previous years, but health officials are keeping a close eye on virus spread.