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Many of the respiratory viruses that normally spike in the United States in the fall and winter are circulating at notably low levels right now — a sunnier side effect of the precautions and policies meant to stem the spread of the coronavirus.

The list of scarcer pathogens includes EV-D68. It’s an enterovirus and one of many bugs that causes colds, but is also believed to be the main driver of a rare, polio-like syndrome that develops in some children called acute flaccid myelitis, or AFM. Children with the condition have muscle weakness, paralysis, and, sometimes, breathing troubles.

Spikes in AFM cases have been documented in 2014, 2016, and 2018, with cases largely occurring between August and November. The original expectation was that 2020 would follow that pattern and see a similar, perhaps even larger, cluster of AFM cases.

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But then came masks, physical distancing, activity restrictions, and, in many communities, still-shuttered schools. The result: There were just 30 confirmed AFM cases in 2020, compared to 238 in 2018 and 153 in 2016, according to data from the Centers for Disease Control and Prevention.

“What we’ve seen really is what we haven’t seen,” said Matthew Vogt, a pediatric infectious diseases expert at the University of North Carolina at Chapel Hill.

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The impact from other respiratory viruses has also been comparatively mild. So far, one child has died from flu in the 2020-2021 season, compared to some 140 or more each in the past three seasons. Low flu levels have also been a relief to health care providers, who have been swamped with Covid-19 patients. A bad flu season on top of that could have been devastating for patient care.

It’s not that the other seasonal respiratory viruses aren’t circulating at all, but trends show levels of some rhinoviruses, enteroviruses, RSV, and others down from past cold and flu periods.

So how is it possible that SARS-CoV-2, the coronavirus that causes Covid-19, has still been running rampant in the U.S. when the measures put in place to control it have kept lids on other respiratory pathogens?

Part of the answer, experts say, is that cases build upon each other. Entering the fall, there was already lots of SARS-2 circulating, while the other viruses had a harder time ramping up with the anti-Covid measures in place.

But it also points to how much better SARS-2 is at spreading, in part because we are much more vulnerable to it.

Because flu and cold-causing viruses circulate every year, some chunk of people gets infected and becomes protected for some amount of time, perhaps a year or two depending on the person and the virus. That means that there is some population-level immunity to all these viruses that can limit their spread. At the individual level, we encounter these viruses again and again over time, generally becoming less likely to get sick from them or be as contagious because our immune systems have some memory against them.

One hypothesis for why AFM spikes occur every two years, for example, is that immunity to EV-D68 lasts longer than a year, meaning the virus can’t circulate all that well during off years when lots of people are protected. In turn, it infects fewer people and leads to fewer cases of AFM. Other viruses that can also cause AFM might follow a similar pattern.

With SARS-2, however, “the virus is just so much more contagious,” Vogt said. Plus, despite the millions of cases it’s caused in the United States, the majority of the country remains unprotected because people haven’t been exposed to the virus before (or have yet to be vaccinated). Essentially, it’s a more infectious pathogen with a wider pool of people to infect.

With Covid-19 vaccines rolling out, it’s more feasible to imagine a time when restrictions get lifted and people start interacting with each other more. That could also open the door to a resurgence of the viruses that have been laid low this season. It’s possible, perhaps, that whatever AFM spike was avoided in 2020 might await this year.

“One of the concerns is that in fall 2021, with kids back in school and enough adults vaccinated that we change our behaviors, could it emerge then?” said epidemiologist Priya Duggal of the Johns Hopkins Bloomberg School of Public Health. “Maybe it just gets pushed.”

More broadly, infectious disease experts will be monitoring what happens with respiratory viruses now that the U.S. population largely missed a round of when it normally reups its protection against the bugs.

With flu in particular, so much of the severity of each season depends on which strains circulate and how well-matched the vaccine is against them. But from the virus’s perspective, it may find more targets next season than in a normal year.

“The number of people who might have gotten infected this year and might be immune to next year’s virus is markedly reduced,” said Walter Orenstein, the associate director of the Emory Vaccine Center. “There will be more susceptibles.”