Viruses that were on hiatus during Covid are back — and behaving in unexpected ways

These viruses are not different than they were before, but we are. For one thing, because of Covid restrictions, we have far less recently acquired immunity; as a group, more of us are vulnerable right now. And that increase in susceptibility, experts suggest, means we may experience some … wonkiness as we work toward a new post-pandemic equilibrium with the bugs that infect us.

Larger waves of illness could hit, which in some cases may bring to light problems we didn’t know these bugs triggered. Diseases could circulate at times or in places when they normally would not.

“I think we can expect some presentations to be out of the ordinary,” said Petter Brodin, a professor of pediatric immunology at Imperial College London. “Not necessarily really severe. I mean it’s not a doomsday projection. But I do think slightly out of the normal.”

Marion Koopmans, head of the department of viroscience at Erasmus Medical Center in Rotterdam, the Netherlands, said she believes we may be facing a period when it will difficult to know what to expect from the diseases that we thought we understood.

“I do think that’s possible,” Koopmans said.

This phenomenon, the disruption of normal patterns of infections, may be particularly pronounced for diseases where children play an important role in the dissemination of the bugs, she suggested.

Little kids are normally germ magnets and germ amplifiers. But their lives were profoundly altered during the pandemic. Most went for stretches of time without attending day care, or in-person school. Many had far less exposure to people outside their households, and when they did encounter others, those people may have been wearing masks.

And babies born during the pandemic may have entered the world with few antibodies passed on by their mothers in the womb, because those mothers may have been sheltered from RSV and other respiratory pathogens during their pregnancies, said Hubert Niesters, a professor of clinical virology and molecular diagnostics at the University Medical Center, in Groningen, the Netherlands.

Koopmans said a study her team did looking for antibodies in the blood of young children showed the impact of what she calls an “infection honeymoon.”

“You really see that children in the second year of the pandemic have far less antibodies to a set of common respiratory viruses. They just got less exposed,” she said.

Such factors may help explain the recent rash of unusual hepatitis cases in young children. Scientists investigating the cases think they may be caused, at least in part, by adenovirus type 41, because it has been found in a significant number of the affected children. The possibility is puzzling, because the virus hasn’t been seen to cause this type of illness in the past.

But some scientists theorize that this virus may have always been responsible for a portion of the small number of unexplained pediatric hepatitis cases that happen every year. Maybe, the thinking goes, there have been a lot more adenovirus type 41 infections over the past eight months because of increased susceptibility among children. That, in turn, could be making visible something that wasn’t spotted before.

“I think sometimes to connect the dots of rare complications of common illnesses you just need enough cases out there to start to put the pieces together,” said Kevin Messacar, a pediatric infectious diseases specialist at Children’s Hospital Colorado. “And there is some suspicion that that could be going on with the hepatitis cases.”

The pandemic-induced disruption of normal mixing patterns means that even adults haven’t been generating the levels of antibodies that would normally be acquired through the regular exposure we have to bugs, creating ever larger pools of susceptible people.