COVID-19 is likely to evolve to become a relatively mild disease that mainly affects children, like many other viruses, the scientists behind a new study say.
When it first emerged from China 18 months ago, the SARS-CoV-2 virus had a rather unusual pattern of attack. Most serious diseases hit the very young and very old hardest, but this one left most children relatively unscathed, if they were even infected at all.
"Following infection by SARS-CoV-2, there has been a clear signature of increasingly severe outcomes and fatality with age," said Ottar Bjornstad of Pennsylvania State University, one of the authors behind the new research.
"Yet, our modeling results suggest that the risk of infection will likely shift to younger children as the adult community becomes immune either through vaccination or exposure to the virus."
They ran various different computer models using demographics from a range of very different countries - some with older populations such as Japan and France, and others with young populations like Brazil and South Africa.
The results were that while the initial waves of infection affected different groups in each country, as long as vaccines and/or infection provided good immunity to later waves, eventually it was very young children bearing the brunt.
"The young are predicted to have the highest rates of infection as older individuals are protected from new infections by prior infection" or vaccines, said Ruiyun Li, postdoctoral fellow at the University of Oslo.
This is what's happened to many past viruses that have become endemic, they said.
"Historical records of respiratory diseases indicate that age-incidence patterns during virgin epidemics can be very different from endemic circulation," said Dr Bjornstad. "For example, ongoing genomic work suggests that the 1889-1890 pandemic, sometimes known as the Asiatic or Russian flu - which killed 1 million people, primarily adults over age 70 - may have been caused by the emergence of HCoV-OC43 virus, which is now an endemic, mild, repeat-infecting cold virus affecting mostly children ages seven to 12 months old."
If that does happen, parents will be hoping the SARS-CoV-2 virus also loses the ability to trigger Long COVID and MIS-C, a rare and mysterious inflammation condition that affects some children infected by the virus.
But then again, it might not. The model's outcomes depend on immunity being strong and long-lasting, and the evidence for that when it comes to COVID-19 is not strong. Relatively new to humans, the virus is evolving to find better ways to infect people - and it's already produced a variant, Delta, which can infect people who've been vaccinated - the vaccines being developed to fight the original strain.
If immunity doesn't last long, countries with older populations "would be expected to have a larger fraction of deaths than those with relatively younger population structures", said University of Oslo ecology and evolution professor Nils Christian Stenseth.
That's even if most of the infections happen in the young, the study - published in journal Science Advances - concluded.
