Restrictions to limit the spread of COVID-19 significantly mitigate the spread of other respiratory diseases. 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 hospitalization rate for influenza in the United States is higher for this time of year than it has been since 2010. Why exactly are these increases happening now? And what does it have in store for the coming winters? “These viruses come back and come back with a vengeance,” says Scott Hensley, an immunologist at the University of Pennsylvania in Philadelphia. “This year is likely to be the granddaddy of them all when it comes to flu.” Hensley 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.”
Immunity debt
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 the 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. On Twitter, the term has taken on a life of its own. Some people take this to mean that a lack of exposure to pathogens like 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 that the increase in hospitalizations could be explained by the large number of people who have lost exposure in recent years. “There is a slightly larger naive population, all of whom are at risk. So you have more numbers coming into the system.” 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 may 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.
Mystery of 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 for a longer time 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 began. 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 valleys may look a lot 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.”
