The flu and colds are back and they’re taking their revenge — Why now?

One of the findings made by the scientific community during the pandemic was that the restrictions for Covid-19 almost eliminate the spread of both its virus fluas well as the simple cold. Indeed, all of us have found that with simple means, such as distance and a mask, we managed to stay away from… inconveniences.

Recently, however, there has been a significant increase in flu and colds around the world. Why now; Is it linked to the lifting of measures for the coronavirus, or is something else, more worrying, going on? And what does it have in store for future winters?

Influenza and respiratory syncytial virus (RSV) — a seasonal virus that usually causes mild cold symptoms but can be dangerous for young children and older adults — have all but disappeared in 2020 and early 2021. Now, in the Northern Hemisphere, the RSV is on the rise and the flu hospitalization rate in the United States is higher for this time of year than it has been since 2010.

“These viruses come back and come back to … get their revenge,” says Scott Hensley, an immunologist at University of Pennsylvania in Philadelphia. “This year is likely to be the worst ever for the flu.”

THE Hensley he says this is because the population “is more immunologically naïve than we would expect in most years.” Normally, children are infected by their second birthday. Now, “you’re going to end up with three, four-year-olds right now who have never seen RSV.”

For older children and adults who have been infected in the past, the problem is a decrease in immunity. In the absence of exposure to a virus, antibody levels decline. In a typical year, “we might be exposed to a tiny bit of the virus and your body will fight it off,” says John Tregoning, an immunologist at Imperial College London. But “this kind of asymptomatic amplification may not have happened in recent years.”

One year after the restrictions were lifted

But the COVID-19 restrictions began to be lifted last year. So why is the wave only starting now? Hensley was concerned that the flu and RSV would rebound last year. But the flu season overall was mild in the Northern Hemisphere. And although RSV infections did increase, the peak was lower than in pre-pandemic years and came in the summer of 2021 — an odd timing that could have helped moderate the spread of the virus. Factors such as temperature and humidity play a role in transmission of the virus, and this peak “was not at a time when the environment was favorable for RSV,” says Virginia Pitzer, an epidemiologist at the Yale School of Public Health in New Haven. Connecticut.

In August 2021, researchers in France coined the term “immunity debt” to describe this decline in immunity at the population level. Some people take it to mean that a lack of exposure to pathogens such as RSV and influenza has permanently damaged the immune system, an idea that Matthew Miller, an immunologist at McMaster University in Hamilton, Canada, calls “nonsense.”

Some scientists have also argued on social media that the rise in RSV hospitalizations may be the result of SARS-CoV-2 infection causing immune deficiencies that make 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 have missed exposure in recent years.

It is difficult to predict what the new normal for seasonal viruses might look like. If many of the susceptible people become infected in the coming months, next year’s flu season could be milder as some of the immune debt is “paid off.” However, it is not yet clear whether COVID-19 will become a seasonal disease, like influenza and RSV, or whether it will continue as it has been, with sporadic peaks throughout the year.

The mystery of the rhinovirus

There’s also a lot researchers still don’t understand about seasonal viruses. For example, the COVID-19 restrictions appeared to have little effect on one type of seasonal virus, rhinoviruses — which are the most common cause of the common cold — for reasons that are not fully understood. This may be due to their resilience, Miller says. They are less prone to desiccation and can remain longer in the environment.

Another open question is how these viruses compete and interfere with each other. Infection with one virus can trigger a strong innate immune response that can prevent infection with another virus. Hensley points out that last year’s first wave of flu subsided soon after the Omicron outbreak. Perhaps the Omicron infection provided some short-lived protection against the flu. Or maybe the Omicron wave just convinced people to cover up and keep their distance.

Pitzer expects that next year’s peaks and troughs may look much more like those that occurred before the pandemic. He doesn’t make any bets. But he says, “I expect this winter will probably be the last unusual winter.”

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