March 26, 2021
Does weather affect the transmission of COVID-19?
Weather was definitely not the primary driver of COVID-19 transmission in the first year of the pandemic. COVID-19 transmission dynamics in 2020 appear to have been controlled primarily by government interventions rather than meteorological factors. Other relevant drivers include changes in human behavior such as physical distancing, mask wearing, and travel routes as well as demographics of affected populations and, more recently, virus mutations. We are not saying that weather is not a factor at all in COVID-19 transmission. Is it an influence? Possibly, but the direct evidence is mixed. Do we expect that a disease like this might, eventually, fall into a seasonal pattern like the flu? Yes, a few studies are pointing toward that, and experience from other diseases suggests that it’s likely.
“Direct evidence is mixed on weather’s role in COVID-19 transmission.”
When we talk about seasonality, what do we mean? Behavior is a huge part of it. Does it mean that cases go up in the winter because the cold outside favors virus transmission, or because when it’s cold outside people spend more time inside with one another? It could be both. Laboratory studies of SARS-CoV-2, the virus that causes COVID-19, have yielded some evidence that the virus survives longer under cold, dry, and low ultraviolet radiation conditions. However, these studies have not yet indicated if direct meteorological influences on the virus have a meaningful influence on transmission rates under real world conditions. People are more susceptible to infection when it’s cold and dry outside, and that’s when we gather indoors. And the buildings we gather in are designed to be low ventilation in order to be more energy efficient. All of that contributes. Most disease transmission happens inside and we live our lives indoors. The Christmas holiday was associated with a COVID-19 spike in the United States, but there was also a Christmas-related spike in South Africa, which celebrates Christmas in their summer.
What should U.S. state and federal agencies take away from the World Meteorological Organization’s task force’s findings as the weather warms up?
At this state, evidence does not support the use of meteorological and air quality factors as a basis for governments to relax their interventions aimed at reducing transmission. We saw waves of infection rise in warm seasons and warm regions in the first year of the pandemic, and there is no evidence that this couldn’t happen again in the coming year.
Why did the World Meteorological Organization convene a task force to examine the science behind environmental influence on COVID-19 transmission?
A staggering number of papers and pre-prints were issued last year based on unreliable data and questionable modelling techniques that did not account for numerous factors that were co-occurring at the beginning of the pandemic. If you don’t account for those other factors then you can incorrectly conclude that a trend in COVID-19 data is a product of Meteorology and Air Quality, or MAQ, effects. For example, if mask mandates and travel restrictions come into place as spring is turning into summer, you might find that COVID-19 spread slows while it’s getting warmer. But that doesn’t mean it slowed because the weather was getting warmer. All of these rapidly posted papers were leading to confusion, so in the summer of 2020 the WMO’s Research Board convened the task team to perform a rapid summary of expert-informed assessment of the peer-reviewed literature before the January 2021 cutoff. We didn’t examine every peer-reviewed study. We included studies that we believed were indicative of the best state of the science. There was a lot of chaff out there.
‘Studies published without peer review can influence public health decisions and public perceptions about disease risk.’
Many researchers were publishing questionable analyses with questionable data, and world leaders like President Trump and President Bolsonaro of Brazil were quoting these studies as if they were scientific fact. We have to avoid situations in the future where leaders can point to flawed studies by scientists in order to claim a disease will get better because of the weather. We don’t want to slow down scientific progress in fast evolving situations like a pandemic but we have to figure out how to do some level of quick vetting of these studies. It’s important, because a study that is promoted before peer review, or one that is published in a rapid publication journal with low publication standards, have the real potential to influence public health decisions and public perceptions of disease risk. For this reason, it is critical that researchers, publishers, and information providers maintain high standards for analysis and evaluation of emerging diseases. Efforts to provide open, timely, and quality-controlled COVID-19 data have been disjointed and point to the need for a reporting infrastructure that supports data management and dissemination for analysis of epidemic diseases.
Could COVID-19 be influenced by the weather in the future if it persists like seasonal flu?
The report looks at the potential role of seasonality. Respiratory viral infections frequently show some form of seasonality, in particular the autumn-winter peak for influenza and cold-causing coronaviruses in temperate climates. This has fueled expectations that, if it persists for several years, COVID-19 will prove to be a strongly seasonal disease. That is still, for the most part, an expectation rather than a demonstrated fact. While some studies have begun to detect seasonality in the COVID-19 data record, these studies are highly debated. Overall, epidemiological studies of COVID-19 have offered mixed results regarding the meteorological sensitivity of the virus and the disease.
‘Results do indicate that chronic exposure to poor air quality has been a major factor in COVID-19 death.’
But that expectation of seasonality does have strong support from those other, similar respiratory viruses. So Meteorology and Air Quality factors may eventually prove to be an important input for monitoring and forecasting of COVID-19 in the coming months and years.
Do pollution and air quality influence the impact of COVID-19?
It’s pretty striking that even with all the limitations that exist in the COVID-19 data record, results do indicate that chronic exposure to poor air quality has been a major factor in COVID-19 deaths. That result is consistent and convincing, and I don’t expect it’s going to go away. It’s important to distinguish between severity and transmission, though. The evidence that chronic and short-term exposure to air pollution exacerbates symptoms and increases mortality rates for COVID-19, and for some other respiratory diseases, is quite strong. But there is much less understanding of how pollution impacts the actual airborne transmission of SARS-CoV-2, the virus that causes COVID-19.