Temperature and latitude are not associated with the spread of COVID-19 disease, according to a global study that found school closures and other public health measures are having a positive effect on containing the novel coronavirus.
The study, published in the Canadian Medical Association Journal, looked at 144 geopolitical areas -- states and provinces in Australia, the US, and Canada as well as various countries around the world -- and a total of over 3,75,600 confirmed COVID-19 cases.
The researchers said China, Italy, Iran and South Korea were excluded because the virus was either waning in the case of China or in full disease outbreak at the time of the analysis in others.
"Our study provides important new evidence, using global data from the COVID-19 epidemic, that these public health interventions have reduced epidemic growth," said Peter Juni from the University of Toronto, and St. Michael's Hospital in Canada.
To estimate epidemic growth, the researchers compared the number of cases on March 27 with those on March 20.
They determined the influence of latitude, temperature, humidity, school closures, restrictions of mass gatherings and social distancing measured during the exposure period of March 7 to 13.
The study found little or no association between latitude or temperature with epidemic growth of COVID-19, and a weak association between humidity and reduced transmission.
The results -- that hotter weather had no effect on the pandemic's progression -- surprised the researchers.
"We had conducted a preliminary study that suggested both latitude and temperature could play a role. But when we repeated the study under much more rigorous conditions, we got the opposite result," said Juni.
The researchers did find that public health measures, including school closures, social distancing and restrictions of large gatherings, have been effective.
"Our results are of immediate relevance as many countries, and some Canadian provinces and territories, are considering easing or removing some of these public health interventions," said Juni.
"Summer is not going to make this go away," said Professor Dionne Gesink, a coauthor and epidemiologist at Dalla Lana School of Public Health in Canada.
"It's important people know that. On the other hand, the more public health interventions an area had in place, the bigger the impact on slowing the epidemic growth.
"These public health interventions are really important because they're the only thing working right now to slow the epidemic," Gesink said.
The researchers noted several study limitations, such as differences in testing practices, the inability to estimate actual rates of COVID-19 and compliance with social distancing.
When deciding how to lift restrictions, governments and public health authorities should carefully weigh the impact of these measures against potential economic and mental health harms and benefits, they said.