‘This is really, really remarkable’: Primary Children’s Hospital reports historically low cases of RSV and flu

SALT LAKE CITY – While COVID-19 transmission rates continue to fall, two diseases that normally result in many pediatric hospitalizations have remained almost non-existent through the winter.

Respiratory syncytial virus, or RSV, and influenza have led to virtually no hospitalizations at Primary Children’s Hospital. That’s a welcome surprise for pediatric specialists who typically see 80 to 120 RSV hospitalizations and dozens of intensive care unit stays per week, plus hundreds of influenza hospitalizations annually.

“This is really remarkable,” said Dr. Andrew Pavia, a pediatric infectious disease specialist at Primary Children’s Hospital, during a news conference Monday on pediatric hospitalization trends for both viruses.

The hospital has seen more cases of the rare SARS-CoV-2 complication called multisystem inflammatory syndrome in children, or MIS-C, than flu or RSV. It is estimated that around 75 cases of MIS-C have been treated in the hospital in recent months.

RSV is something that affects “virtually all children” in the first few years after birth. Usually causes coughing and wheezing; some children end up short of breath and need to be hospitalized.

Adults over the age of 75 also have RSV. Pavia said it ends up resulting in many cases of pneumonia in the elderly. Older children and adults often experience symptoms similar to those of a cold.

Primary Children’s Hospital has yet to report a single RSV hospitalization during the typical season.

“We are seeing something that I have never seen in the last 35 years,” said Pavia. “If you go back in history, it really hasn’t happened except briefly after the 2009 influenza pandemic.”

Then there is the flu, which often affects hundreds of thousands of Americans each year. So far, the Centers for Disease Control and Prevention has only reported slightly less than 1,600 total confirmed cases of influenza in the US as a result of nearly 1 million tests.

The CDC reports that all 50 states and Puerto Rico have “minimal” trends in flu cases. In fact, fewer than two dozen new cases were reported in the past week.

These are similar trends almost unknown in Utah.

The Utah Department of Health Weekly Flu Dashboard shows there have only been 13 total hospitalizations due to flu as of February 13. There were a total of 1,310 influenza hospitalizations last year.

Pavia said there has been one pediatric hospitalization compared to “several hundred” that would normally occur at this point in the flu season.

There have been so few cases lately that there was insufficient data to publish the most recent flu-positive rate. The figures on a graph compared to previous years are staggering.

These graphs show flu transmission and hospitalization rates for the past six years.  The current rates for 2020-2021 are in the red and well below the figures for the previous five years.
These graphs show flu transmission and hospitalization rates for the past six years. The current rates for 2020-2021 are in the red and well below the figures for the previous five years. (Photo: Utah Department of Health)

It is unknown why both viruses essentially disappeared this winter. One theory for the flu is that there were very few cases during the southern hemisphere winter and travel restrictions were in place, so the flu couldn’t really be transmitted 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 puzzling,” Pavia added.

“RSV does not go away completely every summer. There are cases in warmer climates throughout the year,” 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 show up this winter, but it’s not happening.”

Masking, keeping children at home and keeping babies away from fewer possible exposures to RSV are theories that, according to Pavía, could explain its decline. Still, experts have no idea why it is “practically zero” not just in Utah but across the country.

Why RSV and the flu might “roar” back

However, the good news now comes with a caveat. Australian physicians reported similar trends for both influenza and RSV during the southern hemisphere winter. Then RSV numbers saw a sharp increase just before the southern hemisphere summer began.

The Australian Broadcasting Corporation reported last December that RSV cases at all ages went from almost zero during the winter months of New South Wales. In November, there were more than 1,600 cases in the month alone.

“It’s very likely that when both the flu and RSV are gone for a while, more people are completely susceptible to it,” Pavia said. “So when it comes, it spreads more dramatically and we see a more serious disease.”

The reason the flu returns more violently is that the flu adapts and it is difficult to know which strain of virus will arrive. The decline of RSV, on the other hand, means that if it arrives later this year or next, there would be an even larger group of babies who would have to fight it for the first time because they didn’t experience it now.

“The RSV will do something really strange when he returns,” added Pavia. “We really can’t predict it very well. Our hunch is that it will come roaring back and that we will have a bad RSV year when it returns.”

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