Photograph by Liu Haojun Xinhua, eyevine/Redux
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Working amid global coronavirus fears, a villager labors in a field in China's Jiangxi Provence on February 18, 2020. Some viruses, like those that cause the flu, are seasonal, meaning they spread more easily in warm, dry air. It is still unclear if this coronavirus will be similarly impacted.

Photograph by Liu Haojun Xinhua, eyevine/Redux

Will warming spring temperatures slow the coronavirus outbreak?

Flu season generally subsides in April and March, but will the coronavirus go with it? Past coronavirus outbreaks can offer clues.

Whether the coronavirus that’s quickly spreading around the world will follow the flu season and subside with spring’s arrival is unsatisfyingly uncertain, and many scientists say it’s too soon to know how the dangerous virus will behave in warmer weather.

Dozens of viruses exist in the coronavirus family, but only seven afflict humans. Four are known to cause mild colds in people, while others are more novel, deadly, and thought to be transmitted from animals like bats and camels. Health officials have labeled this new virus SARS-CoV-2 and its disease COVID-19.

The prospect that summer could stave off a pandemic is enticing. Last month, President Donald Trump tweeted about China’s efforts to contain the virus, saying they would be successful, “especially as the weather starts to warm.”

Viruses that cause influenza or milder coronavirus colds do tend to subside in warmer months because these types of viruses have what scientists refer to as “seasonality,” so the president’s comments have some scientific backing. But it’s highly uncertain that SARS-CoV-2 will behave the same way. Those currently studying the disease say their research is too early to predict how the virus will respond to changing weather. (See how coronavirus compares to flu, Ebola and other outbreaks.)

“I hope it will show seasonality, but it’s hard to know,” says Stuart Weston, a postdoctoral fellow at the University of Maryland School of Medicine, where the virus is being actively studied.

As of March 6, more than 98,000 coronavirus cases had been confirmed in 88 countries, with experts saying the disease is likely to keep spreading.

Generally, what do we know about viruses?

At the most basic level, you can think of flu and coronaviruses as a collection of proteins and lipids. They pass from person to person via physical contact, but they can also exist on hard surfaces or in the cough of a sick person’s respiratory droplets.

Once outside a human body, external forces will cause the virus to deteriorate. The alcohol in hand sanitizer, for example, breaks down these proteins and lipids, making the virus less stable and less likely to successfully cause an infection. (Once your body's infected, this is what coronavirus does.)

Research about why some viruses are seasonal has been largely centered around those that cause the flu, a disease long associated with winter months. “Flu season” generally lasts from October to April or March. Scientists have a number of theories for why that is.

Some suggest it’s closer quarters—to escape the cold weather, people cluster indoors, where human-to-human transmission becomes more likely. To understand why northern latitudes see an uptick in flu cases during winter, researchers have looked at how the virus spreads in different temperature and humidity levels.

And relatively recent research suggests that dry, cold air may also help viruses stay intact in the air or travel farther as they become airborne.

One of the first studies to test how environmental conditions affect viral transmission was published in 2007, and it looked at how influenza spread through guinea pigs infected in a lab. High temperatures and in particular high humidity slowed the influenza spread, and at very high humidity levels, the virus stopped spreading completely. Warmer air holds more moisture, which prevents airborne viruses from traveling as far as they would in dry air. In humid conditions, the small liquid droplets in a cough or sneeze gather more moisture as they’re expelled. Eventually too heavy to stay airborne, they drop to the ground.

Studies outside the lab show similar results, though some tropical regions have more cases of flu during rainy season, when people also cluster indoors.

Scientists hypothesize that low humidity, which often occurs in winter, might impair the function of the mucus in your nose, which your body uses to trap and expel foreign bodies like viruses or bacteria. Cold, dry air can make that normally gooey mucus drier and less efficient at trapping a virus.

Ian Lipkin, director of the Columbia University's Center for Infection and Immunity, has been studying the novel coronavirus. He says sunlight, which is less abundant in winter, can also help break down viruses that have been transmitted to surfaces.

“UV light breaks down nucleic acid. It almost sterilizes [surfaces]. If you’re outside, it’s generally cleaner than inside simply because of that UV light,” he says.

UV light is so effective at killing bacteria and viruses it’s often used in hospitals to sterilize equipment.

What does that tell us about this coronavirus?

Though the coronavirus and the flu are both respiratory infections, not enough is known about SARS-CoV-2 to predict if it will have the same seasonal patterns.

To better understand this outbreak, scientists are looking at comparable outbreaks like SARS and MERS. SARS, which started its spread in late 2002, shares almost 90 percent of its DNA with the current virus. The SARS outbreak started in November and lasted until July, which only hints at seasonality, says Weston, and containment may have resulted from early intervention. In other words, did it disappear with warmer weather, or did treatment and prevention efforts simply work?

MERS began in September 2012 in Saudi Arabia, where temperatures are generally high. Unlike SARS, it was never fully contained, and new cases are occasionally reported. The novel coronavirus has also begun to circulate locally in the Middle East, namely in Iran and the United Arab Emirates.

“We don’t see too much evidence of seasonality in MERS,” says Weston.

But whether SARS and MERS were truly seasonal or if this virus will imitate SARS is unclear. Weston says their lab is focused on developing treatments and vaccines for the virus, which he cautions won’t likely be available for a least a year, if not several.

What’s next?

Harvard epidemiologist Marc Lipsitch doesn’t think any weather changes will put a big dent in how the virus spreads. COVID-19 has now been documented around the world. If the virus is anything like a typical flu virus, it may worsen in Southern Hemisphere regions as the seasons change.

David Heymann from the London School of Hygiene and Tropical Medicine says not enough is known about this new virus to predict how it will change with different weather conditions.

“The risk of making predictions without an evidence base is that they could, if they prove to be wrong, be taken as verity and give a false security,” Heymann says via email.

“The emphasis today should continue to be on containment to elimination where possible,” he cautions.

According to the Centers for Disease Control (CDC), people are most contagious when they’re showing symptoms. However, some experts suspect official counts may underestimate the number of infected people, saying not everyone infected will develop a severe illness.

“We’re only seeing the most severe cases,” says Weston. “There may be some infection going on that isn’t being detected.”

Many experts are saying SARS-CoV-2 will likely become endemic, joining the other existing four coronaviruses that cause mild colds, or becoming a seasonal health hazard like the flu.

To prevent contracting an illness from any virus, the World Health Organization recommends frequently washing your hands, avoiding close contact with those showing symptoms like coughing or sneezing, and seeking treatment if sick.

Editor’s Note: This story originally published on February 25, 2020. It has been updated to reflect the latest statistics.