Multiple early studies provide evidence of statistical ties between temperature and humidity ranges and the geographic regions where this virus has thrived. While none of these studies has been peer-reviewed, they all point to the same general possibility: The pandemic could ease in parts of North America and Europe during the summer months, although it could then come roaring back in the fall.

Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases (NIAID) and a visible figure on the White House coronavirus task force, said at a Wednesday press briefing that a seasonal cycle to the pandemic is possible, perhaps even likely.

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“... I think it very well might. And the reason I say that is that what we’re starting to see now in the Southern Hemisphere, in southern Africa and in the southern hemisphere countries, is that we’re having cases that are appearing as they go into their winter season. And if, in fact, they have a substantial outbreak, it will be inevitable that we need to be prepared, that we’ll get a cycle around the second time.”

The anecdotal evidence suggesting a possible connection between the rate of spread of covid-19, the illness the novel coronavirus causes, and temperature emerged early on. One of the most puzzling things about the unfolding pandemic in Asia is why many countries and territories in the region have apparently been spared major, rapidly growing outbreaks, despite close contacts with China, where the outbreak began.

The success of places like Taiwan, Singapore and Hong Kong in tamping down the epidemic could be ascribed to quick and decisive action by their governments as well as effective precautions by local residents to enforce social distancing.

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But that doesn’t fully explain why Cambodia, Thailand, Vietnam and the Philippines have largely been spared mass outbreaks of the disease, at least not on the scale that has been seen in Western Europe and the United States. All had huge numbers of Chinese visitors at the start of the year, have crowded cities and relatively simple health-care systems.

Cambodia was criticized early in the epidemic for failing to close its borders with China and even failing to promote the wearing of masks, with Prime Minister Hun Sen accused of putting his relationship with his strongest foreign backer ahead of the needs of his people.

Nearly two months since its first case, the country has recorded only 91 cases of the coronavirus, according to a Johns Hopkins University database, with almost all the infected people thought to have contracted the virus abroad.

Compare that to the United States, which added more than 10,000 cases during each of the past three days.

In recent weeks, there has been a rise in cases in many hotter countries as testing has expanded, although many of the cases have been imported from Europe or the United States. In the Southern Hemisphere, Australia has seen a sharp rise in cases, but most are thought to have been acquired abroad — while a large number remain under investigation.

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There are countless examples where the virus has been transmitted between people within warmer and more humid places, and several examples of infection clusters within hotter countries.

One of the biggest clusters was a four-day religious gathering at a mosque in Malaysia from Feb. 27 to March 1, with more than 800 people testing positive for the virus and carrying the infection to at least a half-dozen other nations.

Hundreds of people in Thailand are thought to have contracted the virus at boxing events held in Bangkok, and others in bars. However, none of these tropical countries has seen the virus grow exponentially as it has in Italy, Spain, France and the United States.

Studies show links between weather and virus transmission, but are not definitive

A new study uploaded to the research site SSRN over the weekend finds that 90 percent of the coronavirus transmissions so far have occurred within a specific temperature (37 to 63 degrees) and absolute humidity range. For areas outside this zone, the virus is still spreading, but more slowly, according to the study by two scientists at the Massachusetts Institute of Technology.

The paper, which was shared with the public before it was peer reviewed for the benefit of public health officials, notes that even in warm parts of the United States, such as Texas and Florida, cases are not exhibiting the same growth rates as they have in New York and Washington state.

The best-case scenario, according to the study’s authors, is that the rate of spread in parts of the Northern Hemisphere will slow as temperatures warm and humidity increases.

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However, they warn that if absolute humidity — the total mass of water vapor in a volume of air — is a more important factor in coronavirus transmission, then many cities seeing a surge in cases, including Boston and Paris, may not get much of a respite in the summer. This is because these areas do not get hot and humid enough to significantly reduce the growth rate, outside of a small window in July.

The researchers caution that even in warm, humid regions, governments need to implement measures to slow the spread of the virus, since warm weather could inhibit but not eliminate the spread of the virus. In these areas, “the general public should not take it as evidence that they can go around with their daily life and not take any precautions,” said study co-author Qasim Bukhari of the McGovern Institute for Brain Research at MIT.

“For many countries there may not be a window at all” with sufficiently high temperatures and humidity to slow the spread of the virus, Bukhari said.

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Study co-author Yusuf Jameel says even tropical countries are seeing this virus spread, albeit at slower growth rates. “You are not safe by any chance,” he warns. “It’s in almost all the countries.”

Jameel says some countries in the Southern Hemisphere, where winter will soon set in, could see their temperature and humidity levels fall within the range associated with the most rapid growth rates. He urged Australia and Brazil in particular to “brace” for what may lie ahead.

The new MIT study is similar in findings to research published this month that also focused on the temperature and humidity link. “Both of our studies show that areas with low absolute/specific humidity and low temperatures are those that are experiencing significant community transmission,” Mohammad Sajadi of the Institute of Human Virology at the University of Maryland School of Medicine, a co-author of a previous study, said via email.

Uncertainties will take time to resolve

Jeffrey Shaman, the director of the climate and health program at Columbia University’s Mailman School of Public Health, said laboratory studies that investigate how the virus behaves in different environments will provide more meaningful insights into covid-19, compared to the statistical studies looking at temperatures and virus spread.

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Regarding the new MIT study, which he was not involved with, Shaman said, “You can’t put a lot of stock in that. That’s a 10,000-foot statistical data set.” He added, “It’s not a smart study from my perspective.”

“We need more laboratory testing,” he said. “The jury is out, [the] evidence from these statistical studies is not convincing to me yet.”

Other coronaviruses do exhibit a seasonal cycle, which helps explain the focus on this topic, he said. “There’s this tantalizing possibility that we’ll catch a break in our part of the world.”

He warns that the susceptibility of the population to the new virus is so high that even if it’s less transmissible during the warm season, it would still spread significantly even in warmer and more humid areas.

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David Heymann, a professor at the London School of Hygiene and Tropical Medicine who was not involved in the MIT research, said the statistical studies don’t hold up compared to how the virus is behaving so far.

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“The virus is transmitting quite easily in Singapore and in Hong Kong where it is now late spring. There is nothing better than observation — if the virus has any characteristics that impede its transmission in hot and humid climates, it has not yet manifested them in the tropics,” he said.

Laboratory studies underway

A team at the University of Utah is beginning work this week with inactive coronavirus particles, which lack the virus’s full RNA and are thus not infectious, to determine how they respond to various environmental conditions.

The research, funded by a rapid response research grant from the National Science Foundation, seeks to examine the mechanical reasons for how droplets of the virus behave and whether they lose structural integrity in different conditions, said Saveez Saffarian, a professor in the university’s Center for Cell and Genome Science. “I am going to poke them and study them,” he said of the virus particles, to try to decipher “what makes the temperature effects work.”

He hopes to have more information on the sturdiness and behavior of the virus in the next month.