A recent study asks which factors impact the progression of the COVID-19 pandemic. The authors investigate meteorological factors and public health measures across multiple geographical locations.

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A new study concludes that public health measures slow the virus, but increases in temperature do not.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

As the COVID-19 pandemic rumbles on, scientists are observing its features from every possible angle. Some scientists are trying to identify factors that reduce the speed of its spread.

The authors of a recent study, published in the Canadian Medical Association Journal, asked whether school closures and other public health interventions result in a slowdown of the COVID-19 pandemic.

They also assessed whether geographical and meteorological factors play a part in curtailing the pandemic, including latitude, temperature, and humidity.

As expected, the authors found that “public health interventions were strongly associated with reduced epidemic growth.” However, more surprisingly, they concluded that the spread of SARS-CoV-2 was not associated with temperature.

Scientists have established that influenza outbreaks — the most well-studied respiratory viral outbreaks — are associated with changes in climate; they tend to occur during colder months. However, questions remain as to why these viruses display such seasonality.

Although scientists are still investigating the matter, reduced case numbers in hotter months are likely to be due to higher temperatures, higher humidity, or higher solar radiation.

As the authors explain, “These three characteristics are all associated with geographic latitude, a measure that can be determined effortlessly and with precision.”

The fact that schools tend to close over the summer months could also play a part in reducing the risk of influenza outbreaks.

For the recent study, the researchers focused on data taken from ‘geopolitical areas with documented outbreaks of COVID-19’ during 2 separate weeks.

They classed the first week — March 7–13, 2020 — as the exposure period. The authors took note of latitude, temperature, humidity, school closures, restrictions of mass gatherings, and physical distancing measures.

Then, they measured the increase in the number of COVID-19 cases 14 days later to allow for the incubation period. They took their measurements for the week, March 21–March 27, 2020. In other words, they looked at the relevant variables during week one and measured how much these variables influenced COVID-19 rates 2 weeks later.

According to the authors, the chosen interval of 14 days reflects “the assumed time between transmission of SARS-CoV-2 and reporting of confirmed COVID-19 cases.”

During their analysis, the scientists controlled the data for a wide range of variables with the potential to skew the results. The variables included altitude, gross domestic product, percentage of inhabitants aged 65 years or older, population density, proximity to regions with established epidemics, such as Wuhan, China, and average life expectancy.

In all, the analysis used data from 144 geopolitical regions, including 375,609 cases of COVID-19. The scientists took data from the World Health Organization’s (WHO) Situation Report 61. This included state-level information for the United States and Australia, province- and territory-level data for Canada, and countrywide data for the rest of the world.

They excluded China because, at that point, the epidemic was waning there. They also excluded Italy, Iran, and South Korea because the epidemic was peaking in those countries.

The authors conclude that COVID-19 epidemic growth “was not associated with geographic latitude, nor with temperature during the exposure period 14 days before.”

This came as a surprise to the researchers. One of the authors, Dr. Peter Jüni from the University of Toronto, Canada, says, “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.”

The scientists did identify a relationship between epidemic growth and relative and absolute humidity. When they carried out more detailed analyses, these relationships weakened. However, the authors believe that both dimensions of humidity may play a minor role but that “this remains hypothetical.”

With some countries considering easing their public health interventions, the findings are important. The authors write:

“[I]t is of considerable importance that we found strong negative associations with three public health interventions commonly used to contain the COVID-19 pandemic: restrictions of mass gatherings, school closures and measures of social distancing.”

Another author, Prof. Dionne Gesink, explains, “Summer is not going to make this go away. 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.”

The authors also note certain limitations to their research. For instance, not every country collects data on COVID-19 in the same way, making it difficult to draw reliable estimates and comparisons. Similarly, the number of SARS-CoV-2 tests each country carried out varies wildly.

Overall, though, the scientists conclude that seasonality is unlikely to play a significant role in the epidemiology of COVID-19.

Their take-home message is that “[o]nly area-wide public health interventions were consistently associated with reduced epidemic growth, and the greater the number of co-occurring public health interventions, the larger the reduction in growth.“