(CNN) There is growing concern among infectious disease and public health experts that the US could face even more respiratory infections in January.
It is "highly likely" that respiratory viruses could spread even more following holiday gatherings and New Year's Eve celebrations, Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center and medical director of the National Foundation for Infectious Diseases, told CNN on Monday.
"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."
At the same time, across the country, there has been a wave of flight cancellations and families stuck at the airport during their holiday travels.
When that happens, "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," Schaffner said, adding that his own granddaughter had four flights canceled over the holidays. He recommends masking up while in the airport and on an airplane.
"I think all of us in infectious diseases and public health would recommend that masks are not perfect, but they are an additional layer of protection," Schaffner said.
Some local health officials are bracing for a possible surge in respiratory illnesses following the winter holidays since that was seen recently following Thanksgiving, Lori Tremmel Freeman, chief executive officer of the National Association of County and City Health Officials, said in an email to CNN on Monday.
"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," Freeman wrote. "Deaths from COVID also rose during that same time period by about 65%."
Flu also surged after Thanksgiving, with more than a third of all flu hospitalizations and deaths at the time this season being reported in the first full week of data post-Thanksgiving, and cases also jumped nearly as much.
Currently, seasonal flu activity remains high in the US, but continues to decline in most parts of the country, according to data published Friday by the US Centers for Disease Control and Prevention. Despite improvements, flu may not have peaked yet.
CDC estimates that, so far this season, there have been at least 18 million illnesses, 190,000 hospitalizations and 12,000 deaths from flu.
As for the current state of Covid-19, increases appear to be relatively mild. Hospitalizations are ticking up in most states, although the overall rate is still just a fraction of what it was during other surges. New hospital admissions have jumped nearly 50% over the past month. Hospitalizations among seniors are nearing the peak from the Delta surge -- and rising fast.
Freeman said it is expected that reports after the winter holidays will continue to show increases in Covid-19 cases and deaths, likely attributable to increased travel across the country, large family gatherings, fewer people being up to date on their Covid-19 vaccinations and flu shots, and fewer people following mitigation measures, such as masking and social distancing.
"Air travel is also back to pre-pandemic levels and there are no more restrictions on mask wearing on airplanes or in airports where viruses can easily circulate. Same for bussing," Freeman said. "Fortunately, we are seeing less RSV in children from our high points earlier in December, so that respiratory illness is stabilizing and becoming less of a part of the triple threat of COVID, flu, and RSV."
As health officials brace for a possible surge in respiratory viruses in the coming weeks, it might not be just flu, Covid-19 and RSV that sickens people, said Dr. Georges Benjamin, executive director of the American Public Health Association.
"We're focusing on those three, but there are others out there -- the common cold and others," Benjamin said.
Overall, "we should expect more respiratory diseases," he said. "The best way to reduce your risk is of course to get fully vaccinated for those that which we have a vaccine, so influenza and Covid, with the new bivalent version, are the two most important right now."
Benjamin added that it also remains important to wash your hands often, wear a mask during holiday travels and stay home when sick.