The Massachusetts health care world was bracing for a winter COVID surge, expecting the disease to strain hospitals and roil holiday plans, as it has done the past two years. But so far this season, COVID hasn’t been the main villain. Instead, it’s been eclipsed by two more familiar foes — RSV and the flu — which both arrived early and, in some cases, hit harder than usual.
Now scientists are trying to understand why. Their theories range from an immunity gap brought on by pandemic precautions, to the way the three respiratory viruses interact, to how COVID infections have impacted people’s immune systems. The answers will determine whether the unusual virus behavior is a blip or the new normal, and may inform ongoing debates about the benefits of mask-wearing.
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Respiratory syncytial virus — or RSV — was the first virus to hit much earlier than normal this season. It flooded pediatric hospitals in October and November instead of during its usual peak in late December to mid-February, creating a dire shortage of pediatric beds.
Normally, the virus poses a particular risk to infants because it can inflame and clog their lungs’ tiny airways.
But this year, RSV hospitalized a much broader group of children, including older kids with no underlying health problems.
“I haven’t experienced the breadth of ages of patients with severe disease that I have been seeing in the last few weeks,” said Dr. Chadi El Saleeby, a pediatric infectious disease specialist at Massachusetts General Hospital.
Many experts have suggested that RSV was worse this year because of an “immunity debt” or “immunity gap” from several years of social distancing and COVID precautions. Dr. Paul Offit, director of the vaccine education center and professor of pediatrics at Children’s Hospital of Philadelphia, said much of the country was still masking, social distancing, and being cautious last winter.
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“That’s not true this year. People are back,” Offit said. “COVID is no longer changing how we live, work, or play.”
With fewer cases of flu and RSV over the last few years, people haven’t received the “booster” of immunity that a typical annual virus would create.
“It may be the same reason flu is definitely worse this year,” he said.
Other scientists are skeptical that mitigation measures played any role in current trends. Michael Osterholm, director of the Minnesota Center of Excellence for Influenza Research and Surveillance, said at a conference in Boston in November that the emergence of H1N1 in 2009 similarly disrupted normal patterns of RSV and flu the following winter. But at the time people didn’t take mitigation measures.
“There was something going on with that virus impacting other respiratory pathogens,” Osterholm said.
Scientists also point out that RSV isn’t just more widespread this year but also appears more severe.
In the past, hospitals would provide hydration, suction, and some oxygen to patients diagnosed with RSV, but this year they had to provide more extensive respiratory support to a larger number of patients, said El Saleeby.
Low population immunity may be part of the answer, but El Saleeby said infection with multiple viruses at one time may also play a role. At MGH, several children tested positive not only for RSV, but also for enterovirus and influenza — which also arrived earlier in the season than normal.
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“These respiratory difficulties can be made worse by having multiple viruses at the same time,” said El Saleeby.
Ryan Gregory, a Canadian evolutionary biologist and professor at the University of Guelph in Ontario, agreed there are likely multiple factors at play. He pointed to a recent small study published in the Lancet Respiratory Medicine that found that contracting COVID during pregnancy could change the lung development of the fetus, resulting in children born with lower lung volume.
“That could account for why so many newborns are getting so sick,” Gregory said.
Though scientists have wondered if the recent surge of RSV was caused by a new strain of the virus, early analysis shows that doesn’t appear to be the case. Looking at the viral genomes of patients coming to MGH and some sites, researchers discovered there were several versions of the existing virus circulating, rather than one predominant new strain, according to a preliminary analysis posted online last Friday.
However, more strains of RSV-A appear to be circulating this year, which is normally more severe than RSV-B, and that probably is contributing to the increased severity of the recent surge, said Dr. Jacob Lemieux, an infectious disease doctor at MGH who co-led the research.
Other scientists suggest that COVID infection may be changing how the human body responds to infections. Sometimes referred to as “immunity theft,” some have hypothesized that COVID could impair people’s immune systems either temporarily or longer term.
One controversial scientific theory, suggested by Anthony Leonardi, an immunologist and graduate student at the Johns Hopkins Bloomberg School of Public Health, posits that COVID infection can wear down a key part of the immune system known as T cells, which contain much of the “memory” a person’s body has of previous infections. Multiple COVID infections could overactivate T cells, either prematurely aging them, engaging them so significantly they accidentally cause organ damage, or taxing them for such a long period of time that they may become less effective against other viruses.
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There are few clear answers, but the University of Guelph’s Gregory said it is an urgent question that the scientific community needs to get right.
“If it’s just an immunity [gap], if it’s just a numbers game and you ride it out, this will be a bad year and we’re caught up, and next year should be fine,” he said. “The consequences of it not being that, if it’s immunity theft, are really serious. This year might not be the only bad year.”
Jessica Bartlett can be reached at jessica.bartlett@globe.com. Follow her @ByJessBartlett.