The United States is not the only place where the off-season wave of RSV can be felt. Australia, South Africa, Iceland and various European countries have also done so. In France, RSV arrived four months late – in April instead of December – according to Jean-Sébastien Casalegno, a doctor and virologist at the Institute for Infectious Agents of Hospices Civils in Lyon and the first author of the book March preprint describing the outbreak.
There aren’t a lot of models that indicate what might happen next. Will RSV return this year and have a smaller, weaker season in its usual time frame? Will he slowly revolve around the calendar until he ends up where he belongs? “Seasonality is likely to return after a few seasons,” Casalegno says. “It’s complicated next season, which is going to happen.”
Viruses are seasonal for complex reasons, not only because they have evolutionary preferences for certain temperatures and humidity, but because winters usually represent a time when people are pushed indoors. But they are also seasonal because it takes some time to build enough vulnerable people – those who have not been previously exposed or have been vaccinated if there is a vaccine – to provide the virus with enough territory to reproduce and forward copies to new hosts.
As this group of “susceptible” spreads, there is little difference for each virus. For RSV, which usually respects the annual cycle, the youngest children are at greatest risk. By school age, most children have acquired immunity from infection or from repeated exposure that did not cause symptoms, but still allowed their immune system to create a defense.
The EV-D68 is also seasonal, but in a more complex way. First, its attacks occur in summer, not winter. Second, as shown in the first analysis of its seasonality, published in March at Science Translational Medicine, and respiratory diseases that also cause diskette paralysis to recur every two years. That analysis revealed that the cycles are triggered by climatic conditions, but also by the immune system: women who are exposed to EV-D68 while pregnant are transmitting antibodies against their baby. Thus, for the first 6 months, babies are protected from disease and become vulnerable as passive immunity declines. This later vulnerability, combined with seasonality, appears to trigger a slower accumulation of vulnerable groups.
The last EV-D68 epidemic was predicted to occur last summer, 2020. As with RSV and influenza, it did not arrive, and for similar reasons: masking, moving away, washing hands and staying at the home of protected children who would then be vulnerable . As with RSV, no one is sure what will happen next.
“Nothing about enteroviruses is what they like even for years – they don’t have a lucky number,” says Kevin Messacar, an associate professor of pediatrics at the University of Colorado and Children’s Hospital of Colorado who co-authored that March analysis. “The model for this whole family of viruses, which is well described, would not predict that we will wait for an outbreak in 2022 because we missed the cycle. It would say that we are constantly growing a multitude of sensitive people who have not seen this virus. “
The national project in which he is involved, he says, has found lower levels of antibodies than EV-D68 in pregnant women because they were not exposed to the virus last year and therefore cannot pass on protection. This could mean that whenever EV-D68 recurs, more children can become infected with the virus or become sicker than they would otherwise be or would catch it earlier in life, in the most vulnerable months of infancy, when they would otherwise be protected.
And there’s the flu – always the most unpredictable of respiratory infections, as it constantly mutates to evade our immune defenses, occasionally replacing its dominant strains with new ones, and sometimes triggering mild years of illness and sometimes devastating ones. It is currently the flu and the future infection that causes the most anxiety. Without a dramatic return to social distancing, “I expect an extremely bad flu season,” says Sarah Cobey, an immunologist and associate professor of ecology and evolution at the University of Chicago. “I expect more people to catch the flu. I also expect a lot of really bad flu infections. “