Will humidity stem spread of coronavirus? It’s too early to say, but one specialist is hopeful

When President Donald Trump asserted last month that the novel coronavirus may dissipate “as the heat comes in” -- that is, in warmer weather -- infectious disease experts responded with skepticism. The virus is less than three months old and, it’s not clear if it will dissipate as the seasons change from winter to spring like some other respiratory viruses tend to.

But researchers now suggest that humidity, more than heat, may prove effective at choking off the person-to-person transmissions that make the disease’s spread so dangerous.

Still, it’s far from settled science.

Dr. Alan Evangelista, a microbiology and virology professor at St. Christopher’s Hospital for Children in Philadelphia, has studied common coronaviruses and influenza particles for eight years.

He says his research indicates that “the size and overall composition of [the novel coronavirus] particle is similar to other coronaviruses we have tested” -- meaning his findings may shed light on how the coronavirus spreads, and possibly how it dies out.

Those findings show that “transmission is highly efficient under drier and colder conditions,” but far less so in a humid environment.

According to the Centers for Disease Control and Prevention, the novel coronavirus is thought to spread mainly from person-to-person through respiratory droplets when an infected person coughs or sneezes. Those droplets can be inhaled into the lungs or land in the mouths or noses of people nearby. The droplets are also believed to linger on hard surfaces, which other people might later touch.

“As humidity increases, the viral droplet size is larger and settles out of the air rapidly,” Evangelista found, according to a statement he provided to ABC News on his research.

“In contrast, in low humidity, there is rapid evaporation of respiratory droplets,” he continued. “They remain airborne for prolonged periods, increasing the time and distance over which transmission can occur.”

Evangelista argues that while “there are obviously no guarantees that COVID-19 will behave exactly like the known coronaviruses … the laws of physics should apply.”

Still, other experts have their doubts.

“While we may expect modest declines in the contagiousness of [the novel coronavirus] in warmer, wetter weather,” according to Dr. Marc Lipsitch, a Harvard epidemiologist, “it is not reasonable to expect these declines alone to slow transmission enough to make a big dent.”

At least one peer-reviewed 2013 study goes so far as to suggest the opposite is true -- that extremely humid conditions are associated with the spread of influenza viruses in areas closer to the equator.

The empirical evidence available so far is inconclusive, and at times confusing. The humid stretches of sub-Saharan Africa have registered a relatively low number positive cases -- fewer than 150 as of Monday, according to Johns Hopkins University. But the conditions in such Southeast Asian countries as Vietnam, where it can at times be humid, have not appeared to put the brakes on the spread with more than 1,000 positive tests there so far.

Skeptics also point out that South Florida is quickly becoming a hot spot for transmissions. Even the mayor of Miami tested positive for the virus -- though Florida has not hit its humid season just yet and there isn’t enough testing being done to get an accurate surveillance of the virus’ spread.

As experts debate the extent to which climate-related factors can mitigate the spread of the novel coronavirus, they agree that Americans should not be resting their hopes on a change in temperature alone.

“Other epidemiologic factors such as social distancing and contact tracing, testing, and quarantining, along with the patient’s immune response will play major roles in reducing the rate of transmission,” Evangelista wrote, “and hopefully, we should see a decrease in cases by mid-April.”

As of Monday, more than 4,600 Americans have tested positive for COVID-19, the disease caused by the novel coronavirus, and the disease has taken the lives of at least 85 people in the United States.

What to know about the coronavirus

• Coronavirus explained : How it started and how to protect yourself
• Coronavirus symptoms: What to do if you have symptoms
• Coronavirus map: Track the spread in the U.S. and worldwide