With governments declaring mass quarantines and travel bans in an effort to stop—or at least slow—the spread of the world’s newest novel coronavirus, many outbreak watchers—Presidents Xi and Trump included—have lodged hope in a bit of circulating conventional wisdom: the idea that, like a seasonal flu, nCoV-2019 will peter out when warmer weather arrives.
Can we count on that? Do we just need to make it to April?
“It’s speculation because this is a new virus, and we have no idea,” says Peter Hotez, Professor and Dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “But it’s educated speculation.”
The new virus that emerged in Wuhan, China late last year—best known as 2019-nCoV—is a coronavirus, one in a large family of viruses that are found in animals and humans. While a handful of coronaviruses commonly circulates among the population and tend to cause mild respiratory illnesses like the common cold, this virus—like SARS, which emerged in 2002 and MERS, in 2012—only recently took hold in humans and is so considered a novel coronavirus.
“Coronaviruses have seasonality,” says Amesh Adalja, an infectious disease specialist with the Johns Hopkins University Center for Health Security. “We do know that certain environmental conditions favor the transmission of viruses and that cold weather, the humidity, all of that, affect trajectory. There’s a good reason to believe [this virus] will have that seasonality.”
When thinking about seasonality, Adalja reminds outbreak watchers that the world has two hemisphere with opposite seasons and cases will likely cluster accordingly. Wuhan, China—considered the epicenter of the outbreak—has temperatures in the 40s and 50s right now, and temperatures typically start rising in March. Tropical countries, meanwhile, tend not to see stark seasonality.
Adalja says it may take years to establish precisely how seasonality factors into this virus’ spread. He notes at the moment, because 2019-nCoV is an emerging pathogen, “there is not much population immunity, and so when it does appear in places, it’s going to find victims.”
Why viruses spread more efficiently in such conditions remains a matter of study. Marc Lipsitch, a professor of epidemiology at Harvard says the going hypothesis for why warmer seasons tend to decrease the spread of viruses includes higher vitamin D levels, resulting in better immune responses; higher absolute humidity which, in the case of flu, impairs virus transmission; and no school in the summer (when children are clustered together, transmission rates of flu and measles increase). Lipsitch notes that while work to better understand the seasonality of coronaviruses is ongoing, current thinking is “speculative.”
Meanwhile, Michael Osterholm, director of the Center for Infectious Disease and Policy at the University of Minnesota, says it’s premature to count on warmer weather impacting this coronavirus’ trajectory.
“A lot of people assume that because the SARS epidemic ended in the summer,” he says. “We have no idea if that was a coincidence of not. That just may have happened to be the time we controlled it.”
In a 2004 Lancet article looking at seasonality and SARS, Scott Dowell and Mei Shang Ho indicated the case wasn’t clear. They wrote: “It is true that most established respiratory pathogens of human beings recur in wintertime, but a new appreciation for the high burden of disease in tropical areas reinforces questions about explanations resting solely on cold air or low humidity.” They added, “newly emergent zoonotic disease such as Ebola or pandemic strains of influenza have occurred in unpredictable patterns.”
Osterholm notes that MERS, another coronavirus that emerged in humans in 2012, does not follow the pattern of expected seasonality at all. “MERS cases continue when temperatures are 110 degrees in the Arabian Peninsula,” he says, noting that “several of the [virus’] peaks have been in the middle of the summer.”
As with so many things with this novel coronavirus, whether it will peter out with warmer temperatures, is something we just don’t know.
While that may make the anxious feel only more so, Adalja warns against that and makes the case that the world is currently overreacting given what we’ve learned about this virus.
“The fact that we now understand this is a community-spreading coronavirus with 40,000 cases in 20-some countries with a spectrum of illness that definitely tilts more towards the mild than the severe, it really argues against these travel bans being enacted by countries, including the United States,” he says. “There has to be consideration of when we’re going to move from containment to mitigation.”
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