There’s a seasonality to many viruses. Flu and cold viruses tend to peak in winter months, then die down with warmer weather.
Will the newly identified coronavirus and the disease it causes — COVID-19 — follow a similar pattern?
Before that question can be answer, let’s consider how seasons and temperature influence the spread of viruses.
“Coronaviruses tend to be associated with winter because of how they’re spread,” explains Elizabeth McGraw, who directs the Center for Infectious Disease Dynamics at Pennsylvania State University. For one thing, in winter months, people may cluster together more indoors, increasing the number of folks at risk of becoming infection by someone who’s contagious.
In addition, there’s the matter of transmission. Viruses spread through respiratory droplets that are released when an infected person coughs or sneezes. And the droplets are more likely to spread under certain conditions. “What we know is that they’re [the droplets] are better at staying afloat when the air is cold and dry, ” says McGraw. “When the air is humid and warm, [the droplets] fall to the ground more quickly, and it makes transmission harder.”
Not every coronavirus hews to the same rules. For instance, the one that causes Middle East Respiratory Syndrome (MERS) has not shown the capacity to spread easily from person to person, says Amesh Adalja, an infectious disease physician and a senior scholar at the Johns Hopkins Center for Health Security: “It doesn’t have that seasonality because it’s really an animal to human virus and not something that that you see causing disease in a seasonal pattern.”
But he says COVID-19 seems more akin to the seasonal cold. And up to a third of common colds are caused by coronaviruses.
“We’ve seen, basically, explosive spread inside China of person-to-person transmission, so — in that sense — it really is behaving like a common-cold causing coronavirus,” says Adalja.
For that reason, he says, “I do think seasonality will play a role. As this outbreak unfolds and we approach spring and summer, I do think we will see some tapering off of cases.”
So as China and the rest of the Northern Hemisphere head into spring, the virus could begin to peter out or plateau. But the southern half of the globe is headed into fall and winter “so we may see this [virus] have increased transmission” in parts of the southern hemisphere, says Adalja — for example, in Australia. That’s similar to what happens with the flu each year.
“It’s not unreasonable to make the assumption” that cases will die down come spring, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NPR. “We hope when the weather gets warmer it will diminish a bit,” he says.
But he sounds a cautionary note: “However, we don’t know that about this [new] coronavirus. We don’t have [a] backlog of history.”
Dr. Nancy Messionnier of the Centers for Disease Control and Prevention sounds a similar note when it comes to predicting a slowdown of cases with warmer weather. “I think it’s premature to assume that,” she said during a call with reporters on Wednesday. “We haven’t been through even a single year with this pathogen.”
Given the uncertainty, public health officials say they must plan for the unexpected and for the possibility that the outbreak drags on regardless of the weather.