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March 26, 2020

Will summer stop the coronavirus? Here's what the evidence says (so far).

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    Researchers have noted that transmission of the new coronavirus so far has been highest in cooler climates, and they're setting out to determine whether an increase in temperature and humidity in the upcoming spring and summer months could curb the virus' spread.

    Your top questions about COVID-19, answered

    Transmission of new coronavirus has been lower in countries with warm weather, high humidity

    Researchers have noted that many viruses have seasonal fluctuations, such as those that cause the flu, which typically surges during the winter, and Chickenpox, which usually peaks in the spring. According to researchers, those viruses seem to thrive and transmit more easily during colder months and months with more mild temperatures, respectively.

    The patterns have inspired researchers to investigate whether warmer temperatures could impact the new coronavirus' transmission rate in countries that are moving toward their summer seasons—and, over the past few days, multiple research groups have published studies on the matter.

    For one study published in the journal SSRN, researchers at Beihang University and Tsinghua University in Beijing focused on 40 known cases of COVID-19, the disease caused by the new coronavirus, that occurred across 100 cities in China. The study focused on the time period between Jan. 21 and Jan. 23, before Chinese officials put in place strict containment measure intended to curb the virus' spread. The researchers calculated the virus' daily effective reproductive number, or the projected number of secondary COVID-19 cases produced from one case of infection, and found that the virus was more contagious in northern China, where temperatures and humidity were relatively low, and less contagious in the warmer and more humid cities in the country's southeast coast.

    Specifically, the researchers found that a 1-degree temperature increase and a 1% increase in relative humidity were associated with a lower daily effective reproductive number. As such, the researchers wrote that "[h]igh temperature and high relative humidity [can] significantly reduce the transmission of COVID-19," meaning "the arrival of summer and rainy season in the Northern Hemisphere" might "reduce transmission of the" disease.

    Separately, researchers at the University of Maryland School of Medicine's Institute of Human Virology mapped outbreaks of the new coronavirus to determine how temperature and specific humidity, which is the mass of water vapor in a volume of air, might have impacted where the outbreaks were most severe. The researchers, led by Mohammad Sajadi, found severe outbreaks of COVID-19 occurred in a band of the Northern Hemisphere that ran through China, South Korea, Japan, Italy, and the Pacific Northwest. According to the researchers, temperatures in the band were consistently between 5 and 11 degrees Celsius, and relative humidity was between 44% and 84%. The researchers found that specific humidity along the band remained low. The researchers are submitting their study for publication.

    Sajadi said, "It couldn't have been bad luck that these particular places were hit." He explained, "This virus is acting like a seasonal respiratory virus. We could be wrong, but with the data we have, we think that is the most likely scenario."

    An early analysis by researchers at the Massachusetts Institute of Technology (MIT) had similar findings, revealing that the majority of transmissions of the new coronavirus have occurred in regions with temperatures between 3 and 17 degrees Celsius, while countries in the Southern Hampshire, which have average temperatures above 18 degrees Celsius, account for fewer than 6% of global cases of COVID-19.

    "Wherever the temperatures were colder, the number of the cases started increasing quickly," said Qasim Bukhari, a computational scientist at MIT and a co-author of the study. "You see this in Europe, even though the health care there is among the world's best," Bukhari said.

    Does this mean COVID-19 transmission will stop in the summer?

    Despite the findings, researchers and public health experts have cautioned that it's not likely COVID-19 transmission will come to a halt this summer.

    Sajadi said, "People want to know if this is going to go away in the summer," but "seasonal respiratory viruses never really go away."

    Bukhari explained that, even if a virus' transmission is lower in warmer temperatures, the virus still could spread. He noted that, for instance, seasonal viruses like the flu are still present in low levels in people's bodies and on surfaces during the summer, and therefore can be spread to others. "Warmer temperatures may make this virus less effective, but less effective transmission does not mean that there is no transmission," Bukhari said, warning, "We still need to take strong precautions."

    Further, researchers noted that it can take years for new viruses to settle into a predictable pattern. And even if the new coronavirus' spread does wane in the Northern Hemisphere during the summer, researchers warned that the virus could reappear in autumn, when humidity and temperatures fall, the Wall Street Journal reports.

    Overall, health experts at CDC and the World Health Organization who weren't involved in the studies said it simply is too soon to determine whether transmission of the new coronavirus will slow in warmer months.

    "We don't know with the [new coronavirus] how it will behave in the warmer weather," said Andy Pekosz, a microbiologist at the Johns Hopkins University Bloomberg School of Public Health who wasn't involved in the studies. Pekosz continued, "Prepare for having to deal with this in the summer months and assume there will be no respite" (Vaidya, Becker's Hospital Review, 3/23; Hotz, Wall Street Journal, 3/23; Sheikh/Londoño, New York Times, 3/22).

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