(NewsNation) — Concerns are growing nationwide for rapidly increasing cases of what health officials have deemed a tripledemic: the flu, RSV and COVID-19, and the Centers for Disease Control and Prevention is once again urging the public to wear face masks indoors.

According to the agency’s December 8 report, 13.7% of Americans now live in communities now rated “high” COVID-19 Community Levels, up from 4.9% of the population last week. An additional 38.1% of Americans are in “medium” areas and 48.2% are in “low” areas. A number of major cities are now mulling a return to masking measures.

In California, more than 10 counties, including Los Angeles, Maricopa, Nassau and San Bernardino, are now in the “high” tier. Los Angeles County health officials are again strongly recommending that everyone wears masks indoors.

Over the past week, Los Angeles hospitals saw an average of 1,245 COVID-positive patients every day — that’s a nearly 20% jump over the previous seven days. “When you put on your mask for these few weeks during this surge, it is about the people of LA County. it is about every individual, every visitor, our health care workers, essential workers and other people who serve. in addition to vaccination, it is one of the easiest things everyone can do right now,” Dr. Barbara Ferrer, Los Angeles County director of public health, wrote in a press release.

In New York, a health advisory notice was sent out alerting hospitals, local health departments, emergency rooms and labs to prepare for rapidly rising cases of respiratory illness. The Bronx, Brooklyn, Queens and Staten Island are in orange, meaning high-risk levels, while Manhattan is in yellow, the medium-risk level, according to the latest CDC data. On December 9, state officials urged schools to return to indoor masking to curb the spread of respiratory illnesses.

As the Christmas and New Year’s holidays approach, when families are expected to gather across the country, health officials fear that could put a significant strain on our health care system if people don’t take the proper precautions. “Our immune system has not been revved up. The vaccine rates are lower. We are a prime sitting target for other respiratory illnesses as we relax our guard down and begin to have contact with other people,” said Dr. Bruce Hirsch, an attending physician in infectious diseases at Northwell Health.

Medical centers across America are reporting higher rates of hospitalizations, and nursing homes are pushing boosters for residents. As for RSV, the ones at greatest risk are children 6 months and younger who haven’t built up strong immune systems yet. An RSV vaccine is reported to become available by this time next year.