Remember those glorious yet all too fleeting days last summer when COVID-19 cases were at their lowest levels and we could actually socialize?

We speculated, and even assumed, the warmer weather and fresh outdoor air was the enabling factor for the short-lived visits with friends. But what does recent Canadian research say about the impact of climate?

In the first Canadian study to use daily meteorological data, researchers looked at the relationship between ambient temperature and COVID-19 incidence in four provinces. They analyzed over 77,700 COVID-19 cases from Alberta, British Columbia, Ontario, and Quebec from January to May 2020. They adjusted for factors like precipitation and wind speed and found a weak association (not statistically significant) between infections and ambient temperature. In other words, their findings did not support the hypothesis that warmer temperatures reduce COVID-19 transmission.

However, we can suspect the study might have been more persuasive one way or another if it had examined data throughout the summer instead of the winter to spring duration. Indeed, study co-authors Teresa To and Kimball Zhang said it’s possible that the relationship between temperature and COVID cases may not be as strong in colder months. In an emailed response to questions, To said:

“We did not find a statistically significant association between total cases or effective reproductive number of COVID-19 and ambient temperature. We were very cautious with our findings because trying to find a relationship between a specific weather factor and COVID-19, independent of other weather, biological, and social factors is very difficult since the effects of other factors simultaneously complicate and muddle the relationship we’re trying to find.

“Asserting a causal link between temperature and COVID requires caution as the spread of COVID  is dependent on many factors and not temperature alone.  That being said, in colder months, when people tend to stay inside more where air circulation is an issue, it may be a contributing factor to the increase of spread of COVID.”

She said that variants of concern may currently be leading to higher contagion. “But I caution you not to jump to conclusions too soon about causation when things are still evolving and knowledge is still new.”

In another study, the authors looked at a broad range of other factors, including pollution, weather, age, and sex. They found that it was actually UV radiation from sunlight that impacted COVID-19 case incidence; stronger UV from sunlight was associated with lower case counts of COVID-19 and a lower reproductive number. Still, the authors are cautious about interpreting too much into it since other factors, like lockdowns, disinfection habits and mask-wearing, also affect results.
Dr. Bonnie Henry, B.C.’s provincial health officer, has often remarked on ambient temperatures as a factor in the pandemic saying things like this:

“Once we get enough vaccine into people, we’re going to see a dramatic drop-off in the virus. We also know it doesn’t seem to spread as easily in the spring and summer months. So we’ve got a lot of things on our side.”

Yet the World Health Organization has tried to counter assumptions about climate and ultraviolet light with this infographic:

The public can be forgiven for feeling confused when public health experts make comments that seem to appear to conflict with the evidence or with each other. Vaccinations – not changing seasons – remain our best hope for reducing cases of COVID, not sunbathing.

Dr. David Fisman

Dr. David Fisman, an epidemiologist and professor at the University of Toronto, said research on associations between seasons, weather and COVID-19 is complex. And there’s long been a search for the interplay between infectious diseases and environmental factors.

“Ground-level ozone, temperature, and UV radiation are all extremely seasonal, as are humidity, precipitation, and other air pollutants.  As well, school terms and holiday vacations all happen with periodicity so teasing apart effects in ecological data can be really challenging.”

Still, he thinks the potential role of natural ultraviolet light in lower COVID transmission rates seems plausible.
“UV radiation has effects both on pathogens. It busts up their genetic material and effects on hosts. It boosts vitamin D levels which have an important role in immune function.  We found an impressive UV effect a few years ago in studies looking at meningococcal disease in Philadelphia ( though we did not find this to be a generalizable effect in other cities. (”