Will warm weather and high humidity stop the coronavirus pandemic, or is this a false hope based on comparisons with common viruses like influenza?
While research results haven’t been consistent, a new report from the National Academies of Science (NAS) suggests that data is leaning toward the conclusion that warm weather won’t stop the virus from spreading.
The report, which analyzes a variety of studies, was authored by David A. Relman, MD, a member of the NAS’ Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats, in a “rapid expert consultation letter” to the White House Office of Science and Technology Policy.
Evidence in favor of the virus slowing in warm weather comes, in part, from observing seasonal patterns of viruses like influenza that build in intensity during the fall and winter and wane in the spring. Even a few other coronaviruses follow this pattern. But Relman says data suggesting that SARS-CoV-2, the virus that causes COVID-19, will follow this same pattern is inconclusive at best.
“Some limited data support a potential waning of cases in warmer and more humid seasons, yet none are without major limitations,” he writes in the report.
In fact, some of the places where the virus has already spread are experiencing warm weather right now.
“Given that countries currently in ‘summer’ climates, such as Australia and Iran, are experiencing rapid virus spread, a decrease in cases with increases in humidity and temperature elsewhere should not be assumed.”
In addition, two of the most notorious coronaviruses, SARS-CoV and MERS-CoV, “have not demonstrated any evidence of seasonality following their emergence.”
Drawing a reliable answer from research to-date is challenging because many studies were conducted in labs with inconsistent conditions, and others are natural history studies—many from China— that don’t line up well with the particulars of this pandemic.
“There are significant caveats in all of the studies presented, mostly related to data quality and the limitation in time and location,” Relman writes. “There are also important confounding factors associated with geography and, hence, with temperature and humidity.”
Even in recent research that suggests higher temperatures and humidity will decrease viral “infectivity,” the rate of infection (known as the average R0 or R naught) “was still close to 2 [each infected person infects two additional people] at maximum temperatures and humidity in their data set, suggesting that the virus will still spread exponentially at higher temperatures and humidity.”
The report adds that relying on research focused on epidemic influenza strains, which are typically seasonal, is also inadequate because “pandemic influenza strains have not exhibited the typical seasonal pattern.”
“There have been 10 influenza pandemics in the past 250-plus years—two started in the northern hemisphere in winter, two in the summer and three in the fall,” Relman writes. “All had a peak second wave approximately six months after the emergence in the human population, regardless of when the initial introduction occurred.”
Relman’s analysis touches on a few other topics related to temperature, including the troubling persistence of the virus on personal protective equipment (PPE), like masks.
“The persistence of infectious virus on PPE is concerning and warrants additional study to inform guidance for healthcare workers,” he writes, noting that a high concentration of viral particles have been found inside masks after a full seven days. This result underscores the need for healthcare workers to use new masks throughout the day to avoid infecting themselves and others.
“Additional studies as the SARs-CoV-2 pandemic unfolds could shed more light on the effects of climate on transmission,” the report concludes.