Communities living in warmer places appear to have a comparative advantage to slow the transmission of coronavirus infections, according to an early analysis by scientists at the Massachusetts Institute of Technology.
The researchers found that most coronavirus transmissions had occurred in regions with low temperatures, between 37.4 and 62.6 degrees Fahrenheit (or 3 and 17 degrees Celsius).
While countries with equatorial climates and those in the Southern Hemisphere, currently in the middle of summer, have reported coronavirus cases, regions with average temperatures above 64.4 degrees Fahrenheit (or 18 degrees Celsius) account for fewer than 6% of global cases so far.
“Wherever the temperatures were colder, the number of the cases started increasing quickly,” said Qasim Bukhari, a computational scientist at MIT who is a co-author of the study. “You see this in Europe, even though the health care there is among the world’s best.”
The temperature dependency is also clear within the United States, Bukhari said. Southern states, like Arizona, Florida and Texas, have seen slower outbreak growth compared with states like Washington, New York and Colorado. Coronavirus cases in California have grown at a rate that falls somewhere in between.
The seasonal pattern is similar to what epidemiologists have observed with other viruses. Dr. Deborah Birx, the global AIDS coordinator in the United States and also a member of the Trump administration’s coronavirus task force, said during a recent briefing that the flu, in the Northern Hemisphere, generally follows a November to April trend.
The four types of coronavirus that cause the common cold every year also wane in warmer weather.
Birx also noted that the pattern was similar with the SARS epidemic in 2003. But she stressed that because the virus outbreaks in China and South Korea began later, it was difficult to determine whether the new coronavirus would take the same course.
At least two other studies published on public repositories have drawn similar conclusions for the coronavirus. One analysis by researchers in Spain and Finland found that the virus seemed to have found a niche in dry conditions and temperatures between 28.3 degrees and 49 degrees Fahrenheit (or minus 2 and 10 degrees Celsius). Another group found that before the Chinese government started imposing aggressive containment measures, cities with higher temperatures and more humid environments reported a slower rate of infection transmission early in the outbreak.
But none of the studies have been peer-reviewed by other scientists, and Bukhari acknowledged that factors such as travel restrictions, social distancing measures, variations in the availability of tests and hospital burdens might have affected the number of cases in different countries.
The possible correlation between coronaviruses cases and climate should not lead policymakers and the public to complacency.
“We still need to take strong precautions,” Bukhari said. “Warmer temperatures may make this virus less effective, but less effective transmission does not mean that there is no transmission.”
Warmer temperatures might make it harder for the coronavirus to survive in the air or on surfaces for long periods of time, but it could still be contagious for hours, if not days, Bukhari said.
Even seasonal viruses like influenza and the viruses that cause the common cold don’t completely disappear during summer. They are still present at low levels in many people’s bodies and in other parts of the world, biding their time until conditions are suitable for infections to spread again.
Some viruses have the opposite pattern. Polio and tuberculosis, for example, tend to spread faster in warmer climes. And some viruses may have no seasonal variation at all.
It will take another 4 to 6 weeks before health officials will have a clearer picture of how weather patterns shape the trajectory of the coronavirus, said Jarbas Barbosa, assistant director at the Pan American Health Organization, the regional office of the World Health Organization that focuses on the Americas.
The fact that local transmission is happening across the global south signals that this virus may be more resilient to warmer temperatures than the flu and other respiratory viruses. That is why World Health Organization officials still urge countries to act urgently and aggressively to try and contain the virus while case numbers are relatively low and close contacts can easily be traced and quarantined.
“One of the big perils in assuming that the virus is less dangerous in warmer temperatures, among particular ages or for any specific group is complacency,” said Julio Frenk, a physician who served as health minister in Mexico and is now president of the University of Miami. “If people fail to heed the warnings and recommendations of public health professionals, the results will be disastrous.”
But because high humidity and heat only align perfectly during mainly July and August in some parts of the Northern Hemisphere, Bukhari cautioned that the effects of warmer weather on reducing transmissions might only last for a brief period in some regions.
“This suggests that even if the spread of the coronavirus decreases at higher humidity, its effect would be limited for regions above 40 degrees North, which includes most of the Europe and North America,” he said.
And because so much is unknown, no one can predict whether the virus will return with such ferocity in the fall.