New Delhi: Results from five early-stage studies from a varied group of researchers suggest that the spread of COVID-19 may slow down with higher average temperatures and humid conditions in countries approaching summers, for example, India.

Epidemiologists, however, warn that making a causal connection between a disease’s progression and temperature is fraught because it does not account for non-climatic factors such as human behaviour, the capabilities of different health systems and variable government/administrative response. They point out that COVID-19 is currently transmitting widely across the world, in widely varying local climates.

All five studies that have come out between March 10 and March 23, 2020, have analysed COVID-19’s relation with temperature and humidity in China and in other countries that are currently dealing with a large number of cases. These studies have come out in ‘SSRN’ and ‘medRxiv’, both well-known global repositories of studies that have not yet been peer-reviewed.

Some epidemics researchers that IndiaSpend spoke to questioned the studies’ methodology and said their results were not conclusive. “We still do not know from empirical laboratory studies how temperature and humidity affects COVID-19 persistence and therefore whether cases are likely to reduce as the temperatures increase in some countries,” Bethan Purse, head of Disease Ecology Group at the Centre for Ecology & Hydrology, UK, told IndiaSpend.

In such studies, it is important to account for non-climatic factors that vary over time, Purse said, such as “the behaviour of people--average social distances in different stages of lockdown--which affects transmission, and the amount of testing effort, that affects the number of cases observed”.

Although the contagiousness of COVID-19 may modestly decline in warmer and wetter weather, “it is not reasonable to expect these declines alone to slow transmission enough to make a big dent”, wrote Marc Lipsitch, the director of the Center for Communicable Disease Dynamics at Harvard’s T.H. Chan School of Public Health in the US, in an analysis on March 11, 2020.

In India, the summer is fast approaching. The number of COVID-19 cases has been rising steadily, as has the number of samples being tested, and states that have tested more samples have detected more cases, we reported on March 25. So far, COVID-19 cases have been detected in 27 states and union territories. India had confirmed 724 COVID-19 cases by March 27.

“How other coronaviruses have reacted to temperature and humidity in the past may not be true for COVID-19,” said an epidemiologist from the Indian Council of Medical Research, speaking on condition of anonymity. “We do not know how the virus will react in Indian summers, it is too early to say anything.”

The five studies

In a paper first posted on SSRN on March 19, 2020, (later updated), researchers from the Massachusetts Institute of Technology (MIT) said that until March 21, 90% of COVID-19 transmissions had occurred in regions that had temperatures between 3 and 17 degrees Celsius (°C) and absolute humidity (the measure of moisture in the air, regardless of temperature) between 4 and 9 grams per cubic meter (g/m3) during the outbreak.

In contrast, fewer than 6% of transmission cases occurred in countries with mean temperatures above 18°C and absolute humidity more than 9 g/m3.

Researchers analysed the COVID-19 cases and the temperature and humidity data from across the affected countries for the time period between January 20 and March 21, 2020.

With summers approaching, the temperature range may go higher, but any reduction in the impact of COVID-19, if at all, may only happen at temperatures above 25°C, the authors wrote.

Similar studies analysing global trends

Other pre-print (not peer-reviewed) papers have drawn similar conclusions, correlating hot and humid regions and COVID-19.

In a paper posted on SSRN on March 9, 2020, researchers from the University of Maryland’s Institute of Virology found that the COVID-19 virus transmits more effectively between humans when the humidity is low and the average temperatures are between 5 and 11°C.

“The distribution of significant community outbreaks along restricted latitude, temperature, and humidity are consistent with the behaviour of a seasonal respiratory virus,” the Maryland study said.

Other coronaviruses, which usually cause common cold symptoms among humans, have been shown to affect people the most between December and April, and are undetectable in summer months in temperate regions, the study added.

Another study, posted on March 16, 2020 in medRxiv and authored by two researchers from Spain and Finland, said 95% of positive cases across the world as of March 10, 2020 occurred at temperatures between -2 and 10°C.

Studies analysing COVID-19 trends specifically in China

Two earlier studies, one by Chinese researchers and another by an independent Indian researcher, also establish similar links. Both the studies posted in SSRN analysed the COVID-19 outbreak specifically in China for different time-periods and persisting weather conditions.

