Restrictions to curb the spread of COVID-19 markedly blunted the spread of other respiratory illnesses. Influenza and respiratory syncytial virus (RSV) — a seasonal virus that usually causes mild cold-like symptoms, but that can be dangerous for young children and older adults — all but disappeared in 2020 and early 2021. Now, in the Northern Hemisphere, RSV is surging, and the hospitalization rate for flu in the United States is higher for this time of year than it has been since 2010. Why exactly are these surges happening now? And what’s in store for future winters?
“These viruses are coming back, and they’re coming back with a vengeance,” says Scott Hensley, an immunologist at the University of Pennsylvania in Philadelphia. “It is possible that this year will be sort of the granddaddy of them all in terms of flu.”
Hensley says that this is because the population “is more immunologically naive than what we would expect in most years”. Normally, children get infected by their second birthday. Now, “you’re going to end up having kids that are three, four years of age right now who have never seen RSV”.
For older children and adults who have been previously infected, the problem is waning immunity. In the absence of exposure to a virus, antibody levels decline. In a typical year, “we might get exposed to a small bit of virus and your body fights it off”, says John Tregoning, an immunologist at Imperial College London. But “that kind of asymptomatic boosting maybe hasn’t happened in the last few years”.
But COVID-19 restrictions started being lifted last year. So why is the surge kicking in only now? Hensley was concerned that flu and RSV would rebound last year. But the influenza season overall was mild in the Northern Hemisphere. And although RSV infections did rise, the peak was lower than in pre-pandemic years and came in the summer of 2021 — odd timing that might have helped to dampen the spread of the virus. Factors such as temperature and humidity play a part in transmitting the virus, and that peak “was not [at] a time that was environmentally favorable to 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 reduction in population-level immunity. On Twitter, the term has taken on a life of its own. Some people have taken it to mean that a lack of exposure to pathogens such as RSV and influenza has irrevocably damaged the immune system, an idea that Matthew Miller, an immunologist at McMaster University in Hamilton, Canada, calls “nonsense”.
Some scientists have also posted on social media that the surge in RSV hospitalizations might be the result of SARS-CoV-2 infection causing immune deficiencies that leave people more susceptible to other infections. But Miller says hasn’t seen any evidence for that either, and the surge in hospitalizations could be explained by the large number of people who missed exposures in the past few years. “There’s a slightly bigger naive population, all of whom are at risk. So you’ve got more numbers going into the system.”
It’s tough to predict what the new normal might look like for seasonal viruses. If many of the susceptible people get infected in the coming months, next year’s flu season could be tamer, as some of the immune debt is ‘paid off’. But it’s not yet clear whether COVID-19 will become a seasonal illness such as flu and RSV, or whether it will continue as it has been, with sporadic peaks throughout the year.
There is also a lot that researchers still don’t understand about seasonal viruses. For example, COVID-19 restrictions seemed to have little impact on one type of seasonal virus, rhinoviruses — which are the most common cause of colds — for reasons that aren’t entirely understood. That might be because of their hardiness, Miller says. They’re less prone to desiccation and can persist for longer in the environment.
Another open question is how these viruses compete and interfere with one another. Infection with one virus can raise a strong innate immune response that might prevent infection with another virus. Hensley points out that last year’s first wave of influenza declined soon after the Omicron surge began. Perhaps Omicron infection provided some short-lived protection against flu. Or maybe the Omicron surge simply convinced people to mask up and keep their distance.
Pitzer expects that next year’s peaks and valleys might look much more like those that occurred before the pandemic. She isn’t placing any bets. But she says: “I do expect that this winter is probably going to be the last unusual winter.”