How will summer weather shape the spread of COVID-19?

By Samantha Black, PhD, The Science Advisory Board staff writer

June 10, 2020 -- New research shows that while higher temperatures and humidity can slow the spread of COVID-19, longer hours of sunlight are associated with a higher incidence of the disease. Human behavior may be at the root of this quandary, according to a June 8 Geographical Analysis publication.

In efforts to control the spread of COVID-19, regional and national governments have recommended or mandated social distancing measures. While these efforts seem to have an impact on mitigation of the disease, they come at a high cost with economic, social, and cultural consequences. Urgency to lift restrictions and resume social contact has become a delicate, politically charged balancing act between public health and the economy.

"There is a lot of pressure to reopen the economy, and many people want to know if it will be safer to do so in the summer months," said lead author Antonio Páez, PhD, a professor in McMaster University's School of Geography and Earth Sciences, in a statement. "Restrictions in movement, which have begun to ease around the world, hinge in part on how SARS-CoV-2 will be affected by a change in season."

Scientists urge against extrapolating the spatiotemporal patterns of other known viruses to SARS-CoV-2. Previous research from the SARS-CoV outbreak from 2002 to 2004 found that SARS lost viability at higher temperatures (greater than 38° C) and high humidity (above 95%). While research on other pathogens suggests that viruses spread better at low temperatures and low humidity, the evidence for SARS-CoV-2 is still inconclusive.

Modeling spatiotemporal effects of SARS-CoV-2 in Spain

Researchers from McMaster University, Polytechnic University of Cartagena in Spain, and Brazil's Federal University of Pernambuco sought to investigate the influence of environmental factors, namely temperature, humidity, and sunshine, on the progression of COVID-19. They used data from provinces in Spain, one of the countries hit hardest by the pandemic. The data were analyzed using a spatial seemingly unrelated regressions (SUR) approach to model COVID-19 as a contagion process.

The researchers combined and analyzed data on reported cases of the disease and meteorological information from March 13 to April 11 -- the time period immediately after a state of emergency went into effect.

The analysis of environmental variables in the SUR approach revealed that humidity and temperature have a negative association with the incidence of COVID-19 in Spain. The researchers explained that this is in line with the literature that describes lower viability and transmission of viruses at higher levels of humidity and temperature. At higher levels of heat and humidity, researchers found that for every percentage increase, there was up to a 3% decline in the incidence of COVID-19.

However, the researchers found a significant and positive correlation between sunlight and incidence of COVID-19. This finding contrasts with observations from previous studies. The researchers interpreted the positive correlation as a behavioral adaptation, whereby compliance with lockdown orders weakens on sunny days.

The control variables in the model also offered interesting insights, according to the researchers. For instance, wealthier regions tended to have a higher incidence of COVID-19, which is in line with the idea that wealthier regions are more connected to a globalized world and more rapid spread of the disease.

Also, regions with large populations of older adults who are naturally socially isolated are negatively correlated to incidence of COVID-19. The researchers noted that this does not detract from evidence that those individuals are more vulnerable, but rather their presence in the community tends to depress transmission of the virus.

The researchers suggested that models like the one used in the current study show that COVID-19 declines as social distancing remains intact, possibly to the vanishing point, which could provide a convincing argument for maintaining distancing despite the onset of pleasant weather.

"We 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," Páez explained. "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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