The flu and colds are back with a vengeance – why now?

Influenza resurgence in Northern Hemisphere, highlighting benefits of vaccination.Credit: Robert Anic/PIXSEL/Alamy

Restrictions to prevent the spread of Covid-19 are significant It blunted the spread of other respiratory diseases. Influenza and respiratory syncytial virus (RSV) — a seasonal virus that usually causes mild cold-like symptoms but can be deadly for young children and adults — all but disappeared in 2020 and early 2021. Now, in the Northern Hemisphere, RSV is on the rise, and hospitalization rates for the flu in the United States are higher this time of year than they have been since 2010. Now why exactly are these upheavals happening? What’s in store for future winters?

“These viruses are coming back, and they’re coming back with a vengeance,” said Scott Hensley, an immunologist at the University of Pennsylvania in Philadelphia. “This year is likely to be kind of the granddaddy of them all in terms of flu.”

“The population is immunologically more naïve than we would expect in most years,” says Hensley. Usually, babies become infected by their second birthday. Now, “you’re going to have three- and four-year-olds who have never seen RSV.”

Older children and adults who have previously been infected can have a weakened immune system. In the absence of viral infection, antibody levels decrease. In a typical year, “we may be exposed to a small amount of the virus and your body will fight it off,” says immunologist John Trekoning of Imperial College London. But “that kind of asymptotic increase has not happened in the last few years”.

Immunity loan

But last year the COVID-19 restrictions were lifted. Why is the uprising kicking in only now? Hensley worried that last year’s flu and RSV would return. But the influenza season is generally mild in the Northern Hemisphere. Although RSV infections increased, the peak was lower than in pre-pandemic years and came in the summer of 2021—the odd timing may have helped slow the spread of the virus. Factors like temperature and humidity play a role in spreading the virus, and that peak “No. [at] “It was a period when the environment was favorable for RSV,” says Virginia Pitcher, an epidemiologist at the Yale School of Public Health in New Haven, Connecticut.

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In August 2021, researchers in France Coined the term ‘immunity loan’ To describe this reduction in population-wide immunity. On Twitter, the term has taken on a life of its own. Some take this to mean that lack of exposure to pathogens like RSV and influenza has irreversibly damaged the immune system, which Matthew Miller, an immunologist at McMaster University in Hamilton, Canada, calls “nonsense.”

There are some scientists Posted on social media The increase in RSV hospitalizations may be the result of SARS-CoV-2 infection causing immunodeficiencies, making people more susceptible to other infections. But Miller says he hasn’t seen any evidence of that, and the increase in hospitalizations could be explained by the large number of people who missed exposure in the past few years. “There’s quite a large number of innocent people who are all at risk. So you’ve got a lot of numbers in the system.

It is difficult to predict what the new normal will be for seasonal viruses. If many of those susceptible become infected in the coming months, next year’s flu season may be mitigated because some of the immune debt is ‘paid off’. But it is not yet clear whether Covid-19 will become a seasonal disease like flu and RSV, or whether it will continue as such.

The rhinovirus mystery

There is much that researchers still do not understand about seasonal viruses. For example, Covid-19 restrictions appeared to have little effect on one type of seasonal virus, rhinoviruses — which are the most common cause of colds — for reasons not fully understood. That may be due to their toughness, Miller says. They are less likely to dry out and last longer in the environment.

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Another open question is how these viruses compete and interfere with each other. Infection with one virus can elicit a strong innate immune response that can prevent infection with another virus. Hensley points out that last year’s first wave of influenza subsided soon after the Omicron outbreak began. Perhaps Omicron provided some short-term protection against infectious fever. Or maybe the Omicron uprising made people wear masks and keep their distance.

Pitcher expects next year’s peaks and troughs to be similar to what they were before the pandemic. She doesn’t bet anything. But he says: “I expect this winter will probably be the last abnormal winter.”

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