Warmer weather unlikely to slow virus, study finds,
Ban on mass gatherings and closings are making a difference, study finds
The pandemic is unlikely to take a summer vacation, according to new University of Toronto-led research — a study that suggests weak impacts of warm weather on COVID-19, but strong reassurance that social distancing works.
The scientists compared epidemics in 144 countries, states and provinces worldwide, and found that global latitude and local temperature were not associated with the spread of COVID-19. But public health measures, including school closings, bans on mass gatherings, and physical distancing, made a significant impact — and the more measures, the better.
The results, published in the Canadian Medical Association Journal (CMAJ), will be surprising and disappointing to some scientists and many policy-makers, who hoped that the newly emerged infectious disease would behave like the flu and be diminished by warm weather.
This question “is a really important thing to consider, especially being in a northern place where we have seasons,” says Dionne Gesink, a professor of epidemiology at U of T’s Dalla Lana School of Public Health and co-author of the new study.
“There’s always this outstanding question: Will things get better when summer comes — or will they get worse as it gets colder? It helps us in public health predict how to move forward.”
In the study, the researchers assembled 144 jurisdictions with an outbreak of COVID-19 for comparison. Over seven days in early March, the researchers collected information on each of those places: the state of their outbreaks, but also their geographic latitude, mean temperature, humidity, and any public health measures they implemented, like school closings and bans on gatherings.
Then the researchers came back 14 days after the beginning of that observation period to see how each jurisdiction had fared. The lag was to give measures time to take effect, since the virus has an incubation period: public health interventions take two weeks to show their effects.
(Countries with epidemics that were much more advanced in March, like China, Iran and South Korea, were excluded from the analysis. Big countries like Canada and the United States were broken into individual provinces and states. All had at least 10 cases.)
Seasonality is a feature of many, if not most, infectious diseases. Turning the heat up or down often has a big effect on transmission. But researchers don’t fully understand why: for influenza’s summer dip, debates still rage over whether the driver is temperature, humidity, solar radiation or schools being dismissed — or all of these together.
The researchers found that humidity seemed to have a very weak effect on COVID-19 transmission, although that effect disappeared in more complex analyses of the data. But temperature and latitude were not associated with epidemic growth.
“A lot of people implicitly or explicitly — also my colleagues sometimes — believe seasons will fix this,” says Dr. Peter Jüni, aprofessor of epidemiology and medicine at the University of Toronto and St. Michael’s Hospital and another of the paper’s co-authors.
But there’s a big difference between the influenza virus and the SARS-CoV-2, the virus that causes COVID-19, Juni notes.
“We all have partial immunity against influenza, but no immunity against SARS-CoV-2. This is the real danger: SARSCoV-2 doesn’t need favourable conditions.”
As more people acquire COVID-19 and recover from it, building immunity in the population, “these seasonal effects may become more marked,” says Dr. David Fisman, another co-author and Dalla Lana School of Public Health epidemiologist.
“But right now, the virus doesn’t need the environmental help, because it’s doing just fine feasting on susceptibles.”
Out of the 144 jurisdictions, 38 had implemented public health interventions. The researchers found a strong association between countries that had instituted these measures in the seven-day observation period and slowing of transmission two weeks later.
“The sacrifices that people are going through and the hard work that the community is doing to shut down COVID is — based on the best available data — making a big difference,” Fisman says.
The study, however, couldn’t disentangle what role each individual public health measure played, since most countries instituted all of them at once or in quick succession. While the study suggests that school closings had an effect, it can’t resolve one of the most difficult policy questions governments are struggling with: how much transmission will tick upwards again if schools are reopened.