While some reports have claimed that cold weather can increase the spread of the deadly Covid-19 virus, a new study by an Indian-origin scientist reveals that hot or cold -- weather alone has no significant effect on Covid-19 spread.
The link between weather and Covid-19 is complicated. Weather influences the environment in which the coronavirus must survive before infecting a new host.
But it also influences human behaviour, which moves the virus from one host to another.
The current study, published in the journal International Journal of Environmental Research and Public Health, found that temperature and humidity do not play a significant role in coronavirus spread.
That means whether it's hot or cold outside, the transmission of Covid-19 from one person to the next depends almost entirely on human behaviour.
"The effect of weather is low and other features such as mobility have more impact than the weather. In terms of relative importance, the weather is one of the last parameters," said study author Dev Niyogi from the University of Texas at Austin in the US.
The study defined weather as "equivalent air temperature," which combines temperature and humidity into a single value.
The scientists then analysed how this value tracked with coronavirus spread in different areas from March to July 2020, with their scale ranging from US states and counties to countries, regions and the world at large.
At the county and state scale, the researchers also investigated the relationship between coronavirus infection and human behaviour, using cell phone data to study travel habits.
The study examined human behaviour in a general sense and did not attempt to connect it to how the weather may have influenced it. At each scale, the researchers adjusted their analyses so that population differences did not skew results.
Across scales, the scientists found that the weather had nearly no influence.
When it was compared with other factors using a statistical metric that breaks down the relative contribution of each factor toward a particular outcome, the weather's relative importance at the county scale was less than three per cent, with no indication that a specific type of weather promoted spread over another.
"We shouldn't think of the problem as something driven by weather and climate. We should take personal precautions, be aware of the factors in urban exposure," the study authors wrote.
The researchers said that assumptions about how coronavirus would respond with weather are largely informed by studies conducted in laboratory settings on related viruses.
This study illustrates the importance of studies that analyse how the coronavirus spreads through human communities.