The United States isn’t the only place to experience an out-of-season RSV push. Australia, South Africa, Iceland and many European countries have done the same. In France, RSV arrived four months late – April instead of December – according to Jean-Sébastien Casalegno, a doctor and virologist at the Institut des Agents Infectieux of the Civil Hospices of Lyon and first author on a March prepress describing the fire.
There aren’t many models to indicate what might happen next. Will RSV return this year and have a smaller, weaker season in its normal time slot? Will it slowly revolve around the calendar until it ends up back where it belongs? “Seasonality will probably return after several seasons,” Casalegno says. “What’s complicated is next season, what will happen.”
Viruses are seasonal for complicated reasons, not only because they have evolutionary preferences for particular temperatures and humidity, but because winters tend to be the time when people flock together at home. But they are also seasonal because it takes some time to build up a large enough number of vulnerable people – those who have not been previously exposed, or vaccinated if a vaccine exists – to provide a virus with enough territory to reproduce and pass. copies of itself to new guests.
Just as that group of “susceptibles” spreads it is slightly different for each virus. For RSV, which generally observes an annual cycle, younger children are at higher risk. At school age, most children have gained immunity from infections, or from repeated exposures that have not caused symptoms, but have allowed their immune systems to create a defense.
EV-D68 is also seasonal, but in a more complicated way. First, their fires occur in summer, not winter. Second, as demonstrated in the first analysis of its seasonality, published of March in Translational Medicine of Science, both the respiratory disease it causes and that floppy paralysis seems to occur every two years. This analysis found that the cycles are driven by climatic conditions, but also by the immune system: women who are exposed to EV-D68 while pregnant pass antibodies against their children. Thus, for their first 6 months, children are protected against the disease, and become vulnerable when passive immunity decreases. This vulnerability then, combined with seasonality, seems to lead to the slower accumulation of susceptibilities.
The last focus EV-D68 is expected to be released last summer, in 2020. As with RSV and influenza, it has not arrived, and for similar reasons: Masking, distance, hand washing, and standing at home protected children who would have been vulnerable then. And as with RSV, no one is sure what will happen next.
“There’s nothing in enteroviruses that makes them love them even years old – they don’t have a lucky number,” says Kevin Messacar, associate professor of pediatrics at the University of Colorado and the Children’s Hospital of Colorado who was co-author of that March analysis. “The model for this whole family of viruses, which is well described, does not predict that we will wait until 2022 for a outbreak because we have missed a cycle. I would say we are constantly growing the group of susceptibles who have not seen that virus.”
A national project in which she participates, she says, finds lower-than-usual levels of EV-D68 antibodies in pregnant women because they were not exposed to the virus last year, and so cannot transmit protection. Which could mean that, every time EV-D68 recurs, more children could contract the virus or become sicker than otherwise, or catch it earlier in their lives, in the most vulnerable months of childhood when they otherwise would have been. protected.
And then there’s the flu – always the most unpredictable of respiratory infections, because it mutates uninterruptedly to bypass our immune defenses, periodically swaps its dominant races for new ones, and sometimes triggers a mild and sometimes devastating disease. The flu is also, now, the future infection that makes you the most anxious. Without any dramatic return to social distancing, “I expect a season of extraordinarily bad influence,” says Sarah Cobey, immunologist and associate professor of ecology and evolution at the University of Chicago. “I expect more people to be infected with the flu. I also hope there will be a lot of really bad flu infections.”