How do seasons influence COVID-19 infections in the Netherlands?

30 March 2022• PRESSRELEASE

Ever wonder why COVID-19 incidence appears to significantly decrease during spring? Together, researchers from the Leiden University Medical Center (LUMC), Open University and Jeroen Bosch Hospital have learned that seasonal human behaviour plays an important role in virus transmission throughout the year.

In a collaborative study between LUMC, Open University and Jeroen Bosch Hospital, researchers have recently shown COVID-19 seasonality can be explained through environmental as well as mobility factors. “For instance, we observed that more hours of sunshine, less indoor recreation visits and more allergens are associated with the reduced transmission of the virus that causes COVID-19”, says Louis Kroes, Professor of Clinical Virology and Head of the LUMC Medical Microbiology Department. Their findings have been published in Environmental Research.

Numerous factors that influence infection

The study looked at the effect of the Dutch Summer season on COVID-19 infections between February and September of 2020. Kroes notes: “Numerous factors that could have an influence on infection rates were taken into consideration. This included solar radiation, temperature, humidity, seasonal allergens (e.g.: pollens) and human behaviour.” Data for human activities were derived from Google Mobility and related to RIVM infection figures. “In this way, we were able to incorporate a very comprehensive set of environmental and mobility parameters in a single model that is significantly more accurate than those which solely rely on environmental indicators”.

Predicting virus infections in spring

With the comprehensive model, the researcher’s main objective was to improve the prediction of virus reproduction during the spring season. “In the Netherlands, this time of the year always coincides with the low-season of flu-like respiratory diseases. It is remarkable that COVID-19 incidence appears to quickly melt away in temperate climate regions. But why?”

Seasonal human behaviours

According to Kroes: “It turns out seasons are more of a trigger for certain human behaviours than absolute temperatures.” The researchers thus discovered that in zones where large seasonal variations occur – like in the Netherlands - virus transmission is not only determined by sunlight and temperature, but also through the behaviour of the population. For example, people tend to spend rather suddenly more time outdoors when the weather becomes nice in spring. Such an explosion in outdoor activities apparently isn’t proportional to the modest temperature changes. “But such aspects of mobility are often overlooked, which makes seasonal factors less accurate in predicting an effect on virus infections than they actually can be!”

Kroes concludes on a high note: “Due to a successful vaccination campaign, higher immunity amongst our population and less clinical disease by the last variants, we can look forward to a positive spring period followed by summer.”

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