As more people stopped following the safety measures meant to slow the spread of Covid-19, other viral and bacterial diseases that seemed to have dissipated over the past two years have returned—and many of them are "behaving in unexpected ways," Helen Branswell writes for STAT News.

Now is the time to move the needle on flu vaccinations

Why are we suddenly more vulnerable to these other diseases?

Over the past two years, many people have strictly adhered to Covid-19 prevention measures intended to stop the spread of the coronavirus. While doing so, other diseases also dissipated.

For instance, the influenza seasons have been some of the mildest on record. However, in recent weeks, flu hospitalizations have increased—an unusual pattern for a virus that typically peaks during the winter months.

Similarly, respiratory syncytial virus (RSV), a virus that typically peaks in the winter, triggered large outbreaks last summer and in the early fall in the United States and Europe.

In addition, there is currently an unprecedented monkeypox outbreak in more than a dozen countries. While monkeypox is typically found in Central and West Africa, cases have recently been reported in several European nations, the United States, Australia, and Canada.

According to Branswell, "[t]hese viruses are not different than they were before, but we are."

Because of Covid-19 restrictions, we do not have as much recently acquired immunity, which makes more of the population vulnerable to disease. With this increased susceptibility, experts have suggested that "we may experience some … wonkiness as we work toward a new post-pandemic equilibrium with the bugs that infect us," Branswell writes.

As a result, larger spikes of disease could hit, and diseases could circulate at odd times or in areas 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."

According to Marion Koopmans, head of the department of viroscience at Erasmus Medical Center in the Netherlands, we may face a period of uncertainty, making it difficult to predict diseases experts previously understood.

What can we expect to see moving forward?

Moving forward, many flu experts are worried that when a serious strain emerges, the number of people who do not have recent immunity could result in a "very bad flu season," Branswell writes.

According to Koopmans, some studies have suggested that after a one- or two-year period of low flu transmission, there could be a significant decrease in the number of people with enough flu antibodies to provide immunity. "So also, potentially, a bigger, more susceptible group in adults," she said.

"We're talking about endemic diseases that had a certain pattern of predictability. And that pattern in part was seasonal but in part was also driven by the size of the immune or non-immune population. And the last bit has, of course, increased," Koopmans added.

Branswell notes that experts will be closely watching children's hospitals this fall to see whether there will be a surge in acute flaccid myelitis (AFM), which is believed to be caused by infection with enterovirus D68.

Kevin Messacar, a pediatric infectious diseases specialist at Children's Hospital Colorado, has studied AFM for the past eight years, since the first of a series of biennial waves of cases emerged in the late summer and early fall of 2014, 2016, and 2018.

However, in 2020 and 2021, there was practically nothing. "Does that mean the fall of 2022 could see a much higher crest of cases, because more children are potentially susceptible to enterovirus D68?" Branswell asks. We must be prepared for that possibility, Messacar noted, while emphasizing he still does not know what to expect.

"Now we have four years of children who haven't seen that virus. We don't know what's going to happen. We don't know when it comes back. But when it does come back, there are more susceptible children out there that would not be expected to have immunity," Messacar said. "That's what we're watching with a variety of different viruses."

In addition, babies born during the pandemic seem to have fewer innate antibodies because their mothers may have been sheltered from RSV and other respiratory pathogens during their pregnancies amid the pandemic.

"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," Koopmans said.

According to Thomas Clark, CDC's deputy director of the division of viral diseases, public health experts are concerned that we could see outbreaks of vaccine-preventable diseases since so many children missed routine childhood vaccinations during the pandemic.

However, he noted that he now understands that isn't the only pandemic-related factor that could influence infectious diseases.

"We're very focused on under-vaccinated children with routine childhood immunizations because it's the set-up for introduction of measles. But then there have also been a lot of kids who haven't gotten the usual kind of viruses they might have been exposed to," Clark said.

We could see differences in the severity of certain illnesses, because young children who were not exposed to diseases during the early stages of the pandemic could now catch them when they are older, Clark said. Notably, some illnesses trigger more serious symptoms when they are contracted as an older child.

"Whether we will see that kind of thing over such a short period of time I think is a big question mark," said Koopmans. "But I think it is certainly something that is worth really watching closely." (Branswell, STAT News, 5/25)