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.
AUDIE CORNISH, HOST:
There's a seasonality to many viruses. Flu and cold viruses tend to peak in winter months, then die down with warmer weather. And some experts say there could be a similar pattern with the novel coronavirus. NPR's Allison Aubrey reports.
ALLISON AUBREY, BYLINE: No one knows for certain how the coronavirus outbreak plays out from here. But scientists say there's a lot to be learned from other coronaviruses in circulation.
AMESH ADALJA: I do think seasonality will play a role. We know that coronaviruses tend to peak in the winter and the spring.
AUBREY: That's Amesh Adalja, an infectious disease physician and a senior scholar at the Johns Hopkins Center for Health Security.
ADALJA: As this outbreak unfolds and we approach spring and approach summer, I do think we will see some tapering off of cases as this virus kind of obeys the general rules of coronaviruses.
AUBREY: He says, unlike the coronavirus that causes the Middle East Respiratory Syndrome, or MERS - which didn't have the capacity to spread easily from person to person the way seasonal cold viruses do - this one seems to.
ADALJA: We've seen basically explosive spread inside China of person-to-person transmission. So in that sense, it really is behaving like a community-acquired common-cold-causing coronavirus.
AUBREY: He says as China and the rest of the Northern Hemisphere head into spring, the virus could begin to peter out.
ADALJA: That said, the Southern Hemisphere has the opposite. So we may see this have increased transmission during our summer months, which are the winter months for, for example, Australia.
AUBREY: Like what happens with the flu. Elizabeth McGraw directs the Center for Infectious Disease Dynamics at Penn State. She says weather and seasons can play a role in a few ways.
ELIZABETH MCGRAW: Coronaviruses, like cold and flu viruses - they tend to be associated with winter because of how they are spread.
AUBREY: In winter months, people may cluster together more indoors. And she says there's more to it. The viruses are typically spread through respiratory droplets that hit the air when an infected person coughs or sneezes. And these droplets are much more likely to spread under certain conditions.
MCGRAW: What we know is that they're better at staying afloat when the air is cold and dry - versus when the air is humid and warm, they tend to fall to the ground more quickly. And so it makes transmission harder.
AUBREY: Anthony Fauci is director of the National Institute of Allergy and Infectious Diseases.
ANTHONY FAUCI: Put it this way - it is not unreasonable given the history of viruses that are respiratory-borne, like influenza and coronavirus.
AUBREY: Clearly, history shows that some common cold coronaviruses have a seasonal ebb and flow.
FAUCI: But since we don't have that backlog of history with this particular coronavirus, I don't think you can say that with certainty.
AUBREY: And that's why Fauci says you can hope for the best, but you've got to plan for the unexpected and the possibility that the outbreak drags on. Allison Aubrey, NPR News. Transcript provided by NPR, Copyright NPR.