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SALT LAKE CITY — While COVID-19 transmission rates continue to fall, two diseases that normally result in many pediatric hospitalizations have continued to be almost nonexistent throughout the winter.
Respiratory syncytial virus, or RSV, and influenza have resulted in virtually no hospitalizations at Primary Children's Hospital. That's a welcomed surprise for pediatric specialists who typically deal with 80 to 120 RSV hospitalizations and dozens of intensive care unit stays per week on top of hundreds of yearly influenza hospitalizations.
"This is really, truly remarkable," said Dr. Andrew Pavia, a pediatric infectious disease specialist with Primary Children's Hospital, during a press briefing Monday regarding the pediatric hospitalization trends for both viruses.
The hospital has seen more cases of the rare complication from SARS-CoV-2 called multisystem inflammatory syndrome in children, or MIS-C, than either flu or RSV. It's estimated that about 75 MIS-C cases have been treated at the hospital over the past few months.
RSV is something that affects "virtually every child" in their first couple of years after birth. It typically results in coughing and wheezing; some children end up with shortness of breath and they end up needing to be hospitalized.
Adults over 75 also suffer from RSV. Pavia said it ends up resulting in many elderly pneumonia cases. Older children and adults typically experience cold-like symptoms from it.
Primary Children's Hospital still hasn't reported a single RSV hospitalization over the typical season.
"We're seeing something that I've never seen in the last 35 years," Pavia said. "If you go back to history, it hasn't really occurred except briefly after the 2009 influenza pandemic."
Then there's influenza, which often affects hundreds of thousands of Americans every year. So far, the Centers for Disease Control and Prevention has only reported a little less than 1,600 total confirmed flu cases in the U.S. as a result over nearly 1 million tests.
The CDC reports that all 50 states and Puerto Rico have "minimal" flu case trends. In fact, fewer than two dozen new cases were reported in the most recent week.
These are similar nearly unheard of trends in Utah.
The Utah Department of Health's weekly influenza dashboard shows there have only been 13 total hospitalizations due to influenza through Feb. 13. There were 1,310 total influenza hospitalizations last year.
Pavia said there has been one pediatric hospitalization as compared to "several hundred" that normally would happen at this point in the flu season.
There have been so few instances lately that there wasn't sufficient data to post the most recent positivity percentage for the flu. The numbers on a graph compared to the previous years is staggering.
It's unknown why both viruses essentially vanished this winter. One theory for the flu is that there were so few cases during the Southern Hemisphere winter and travel restrictions were in place so the flu couldn't really carry over from that region of the world, Pavia explained. The other is that masks and other guidelines to prevent the spread of COVID-19 work to stop other respiratory illnesses.
RSV, on the other hand, is "somewhat more perplexing," Pavia added.
"RSV doesn't go away completely every summer. There are cases in the warmer climates year-round," he said. "Places like New Orleans and Miami have some RSV year-round, so you'd think that would be our reservoir that would seed it and make RSV pop up this winter but it's not happening."
Masking, keeping children at home and having infants away from fewer possible exposures to RSV are theories that Pavia said could explain its decline. Still, experts have no clue why it's "practically zero" not just in Utah but across the country.
Why RSV and flu could come "roaring back"
The good news now comes with a caveat, however. Australian doctors reported similar trends for both influenza and RSV during the Southern Hemisphere winter. Then RSV numbers took a sharp increase right before the Southern Hemisphere summer began.
Australian Broadcasting Corporation reported last December that RSV cases in all ages went from nearly zero throughout New South Wales's winter months. By November, there were over 1,600 cases in the month alone.
"It's very likely when both the flu and RSV have been away for a while, you have more people fully susceptible to it," Pavia said. "So when it arrives, it spreads more dramatically and we see more severe disease."
The reason that flu returns more fiercely is that influenza adapts and it's difficult to know which strain of the virus will arrive. RSV's decline, on the other hand, means that if it does arrive later this year or next year, there would be an even larger group of infants who would have to battle it off for the first time because they didn't experience it now.
"RSV is going to do something really strange when it comes back," Pavia added. "We really can't predict it very well. Our gut feeling is that it will come roaring back and that we'll have a bad RSV year when it does return."