Many viruses show seasonal incidence and are more common during the colder winter months, but it is not yet known whether SARS-CoV-2, the novel coronavirus responsible for COVID-19, follows a similar pattern. Previous studies have shown that viral transmission appears to decrease as temperature and humidity rise, suggesting a seasonal pattern, but the results are inconsistent.

In this pre-print study, researchers analysed data from nearly 7,000 patients admitted to hospital with COVID-19 in Croatia, Spain, Italy, Finland, Poland, Germany, the UK and China.

Mapping this information against local temperature and estimated indoor humidity revealed that severe COVID-19 outcomes (hospitalisation, admittance to ICU or need for ventilation) decreased in most European countries over the course of the pandemic, covering the transition from winter to early summer.

There was also a corresponding decrease in the rate of deaths (mortality) from the disease, with a roughly 15% drop in mortality for every one degree Celsius rise in temperature. By contrast, the severity of symptoms and mortality rate remained constant in China during the first wave of the pandemic, which occurred solely throughout the winter.

Analysis of data from more than 37,000 UK users of the COVID Symptom Study app reporting symptoms consistent with COVID-19 showed a similar decrease in the severity of reported symptoms from March through May as UK temperatures rose.

These changes were too large to be explained by improvements in treatment of COVID-19, patient age or hospitals becoming overwhelmed during this time, suggesting that there is a seasonal influence on the virus.

Although SARS-CoV-2 can clearly spread in hot, humid countries in East Asia, it is notable that the severity and mortality from the disease have been lower than in Europe and other more temperate climates.

The researchers suggest that indoor heating during the winter months may also contribute to the spread of the disease by drying out the protective mucus barrier within the nose and airways, making viral infection easier. Additionally, very dry indoor environments created by air conditioning in hot countries (such as in the Southern US) could also contribute to the severity of the disease - a hypothesis that needs further investigation.

Professor Tim Spector from King’s College London added: “This study highlights the importance of gathering long-term data about the incidence, symptoms and progression of COVID-19 from as many people as possible. By understanding the many factors that contribute to the severity and spread of the disease, we can implement effective measures to control it over the coming months.”