Coronavirus may be more severe in colder months than warmer ones, and dry indoor air may encourage its spread, new research suggests. Severe COVID-19 outcomes decreased as the pandemic progressed from winter to the warmer months, analysis indicates.


Experts warn that their findings paint a grim picture for the colder weather, when it is thought the coronavirus may re-emerge.

Researchers analysed data from 6,914 patients admitted to hospital with COVID-19 in Croatia, Spain, Italy, Finland, Poland, Germany, the UK and China.

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They mapped this against local temperature and estimated indoor humidity and found that severe outcomes – being taken to hospital, admittance to ICU or the need for ventilation – dropped in most European countries over the course of the pandemic, covering the transition from winter to early summer.

The study, which has not been peer-reviewed, suggests there was a corresponding decrease in the rate of deaths from the disease.

There was roughly a 15 per cent drop in mortality for every 1°C rise in temperature, the King’s College London researchers say. 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.

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

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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
Dr Gordon Lauc

According to the researchers, the changes were too large to be explained by improvements in treatment of the disease, patient age or hospitals becoming overwhelmed during this time. This suggests there is a seasonal influence on the virus.

Although the virus 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 paper suggests.

The researchers suggest 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.

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Dr Gordan Lauc, professor of biochemistry and molecular biology at the University of Zagreb and honorary visiting professor at King’s College London, is senior author of the study.

He said: “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.”

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.

We could experience a double-whammy of serious respiratory diseases, which could cause the NHS to grind to a halt
Dr Simon Clarke

He added: “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.”

Dr Simon Clarke, associate professor of cellular microbiology at the University of Reading, said: “This study shows an association between temperature and severity of symptoms, but it does not demonstrate why that happens. It may be because low temperatures slow down the normal mucus clearing of infecting viruses in our nasal passages.

“If there is indeed a wave of infections in the UK this winter, it could coincide with the annual flu season, which itself puts significant stress on the NHS. We could experience a double-whammy of serious respiratory diseases, which could cause the NHS to grind to a halt.


“It’s essential that the government mount an extensive publicity campaign to ensure that uptake of this year’s flu vaccination is as extensive as possible.”

How can I protect myself from the coronavirus when shopping?

You’ll have seen signs in your local supermarket advising you to keep two metres from others while moving around the store. This is key to reducing your chances of catching the virus while shopping.

The coronavirus SARS-CoV-2 is spread through respiratory droplets that leave our mouth and nose when we cough, sneeze, or sometimes even talk. The droplets sprayed out by an infected person will contain the virus, which could then enter your body via your mouth, nose or eyes (this is why you shouldn’t be touching your face).

Respiratory droplets don’t usually travel more than one metre, so by keeping two metres from others, you’ll reduce the likelihood of being in the firing line. To make it easier to keep your distance, try to shop during off-peak hours, choose a store that’s limiting the number of people who can be inside at any one time, and use self-checkout if you can.

Keeping your hands clean is the other main thing you can do. If possible, wipe the trolley or basket handles with a disinfectant wipe when you arrive at the store. When you get home, wash your hands or use hand sanitiser before and after unpacking your bags.

A US study found that the coronavirus can survive for up to 24 hours on cardboard, and up to three days on hard, shiny surfaces such as plastic, so wiping down your purchases with a disinfectant spray or a soapy cloth before you put them away is another good habit to get into.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.