The usual broken bones and other trauma are being treated this winter at the emergency department of St. Christopher's Hospital for Children. But one category of medical complaints has all but disappeared: infectious disease.

Take influenza, for example. Typically between December and April, nurses and doctors at the Philadelphia hospital see hundreds of children with the flu. So far this winter, the number is zero.

Flu numbers for Pennsylvania, New Jersey and the United States as a whole also are well below normal across all ages, though far from zero. Likewise, the country has seen few cases of acute flaccid myelitis, a form of childhood paralysis that is thought to be caused by viral infection. The rate of that illness typically spikes in the fall of even-number years, with 238 confirmed cases in 2018, according to the Centers for Disease Control and Prevention. Yet in 2020, there were just 29.

Call it the upside of life in a pandemic. While several factors could be at work in these downward disease trends, a key driver is almost certainly the various precautions that remain in place for COVID-19, physicians say.

"Wearing masks, social distancing, staying home, children not being in school together," said Emily Souder, an infectious diseases specialist at St. Chris. "All that happening at the same time has definitely impacted lots of respiratory viruses."

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In Pennsylvania, 2,195 confirmed cases of flu were reported to the state Department of Health from late September through Jan. 16. In many years, the state sees three times that many flu cases in just one week.

When flu rates are down in any given year, a variety of factors can play a role.

The predominant strains may be similar to strains that have circulated in the recent past, meaning more people have at least partial immunity. Or the vaccine may be an especially good match for the strains in question, and perhaps more people than usual choose to get it.

That may be happening this winter. Concerned that emergency rooms would be swamped with a double whammy of flu and COVID cases, public health officials have made an especially strong effort to encourage the flu vaccine. As of Jan. 1, 192.5 million flu vaccine doses already had been distributed in the United States, compared with 174.5 million doses for the entire 2019-20 season, the CDC says.

And two late-December surveys found that 53% to 54% of adults had gotten the flu vaccine, compared with 42% in December 2019, though coverage was lower in some minority groups, the agency said.

But could a flu surge still be on the way this winter? The disease is generally in full swing by this time of year, but not always. The low level of flu so far this season is similar to what was seen in the winter of 2015-16, when cases started to rise in late February.

Still, evidence so far suggests the COVID precautions are helping to keep the flu in check, said Ray Barishansky, deputy secretary of health preparedness and community protection at the Pennsylvania Department of Health.

"I do think that the washing of hands, the avoidance of gatherings, wearing masks, these are things that definitely reduce the spread of flu," he said.

Another possible explanation for low reported flu numbers this winter could be that people are reluctant to seek medical care for fear of contracting COVID. In other words, some people with flu symptoms may simply be staying home, and will never be counted. That's true in any year, as most people with flu symptoms are not tested, even if they do go to the doctor's office.

But Souder, the St. Christopher's physician, doubts that phenomenon is much greater this winter than in others. Because symptoms of the flu overlap somewhat with those of COVID (such as fever), anyone with those symptoms is likely to seek medical care — or at least get tested.

And many hospitals are testing nasal swabs for both illnesses. If patients turn out to have one or the other, they will be counted in official statistics.

Yet another clue suggests that the apparent decline of flu is real: the decline in other infectious diseases such as acute flaccid myelitis, said Ignacio Valencia, chief of the neurology section at St. Christopher's.

The rare disease is thought to be caused by a type of virus called an enterovirus, distantly related to the one that causes polio. It typically strikes children of preschool age, causing sudden weakness or paralysis in the arms and sometimes legs. Case numbers spiked in the late summer and fall of 2014, 2016 and 2018, due to the biology of how the virus spreads through the population. Yet in 2020, numbers were down.

Epidemiologists who track the disease have warned that it could return in 2021, which would otherwise be an off-year — assuming preschools and day-care centers resume normal operations. But in the meantime, the low numbers of the condition are cause for relief, Valencia said.

And social distancing may also be contributing to lower numbers of garden-variety viral afflictions, he said.

"I haven't had a cold in one year, and usually I get one cold per year," he said. "I think it's all these measures combined that have led to the decreases in other viruses."

The open question is whether hand-washing and other disease-avoidance measures will become more common in future years, once COVID is behind us.

"If we could at least do the hand-washing piece, that would be good," Valencia said. "But you cannot keep people separated. We are a social entity."

And masks? Even before COVID, clinicians at St. Christopher's wore masks when treating patients with respiratory illness, and that will continue in future years, said Souder, an assistant professor of pediatrics at Philadelphia's Drexel University College of Medicine. Whether mask-wearing becomes more common among the general public remains to be seen.

But this year, at least, it seems to be helping, she and Barishansky said. And with flu season still underway for months, and COVID still widespread, they urge everyone to keep it up.

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