One team led by researchers from Beihang University in China studied COVID-19 transmission specifically in 100 Chinese cities with more than 40 cases, between January 21 and January 23, 2020, before the government announced interventions on January 24.

Hot and humid cities saw a slower rate of spread than cold and dry ones, the study said. High temperature and high relative humidity significantly reduce the transmission of COVID-19, it said--an increase of just 1°C in temperature and 1% in relative humidity substantially lowered the virus’s transmission, said the study posted on March 10, 2020.

“In July, the arrival of summer and rainy season in the northern hemisphere can effectively reduce the transmission of the COVID-19; however, risks remain in some countries in the southern hemisphere (e.g. Australia and South Africa),” the authors wrote.

An Indian researcher subsequently studied the relation between COVID-19’s infectiousness and weather data for 24 provinces of China (of 26--excluding Hubei and Tibet) for cases that came up between January 21 and February 16, 2020. This study came out on March 23, 2020.

On average, each additional centigrade above 5°C was found to be associated with a 1.2-percentage-point decrease in the rate of increase in confirmed cases, the study said. It did not find any substantial link between humidity and COVID-19 infectiousness, however.

While the Chinese researchers tried to understand COVID-19’s transmission before the government interventions to try and assess the true transmission rates, the Indian researcher and other studies mentioned above took into account the government interventions too.

“Our data sample was limited in average temperature from 6-20 degrees,” the author wrote. “It is quite possible that these results do not hold true at above 20 degrees.”

Do these studies change our understanding of COVID-19?

Experts say several factors in addition to temperature and humidity will affect how COVID-19 plays out in different countries, including their density of population, healthcare infrastructure, and especially the different policy interventions. All of these factors make it difficult to predict how COVID-19 will spread in different climatic conditions.

The fact that COVID-19 is currently transmitting so widely across the world, in widely varying local climates, suggests that transmission is not particularly limited by ambient conditions of temperature and relative humidity but will instead be limited by local human behaviour, health system factors and government policies, said Purse of Centre for Ecology & Hydrology.

Many experts are making conclusions about COVID-19 based on the behaviour of closely related viruses such as SARS (an earlier coronavirus) that seem to persist for longer in cooler and drier conditions, she added.

All five studies rely on an understanding of how other types of coronaviruses including SARS (see this and this) and common influenza coronaviruses (read this) have shown seasonality in the past.

Experts, however, say that drawing a parallel between a new coronavirus and past outbreaks could lead to incorrect results. “SARS did not die of natural causes. It was killed by extremely intense public health interventions in mainland Chinese cities, Hong Kong, Vietnam, Thailand, Canada and elsewhere,” Lipsitch of Harvard wrote in his analysis.

With an example of the resurgence of SARS in Toronto after the initial wave was over, Lipsitch said the resurgence was eventually linked to a case from the first wave. It confirmed that control measures had stopped transmission the first time, not warmer weather.

“Predicting how a novel virus will behave based on how others (influenza coronaviruses) behave is always speculative,” Lipsitch wrote, adding that “seasonal viruses that have been in the population for a long time behave differently from viruses that are newly introduced into the population”.

Even if COVID-19 is proven to transmit somewhat more efficiently in winters than in summers, the mechanisms responsible for this are unknown. The size of the change is expected to be modest, and not enough to stop transmission on its own, Lipsitch said. “We expect that SARS-CoV-2 (COVID-19), as a virus new to humans, will face less immunity and thus transmit more readily even outside of the winter season.”

Scientists should resist the urge to draw conclusions from analyses that simply match patterns in disease with patterns in climate over space and time, but should rather integrate a wider range of behavioural, political factors and status of health systems into their analyses, Purse said.

(Tripathi is an IndiaSpend reporting fellow.)

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Bhasker Tripathi

    For the last eight years, Bhasker has specialised in development journalism, writing on climate policies, environment, agriculture, energy and gender and their intersections. He started his career with Gaon Connection, a rural-media platform, which he helped build as a founding team member. His last job was coordinating Land Conflict Watch, a non-profit data journalism project that analyses land conflicts across India. In 2014, he was named as one of the best young journalists from developing countries by the United Kingdom’s Foreign Press Association & Thompson Foundation. In 2019, he was awarded as the 'Young Environment Journalist' of the year by Center for Media Studies & Indian Himalayas Climate Adaptation Programme, a joint programme of Indian and Swiss governments.