Why the Coronavirus Has Been So Successful

During a cytokine storm, the immune system isn’t just going berserk but is also generally off its game, attacking at will without hitting the right targets. When this happens, people become more susceptible to infectious bacteria. The storms can also affect other organs besides the lungs, especially if people already have chronic diseases. This might explain why some COVID-19 patients end up with complications such as heart problems and secondary infections.

But why do some people with COVID-19 get incredibly sick, while others escape with mild or nonexistent symptoms? Age is a factor. Elderly people are at risk of more severe infections possibly because their immune system can’t mount an effective initial defense, while children are less affected because their immune system is less likely to progress to a cytokine storm. But other factors—a person’s genes, the vagaries of their immune system, the amount of virus they’re exposed to, the other microbes in their bodies—might play a role too. In general, “it’s a mystery why some people have mild disease, even within the same age group,” Iwasaki says.

Coronaviruses, much like influenza, tend to be winter viruses. In cold and dry air, the thin layers of liquid that coat our lungs and airways become even thinner, and the beating hairs that rest in those layers struggle to evict viruses and other foreign particles. Dry air also seems to dampen some aspects of the immune response to those trapped viruses. In the heat and humidity of summer, both trends reverse, and respiratory viruses struggle to get a foothold.

Unfortunately, that might not matter for the COVID-19 pandemic. At the moment, the virus is tearing through a world of immunologically naive people, and that vulnerability is likely to swamp any seasonal variations. After all, the new virus is transmitting readily in countries like Singapore (which is in the tropics) and Australia (which is still in summer). And one recent modeling study concluded that “SARS-CoV-2 can proliferate at any time of year.” “I don’t have an immense amount of confidence that the weather is going to have the effect that people hope it will,” Gralinski says. “It may knock things down a little, but there’s so much person-to-person transmission going on that it may take more than that.” Unless people can slow the spread of the virus by sticking to physical-distancing recommendations, the summer alone won’t save us.

“The scary part is we don’t even know how many people get normal coronaviruses every year,” Frieman says. “We don’t have any surveillance networks for coronaviruses like [we do for] flu. We don’t know why they go away in the winter, or where they go. We don’t know how these viruses mutate year on year.” Until now, research has been slow. Ironically, a triennial conference in which the world’s coronavirus experts would have met in a small Dutch village in May has been postponed because of the coronavirus pandemic.

“If we don’t learn from this pandemic that we need to understand these viruses more, then we’re very, very bad at this,” Frieman says.

Ed Yong is a staff writer at The Atlantic, where he covers science.