Flu and colds are back with a vengeance — why now?
Restrictions to significantly suppress the spread of COVID-19 dampened 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 dangerous for young children and older adults — have all but disappeared in 2020 and early 2021. Now, in the Northern Hemisphere, RSV is on the rise, and the rate of flu hospitalizations in the United States is higher for this time of year than it has been since 2010. Why exactly are these waves happening now? And what is in store for future winters?
“These viruses are coming back, and with a vengeance,” says Scott Hensley, an immunologist at the University of Pennsylvania in Philadelphia. “It’s possible that this year will be kind of the granddaddy of them all in terms of the flu.”
Hensley says this is because the population is “more immunologically naïve than we would expect in most years.” As a rule, children are infected by their second birthday. Now, “you’re going to end up having kids who are three, four years old right now who have never seen RSV.”
In older children and adults who have already been infected, the problem is a weakening of immunity. In the absence of exposure to the virus, antibody levels decline. In a typical year, “we can be exposed to a small virus and your body fights it off,” says John Tregoning, an immunologist at Imperial College London. But “that kind of asymptomatic amplification may not have happened in the last few years.”
But restrictions related to COVID-19 began to be lifted last year. So why did the wave come in now? Hensley was concerned that the flu and RSV would rebound last year. But the flu season has generally been mild in the northern hemisphere. And although RSV infections increased, the peak was lower than in the years before the pandemic and came in the summer of 2021 — an odd time that may have helped slow the spread of the virus. Factors such as temperature and humidity play a role in virus transmission, and that peak “wasn’t [at] a time that was environmentally 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 decrease in immunity at the population level. On Twitter, the term took on a life of its own. Some people have taken this to mean that lack of exposure to pathogens like RSV and influenza has irreversibly damaged the immune system, an idea that Matthew Miller, an immunologist at McMaster University in Hamilton, Canada, calls “nonsense.”
Some scientists too published on social networks that the rise in RSV-related hospitalizations could 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 either, and that the increase in hospitalizations could be explained by the large number of people who have missed exposures in recent years. “There is a slightly larger naive population, everyone is at risk. So you have more numbers going into the system.”
It is difficult to predict what the new normal might look like for seasonal viruses. If many susceptible people become infected in the coming months, next year’s flu season could be calmer, as some of the immune debt has been ‘paid off’. But it is not yet clear whether COVID-19 will become a seasonal disease like the flu and RSV, or whether it will continue as it has been, with sporadic peaks throughout the year.
The rhinovirus mystery
There is also much that researchers still do not understand about seasonal viruses. For example, restrictions related to COVID-19 appear to have little effect on one type of seasonal virus, rhinoviruses — which are the most common causes of the common cold — for reasons that are not entirely clear. This could be because of their resilience, says Miller. They are less prone to drying out and can last 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 could prevent infection with another virus. Hensley points out that last year’s first flu wave subsided shortly after the Omicron wave began. Perhaps the Omicron infection provided some short-term protection against the flu. Or maybe the Omicron wave simply convinced people to mask themselves and keep their distance.
Pitzer expects that next year’s peaks and troughs could be much more similar to those that occurred before the pandemic. She doesn’t bet. But she says, “I expect this winter will probably be the last unusual winter.”