The arrival of warmer, drier weather may slow the spread of Covid-19 in temperate climates like Europe and America, according to new research which suggests the illness will wax and wane with the seasons.
The new coronavirus which by Friday had killed more than 24,000 people worldwide spreads best in cold, dry conditions an analysis of the first three months of cases found.
Researchers behind the study said their findings joined a growing body of work suggesting the illness could be seasonal like other viruses including flu and polio.
Yet scientists also warned that the picture was unclear only four months into a pandemic, and lack of immunity to a new virus meant the disease many not settle into a seasonal pattern immediately.
Any seasonal effect may also not protect hot and tropical countries from the disease either.
Prof Mohammad Sajadi, of the University of Maryland’s Institute of Virology, said: “What we noticed was that the areas that were hardest hit in terms of deaths as of early March had a similar kind of climate, with low humidity and low temperature. Based on that, and knowing what we know about seasonal respiratory viruses, we hypothesised that this might be acting in a seasonal way.”
Analysis of cases found that the virus spreads best between people in places where the humidity is low and the average daily temperature is between 5C and 11C.
“It is hard to predict,” Prof Sajadi told the Telegraph. “We are hypothesising that it is seasonal, and that in temperate areas we are going to see decreasing cases.”
Other research has had similar findings, though as yet none of the papers have been peer-reviewed, he said.
Scientists at Beijing's Beihang and Tsinghua universities earlier this month reported that “high temperature and high relative humidity significantly reduce the transmission of Covid-19". An increase of just a single degree Celsius could have a significant effect, they found.
Yet Prof Sajadi cautioned against hopes in some warm countries that they will be protected against the virus. Viruses that are seasonal in temperate climates can sometimes be year-round in tropical, or sub-tropical areas, he said.
For a new disease like Covid-19, the fact the population does not have widespread immunity could also mean it can spread quickly because so many are vulnerable, even if the atmospheric conditions are not ideal. Populations densities and the effectiveness of different health systems may also have a large effect.
Dr Michael Skinner, a reader in virology at Imperial College London, said the physical structure of the virus meant it would probably become seasonal, like other human respiratory coronaviruses, when more of the population is immune.
“That does not mean that it will follow seasonal dynamics during the larger epidemics we are currently seeing in many countries,” he said. “There may be just too many people infected, so that most transmission is short range and less subject to environmental constraints.
“It’s also worth appreciating that flu shows seasonal dynamics in the temperate regions of each hemisphere, but is not seasonal in tropics/sub-tropics (where humidity might benefit the virus and where seasons are far less evident, at least in terms of temperature).”
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