Explained: Will summer heat kill coronavirus? Some trends, but experts still advise caution

AIIMS: AIIMS Director Randeep Guleria, a member of the high-level technical committee to guide strategies against COVID-19, told The Indian Express in a recent interview: “The virus probably will not survive for a long duration in an outdoor environment, if the temperature is above 40°. But having said that one must remember two things : we are still having outbreaks in (tropical) areas; second, a lot of us spend time indoors, where the temperature is air-conditioned… Therefore, summer may help in preventing the transmission outside but possibly not indoors.”

Temperature map for Nov 2018-March 2019. Black circles represent countries with significant community transmission (as of March 10, 2020). (Source: Climate Reanalyzer, University of Maine, via Sajadi et all, SSRN)

What research has projected

Researchers at the University of Maryland School of Medicine used weather modelling data to predict that COVID-19 is likely to follow a seasonal pattern. In an online paper in Social Science Research Network, the team led by Dr Mohd Sajadi observed a significant community spread along a east-west distribution approximately between latitudes 30°N and 50°N at similar weather patterns (temperature between 5-11°C and humidity between 47-79%).

These include Wuhan, South Korea, Japan, Iran, Northern Italy, Seattle, and Northern California. Using 2019 temperature data for March and April, community spread is likely to reach north of the current areas at risk, the paper predicts. These include Manchuria, Central Asia, the Caucuses, Eastern and Central Europe, the British Isles, Northeastern and Midwestern US, and British Columbia.

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“Although it would be even more difficult to make a long-term prediction at this stage, it is tempting to expect COVID-19 to diminish considerably in affected areas (above the 30 degrees N”) in the coming months. It could perhaps prevail at low levels in tropical regions and begin to rise again in late fall and winter in temperate regions… . One other possibility is that it will not be able to sustain itself in the summer in the tropics and Southern Hemisphere and disappear,” it states.

Limitations to projections

The researchers acknowledge: “The above factors, climate variables not considered or analyzed (cloud cover, maximum temperature, etc.), human factors not considered or analyzed (impact of epidemiologic interventions, concentrated outbreaks like cruise ships, travel, etc.), viral factors not considered or analyzed (mutation rate, pathogenesis, etc.), mean that although the current correlations with latitude and temperature seem strong, direct causation has not been proven and predictions in the near term are speculative and have to be considered with extreme caution.”

In another study, Massachusetts Institute of Technology researchers Qasim Bukhari and Yusuf Jameel, too, discuss the limitations of correlating the virus spread with temperature and humidity. Their analysis showed that for each 10-day period during January 22-March 21, the maximum number of new cases happened in regions with mean temperature between 4-17°C and absolute humidity between 3-9 g/cubic metre. However, they underline that the spread depends on multiple factors including testing, social dynamics and government policies. “Our results in no way suggest that 2019-nCoV would not spread in warm humid regions,” the paper notes.

The MIT paper too observes that countries and states experiencing high COVID-19 growth such as Italy, Iran, South Korea, New York and Washington exhibit weather patterns similar to original hotspots of Hubei and Hunan. Countries with warmer humid climates such as Singapore and Malaysia had a lower growth rate.

Looking for reasons

The MIT paper discusses probable reasons for the fewer cases in the tropics. “First, it could solely be due to less testing as many of the countries lack good healthcare facilities and may have not done enough testing to detect the actual spread… Indeed, so far, the number of testing in several densely populated tropical countries (Brazil, India, Indonesia etc.) have been very low ,” it states.

“Second, it could be argued that human mobility between China and Europe and between China and the US is high, therefore the number of cases in these regions are high. However… human mobility between China and South-East Asia is also high and therefore the lower growth rate in these countries is perplexing… Sophisticated infrastructure does not exist in Malaysia, Thailand, Philippines, Cambodia and the lower growth rate in South-East Asia cannot be explained by lower human mobility with China or robust health infrastructure,” the paper states.

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“Third, it could also be argued that the government in these countries is taking exceptional measures to stop the spread… which we also know is not true,” it states.

The paper concludes that the lower counts in densely populated countries between 0-30°N (combined population almost 3 billion) “may be due to natural factors that warrant investigation”.

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