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Nurses pose for photos as they wait to test people for coronavirus at a drive-thru operation at Ramkhamhaeng Hospital on March 19, 1919 in Bangkok, Thailand.

Tim de Waele/Getty Images

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On Jan. 13, two days before Chinese authorities played down the risk of a new virus spreading between people, authorities in Thailand confirmed their first case of what is now called COVID-19.

Bangkok is one of Asia’s most important travel hubs. Before the outbreak, its airport had more scheduled flights from China than any other country, including 45 a week direct from Wuhan, the epicentre of the deadly pandemic. Flights to Bangkok from Wuhan continued until the city began a full lockdown on Jan. 23.

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And yet epidemiologists have been puzzled by the fact that Thailand has seen so few cases of COVID-19. While it spread at terrifying speed to tens of thousands of people in South Korea, Italy and now most other countries, the number of confirmed cases in Thailand remains comparatively small: 322, including 50 announced Friday, and just a single death. In nearby Malaysia, authorities have confirmed 1,030 cases and three deaths. Taiwan, Singapore and Hong Kong, meanwhile, have ranked among the world’s most effective jurisdictions in combating the spread of the virus.

Those places all have relatively advanced systems for epidemic detection – Singapore is considered the global leader, while both Malaysia and Thailand have led their peers in Southeast Asia.

But those jurisdictions also share a geography of more southerly latitudes that bring hotter, more humid weather.

The most rapid spread of the virus has taken place in areas with cooler climates: central China, South Korea, Iran, Northern Italy and continental Europe.

It appears that cooler climates “allow the virus to spread further and faster than in the tropics,” said Lam Sai Kit, an emeritus professor at the University of Malaya who is one of Malaysia’s most prominent epidemiologists. "Of course, a lot of other factors are involved. Climate is one. But containment and mitigation measures – lockdowns – all these play a role in controlling the spread of the virus.”

The science on COVID-19 and climate is not settled. At least one paper from Chinese researchers found no link between temperature and transmissibility. Thailand, Malaysia, Hong Kong, Singapore and Taiwan have all reported a surge of cases in recent days, as the global spread of the virus intensifies.

But several other research groups, including another in China, have examined the data and found a less vigorous spread in warmer climates. It’s a critical question, as the speed of spread of the annual flu is typically influenced by the changing of the seasons. If COVID-19 similarly proves to be transmitted less easily in warmer weather, the northern countries struggling against an out-of-control epidemic may have reason to hope for some reprieve as spring arrives.

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A study led by researchers at the University of Maryland found exactly that, saying “significant community spread” of COVID-19 has taken place in a latitude window – between 30 and 50 degrees north – while there has been “a lack of significant community establishment in expected locations that are based only on population proximity and extensive population interaction through travel.”

“If our hypothesis is correct, then there should be a significant decrease in cases in the areas which are currently heavily affected, especially with the combined public health interventions,” said Mohammad Sajadi, a scholar at the University of Maryland’s Institute of Human Virology. He cautioned, however, that because COVID-19 is a novel virus it could, like H1N1, “cause outbreaks even in the offseason.”

But another report led by researchers at China’s Beihang University examined 100 Chinese cities and found that “high temperature and high relative humidity significantly reduce the transmission of COVID-19.”

Warmer temperatures can reduce the contagiousness of the virus by roughly 20 per cent, one of the researchers said in an interview. The Globe and Mail is granting the person anonymity because his institution does not permit interviews with foreign media.

“In places with higher temperature and humidity, the transmissibility will be lower,” the researcher said. But, he stressed, “temperature and humidity are not enough in themselves to make the virus disappear.”

Indeed, while warmer weather may bring “modest declines in the contagiousness” of COVID-19, it’s unlikely to be “enough to make a big dent” in places already in an epidemic situation, Marc Lipsitch, a professor of epidemiology at the Harvard T.H. Chan School of Public Health, has written.

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But a warmer climate may help explain the relatively slow spread of the virus in Thailand, which has designated 35 assessment centres for COVID-19 and is considered by Western epidemiologists to be testing as assiduously as many of the large Western democracies engulfed by the outbreak.

Taiwan, Singapore and Hong Kong have also seen relatively limited epidemics, although all three have also been held up as global examples of how the virus can be countered through aggressive testing, contact tracing and isolation measures.

At the same time, even in the warmest areas of Asia, anxiety about COVID-19 is rapidly becoming more intense. Malaysia, where 60 per cent of cases have been linked to a single mass religious gathering, will dispatch its armed forces on Sunday to enforce an order that bans all movement except for official duties and essential services. Thailand has suspended schools and begun requiring certificates of health clearance from many arriving travellers – including Thai citizens – after outbreaks related to people who went to bars and boxing matches. A rise in cases has prompted stricter measures in both Singapore and Taiwan, which began denying entry to foreign travellers this week.

Australia, too, has barred entry to international visitors, and its authorities are considering suburb-by-suburb lockdowns after the country said Friday that it now counts 876 cases and seven deaths. The influence of climate on COVID-19 is especially important for Australia, where it will soon be winter.

“There is very likely to be seasonality,” said Joel Negin, a Canadian who is head of the School of Public Health at the University of Sydney. “I think as Canada approaches summer, there will be a reduction. And Australia is entering its winter, and I suspect we will see a much higher caseload.”

Part of the reason lies in human behaviour: “In general, flus do worse where people are outside, where they are getting more fresh air, where there’s a breeze,” Prof. Negin said. “When people are inside and confined, flus do quite well in general.”

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For now, however, COVID-19 seems “to be spreading in both northern and southern hemispheres,” said David Issacs, a prominent Australian pediatric specialist in infectious diseases.

“At the moment, this particular coronavirus seems to be spreading regardless of seasonality,” he said. ”The thing that seems to make the most difference is whether you recognize cases early and intervene early. And if you do that, that limits spread.”

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