Seasonal influenza and common human coronaviruses typically fall off in warmer seasons, as heat and humidity diminish transmission sharply. There has been some hope that this will happen with SARS-CoV-2 novel coronavirus, giving the United States a summer respite against a relentlessly accelerating pandemic. The National Academy of Sciences looked at this question in a recently issued report, concluding in so many words, “Don’t count on it.”1
“There is some evidence to suggest that SARS-CoV-2 may transmit less efficiently in environments with higher ambient temperature and humidity,” the report stated. “However, given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread without the concomitant adoption of major public health interventions. Furthermore, the other coronaviruses causing potentially serious human illness, including both [the original] SARS-CoV and MERS-CoV have not demonstrated any evidence of seasonality following their emergence.”
Again, limited data support a waning of cases in warmer and more humid seasons, yet none are without caveats. Indeed, Australia and Iran reported outbreaks of the novel coronavirus in their typical “summer” climates.
“A decrease in cases with increases in humidity and temperature elsewhere should not be assumed,” the report concluded. “Given the lack of immunity to SARS-CoV-2 across the world, if there is an effect of temperature and humidity on transmission, it may not be as apparent as with other respiratory viruses for which there is at least some preexisting partial immunity.”
Moreover, pandemic influenza strains have not historically fallen into seasonal patterns, but have demonstrated a second wave of infections before receding.
“There have been 10 influenza pandemics in the past 250-plus years — two started in the northern hemisphere winter, three in the spring, two in the summer, and three in the fall,” the Academy reported. “All had a peak second wave approximately six months after emergence of the virus in the human population, regardless of when the initial introduction occurred.”
- National Academies of Sciences, Engineering, and Medicine 2020. Rapid Expert Consultation on SARS-CoV-2 Survival in Relation to Temperature and Humidity and Potential for Seasonality for the COVID-19 Pandemic (April 7, 2020). Washington, DC: The National Academies Press. https://doi.org/10.17226/25771