In a recent study led by McMaster University, researchers found that while higher heat and humidity can slow the spread of COVID-19, longer hours of sunlight are linked to a higher incidence of the disease.
In addition, sunny days can tempt more people out even if this means a higher risk of infection.
The findings inform the widespread scientific debate over how seasonal changes, specifically warmer weather, might shape the spread of COVID-19.
The study is published in Geographical Analysis. The lead author is Antonio Páez, a professor and researcher at McMaster’s School of Geography & Earth Sciences.
While previous research has shown that pathogens such as influenza and SARS thrive in lower temperatures and humidity, little is known about SARS-CoV2, the agent that causes COVID-19.
In the study, the team examined climate factors in the spread of COVID-19 in several provinces in Spain, one of the countries hardest hit by the pandemic, with more than 270,000 cases.
They combined and analyzed data on reported cases of the disease and meteorological information over a period of 30 days that began immediately before a state-of-emergency was declared.
At higher levels of heat and humidity, the researchers found that for every percentage increase, there was a 3% decline in the incidence of COVID-19, possibly because warmer temperatures curtail the viability of the virus.
The opposite was true for hours of sunshine: more sun meant greater spread. The researchers speculate the increase may be related to human behavior since compliance with lockdown measures breaks down in sunnier days.
They were also surprised to find rates of transmission dropped among more dense populations and in areas with more older adults.
This suggests those populations regard themselves as being at greater risk, and so are more likely to adhere to lockdown guidance.
While older adults are more vulnerable to the disease, the researchers believe they are less likely overall to contribute to the spread of the disease because they are more apt to be isolated from others because of health or mobility issues.
The team says they will likely see a decrease in the incidence of COVID-19 as the weather warms up, which is an argument for relaxing social distancing to take advantage of the lower incidence associated with higher temperatures.
But a more conservative approach would be to use the months of summer to continue to follow strict orders to remain in place and to crush this pandemic.
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