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Warmer weather may slow spread of disease, studies suggest

Recent research indicates spring and summer could bring a fall in transmissions, but scientists warn this alone will not be enough to stop the outbreak

Warmer weather could help slow down the spread of coronavirus, research suggests, although scientists have warned the changing of the seasons will not put a stop to the pandemic by itself.

Multiple early studies indicate Covid-19 may find it harder to transmit between people in summer than in winter.

Cases of similar viruses such as influenza tend to wane during the summer, with warmer temperatures, increased humidity, and changes to human behaviour and our immune systems thought to play a role in making those diseases less infectious.

This has fuelled hopes that the arrival of spring in the northern hemisphere, which has been worst-hit by coronavirus, will bring respite in the rapidly spreading coronavirus pandemic.

US president Donald Trump and the UK government’s chief scientific adviser, Sir Patrick Vallance, have both suggested there may be fewer transmissions of Covid-19 during the summer.

While there is limited data available, several recent studies provide evidence this could be the case.

Scientists at China’s Beijing and Tsinghua University, who examined the spread of the disease in 100 Chinese cities, found “high temperature and high relative humidity significantly reduce the transmission of Covid-19”.

The number of people infected per carrier of the disease fell relative to increased temperature and humidity, their study found.

Researchers at the University of Maryland and Massachusetts Institute of Technology (MIT) in the US came to similar conclusions.

“Based on what we have documented so far, it appears that the virus has a harder time spreading between people in warmer, tropical climates,” said Mohammad Sajadi, associate professor at the University of Maryland’s Institute of Human Virology, who lead a study which used weather data to predict the spread of Covid-19.

The researchers found all cities with significant outbreaks of the coronavirus – such as Wuhan, Tokyo and Milan – have similar winter climates with average temperatures of 5C to 11C and humidity between 47 and 79 per cent.

Their paper, published on the open-data website SSRN, warns the epicentre of the outbreak could edge north to northern Europe and Canada over the next few weeks. uploaded to the same site, found that 90 per cent of coronavirus transmissions up to 22 March occurred in regions with an average temperature between 3C and 17C and a particular humidity range.

The paper notes that warmer US states such as Arizona, Texas and Florida have not seen coronavirus outbreaks on the same scale as Washington and New York, which has recorded more than 5 per cent of global cases.

But the MIT researchers warned their results “in no way suggest” Covid-19 cannot spread in warm, humid regions and said “effective public health interventions should be implemented across the world to slow down the transmission”.

They added the impact of warmer weather in slowing the outbreak could be “limited” in northern US and European cities,” where average temperatures remain 20C or lower for most of the year.

Other scientists have sounded notes of caution about the prospects of spring and summer slowing the coronavirus outbreak.

Marc Lipsitch, professor of epidemiology at the Harvard School of Public Healh, said “while we may expect modest declines in the contagiousness of [the virus] in warmer, wetter weather … it is not reasonable to expect these declines alone to slow transmission enough to make a big dent”.

Because Covid-19 is a new disease, he added, it will be “face less immunity and thus transmit more readily even outside of the winter season” compared to seasonal flu.

Academics at the University of Oxford said emerging data “appears to suggest that cold and dry conditions may facilitate the spread of the novel coronavirus”, but urged caution as most of the existing studies have yet to be peer-reviewed.

In an analysis of the available research, they added: “Although warmer climates may slow the spread of [coronavirus], relying on weather changes alone to slow the transmission of Covid-19 [is] unlikely to be sufficient.”

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