Something is changing the seasonal strains of common cold and flu viruses. They slowed to a trickle in the early stages of the Covid pandemic only to burst through the human population this year. Some public health experts have called it a “triple pandemic,” but it could even be described as a quadruplet.
In the Northern Hemisphere, flu begins to increase in October, months before the normal season. This year also saw a sharp and early rise in two other viruses, RSV (respiratory syncytial virus) and adenovirus. These often cause colds, but RSV can be dangerous for young children and has recently led to overcrowding at children’s hospitals. Adenovirus is also usually mild, but this month there were reports of the virus sending college athletes to the ICU and it was linked to groups of dangerous childhood hepatitis.
Why now? The easy answer is that wearing a mask for more than two years has reduced the incidence of these viruses, and then people lose their immunity – what the press famously called an “immunity debt.” “. But that’s not necessarily the whole story. Michael Osterholm, director of the Center for Infectious Diseases Research and Policy and host of the Covid podcast, said the circulation patterns of different viruses appear to be influencing each other in incomprehensible way.
“When multiple respiratory viruses are circulating in a given season, one of them will prevail for reasons we don’t understand,” he said on the podcast. “There is something going on there that cannot be attributed to personal protection alone [or] away.”
A few other researchers made a similar observation this week in Science. Virologist Richard Webby told the journal: “The flu and other respiratory viruses and SARS-CoV-2 don’t get along very well. Or, as epidemiologist Ben Cowling of the University of Hong Kong put it: “One virus tends to bully others.”
That means viral interference may be a more important factor than immune debt.
Osterholm points out that similar containment for other viruses occurred in 2009 during the H1N1 flu outbreak. “For the first time in years, we don’t see other influenza strains like H3N2 and or any RSV activity. It just disappeared,” he said. “It can’t be due to mitigation, because we didn’t mitigate.”
He said he also doubts mitigation at all works for RSV and flu because it’s not very effective against Covid – more than 75% of children have been infected as of February 2022. Osterholm pointed to previous research showing that although N95 respirators still work, there is less evidence behind the widespread fabric masks prevalent in schools (although school closures may have disrupt the spread of other seasonal viruses).
Viral interference may offer a more complete explanation. Our innate immune system includes disease-fighting substances called interferons, which can protect people infected with one virus from infection with another. That may be why, as a large study from the University of Glasgow published in 2019 showed, cold viruses sink as flu viruses increase.
Others, such as infectious disease expert Jeremy Luban of Harvard Medical School, say human behavior is still likely to play a role in changing our virus strain. The lockdown, although short-lived in many places, may have been enough to change the seasonal pattern. And many schools in the United States have been closed for much longer than businesses. If we avoid a flu season or two, RSV and adenovirus, populations may have less immunity to them later on.
He says that could help explain mysterious clusters of hepatitis that emerged in 35 countries around the world starting last spring – in some cases, leading to liver transplants. The latest thinking, presented at a large meeting earlier this month, points to the co-infection of the two viruses interacting with each other.
One is an adenovirus and the other is an adeno-associated virus. This virus, called AAV2, needs adenovirus to replicate. In several recent studies, nearly all children with hepatitis were AAV2-positive, but none of the children in the control group were AAV2-positive. Similar infections may have occurred before the Covid era, but only became noticeable when the adenovirus increased abnormally after Covid mitigation measures were lifted.
Biologist Andrew Read, who studies the evolution of pathogens at Penn State University, said he would not rule out the possibility that the shutdown of mitigation measures was a factor in the rise of the virus. withdrawal that we are seeing today. He said it is possible that these other viruses are inherently less transmissible than SARS-CoV-2, so their spread has been contained for several years by the same measures that have failed to contain them. block Covid. But it’s not known whether a year or two of reduced transmission will have a significant impact on people’s immunity. “We really don’t have good data on that,” he said.
He said he’s particularly concerned about cases of adenovirus that are believed to have sent several University of Michigan hockey players to the hospital and at least one to the ICU. “The idea that there’s a problem with young people with adenoviruses – that’s really striking,” he said. It could be a stroke of luck – the tip of the big iceberg in terms of mild cases of adenovirus. But it can also be something more unsettling. “We are in new territory,” he said.
That raises the question of whether all these flu, RSV and adenovirus cases that flood the population with interferon could stave off the next wave of Covid. Something strange is going on — a new wave of omicrons called BQ.1.1 has begun, but as Dr. Eric Topol writes in his newsletter, this is the first new variant to become dominant without causing a new wave of cases or hospitalizations.
I am cautiously optimistic that we will not see the pandemic quadruple this winter, if only because Covid can be overwhelmed by other viruses. But as we know just a year ago, when the omicron strikes, Covid can always hit us with something from the left field.
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