Researchers investigate if warmth, humidity slows coronavirus

PHOENIX – New research from leading scientists raises questions about President Donald Trump’s assertion that summer temperatures will drive away the novel coronavirus that causes COVID-19.

At a New Hampshire rally on Feb. 10, Trump told Americans the virus would, “go away in April, with the heat,” later saying it was Chinese President Xi Jinping who told him “the heat generally kills this kind of virus.”

That may have raised hopes in Arizona, where temperatures in the lower deserts normally begin to approach 100 by May.

Because the virus is novel, researchers still are learning things about the virus on a weekly, if not daily, basis. The Centers of Disease and Control and Prevention states on its website that it is “not yet known whether weather and temperature affect the spread of COVID-19.”

Although little if any research definitively connects the weather’s effect on COVID-19, new studies from Harvard University and Massachusetts Institute of Technology researchers could provide some clarity.

“Summer may not save us, and repeated periods of social distancing may be needed to keep serious cases from overwhelming the hospital system,” Harvard researchers wrote.

They looked at how coronaviruses that cause the common cold are affected by the weather and concluded there was no seasonal effect on the virus due to large populations getting sick and building temporary immunity by spring.

However, the MIT study – which looked into global cases of COVID-19 from Jan. 22 through March 21 – came to a more hopeful conclusion.

“The data so far clearly shows that the number of cases for temperatures > 17°C (63°F) and absolute humidity > 9 g/m3 (about 60% relative humidity) is low,” meaning that the virus was not spreading as rapidly in warm and humid climates.

The MIT researchers said 83% of global testing has been done outside the tropics – north of 30 degrees N – where temperatures on average have ranged from 37 degrees F to 63 degrees F, and absolute humidity values ranged from 4 to 9 grams per cubic meter, which is dry air.

Absolute humidity tells us how much actual water vapor is in the air, and this is a key factor in virus transmission, according to the MIT study. Drier climates tend to evaporate the layer of moisture that surrounds a virus when it is expelled by a cough or sneeze. A humid environment would prevent that layer of moisture from evaporating, further preventing exposure.

Absolute humidity values that exceed 10 grams per cubic meters may cause “a slowdown in transmissions,” which MIT found to be the case in such tropical nations as Taiwan, Qatar, Singapore and Australia, all of which have done extensive coronavirus testing per capita.

Monsoon in the Southwest, which runs June 15 to Sept. 30 in Arizona, can at times reach these high absolute humidity values when moisture begins to well up from the Gulf of California.

But because the majority of testing across the globe has been done outside the tropics, it is hard to accurately say the effect a warm, moist climate has on COVID-19, the MIT study said.

“Our results in no way suggest that 2019-nCoV would not spread in warm humid regions and effective public health interventions should be implemented across the world to slow down the transmission of 2019-nCoV,” it said.

Marc Lipsitch, director at the Center for Communicable Disease Dynamics at Harvard, wrote, “changing seasons and school vacation may help, but are unlikely to stop transmission. Based on the analogy of pandemic flu, we expect that COVID-19, as a virus new to humans, will face less immunity and thus transmit more readily even outside of the winter season.”