Environmental temperature and humidity have an effect on the severity of COVID-19 symptoms, researchers from Europe and China have concluded.

Looking at outcomes relating to than 40,000 patients with COVID-19 amid the pandemic, the research indicates that disease caused by the virus is worse in colder months than warmer ones, and that dry indoor air could encourage its spread.

Data from nearly 7,000 patients admitted to hospital with COVID-19 in Croatia, Spain, Italy, Finland, Poland, Germany, the UK and China was analysed and mapped against local temperature and estimated indoor humidity.

The findings revealed that severe COVID-19 outcomes, including hospitalisation or need for ventilation, decreased in most European countries over the course of the pandemic, alongside the transition from winter to early summer.

Also, there was a corresponding decrease in deaths from the disease, with an approximate 15% fall in mortality for every one degree Celsius rise in temperature.

In direct contrast, severity of symptoms and mortality stayed consistent in China, where the first wave of the pandemic occurred solely throughout the winter.

Data from more than 37,000 people in the UK using the COVID Symptom Study app revealed a similar drop in the severity of reported symptoms from March through May, as the country's climate warmed, which researchers say was too significant to be explained by other factors such as treatment improvement and so indicates a seasonal affect on the virus.

“Our findings point to a role for seasonality in the transmission and severity of COVID-19, and also argue for increased humidity and hydration as a way to combat the virus. This paints a grim picture for the next winter in Europe when more severe “winter” COVID-19 is expected to return - something we are currently observing in the southern hemisphere,” said Dr Gordan Lauc, honorary visiting Professor at King’s and lead author of the study.

“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,” added Professor Tim Spector from King’s College London. “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.”

The findings are particularly pertinent given recent warnings about the ability of the NHS to cope with a second wave of the pandemic in the winter.

A recent report by the Academy of Medical Sciences indicates that there could be a peak in hospital admissions and deaths in January and February 2021 similar to or worse than the first wave in spring 2020, which would coincide with a period of peak demand on the NHS.

It warns that the number of COVID-19-related hospital deaths (excluding care homes) between September 2020 and June 2021 could hit 119,900.