Health experts are raising concerns about a "highly likely" surge in respiratory viruses, including flu and COVID-19, among the public after the holiday season.

Dr. William Schaffner, the director of the National Foundation for Infectious Diseases and a professor at Vanderbilt University Medical Center, told CNN that respiratory viruses are "highly likely" to spread after large gatherings.

"These are highly contagious viruses — and people have generally put COVID-19 and COVID vaccination behind them. They haven't been all that attentive to flu. They're not wearing masks," Schaffner said. "And if you're close together with other people, it's an opportunity for all three of these viruses — flu, COVID, and even RSV — to spread from one person to another. So, we do expect a post-holiday surge in these viruses."

He also noted the large number of flights that take place this time of year, saying that while in airports, "People are together for very long periods of time, and they're not wearing masks, and they're weary and tired and stressed, and those are occasions where people are more apt to spread the virus."

Lori Tremmel Freeman, who heads the National Association of County and City Health Officials, added in an email that "After the Thanksgiving holiday period, we saw an uptick in COVID cases by about 58% through the beginning of the Christmas holiday, on December 21. Deaths from COVID also rose during that same time period by about 65%."

"I really recommend masking indoors in public places, grocery stores, movie theaters — any place indoors," Dr. Sharon Welbel, director of Hospital Epidemiology and Infection Control for Cook County Health, told NBC Chicago. "I'd really still consider masking, even if one isn't immunocompromised or at risk for having severe disease, because one can always spread it to others. So it would be to protect other people as well as to protect yourself."

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