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Updated: Mar 10, 2020 01:26 IST

The summer heat may not necessarily kill or significantly weaken Sars-CoV-2, the virus that causes the novel coronavirus disease (Covid-19), say some experts, who have warned against “false hope” following an analysis from China that said the number of cases decreased after average temperatures crossed 8.72 degrees Celsius.

Epidemiologists say that while rising temperature and humidity may lead to modest declines in the potency of Sars-CoV-2, countries should not depend on warm weather to slow transmission, as the new virus may not react to seasonal changes in the way that other seasonal viruses causing flu and common cold do.

The China study found that in cold regions, every 1°C rise in average temperatures led to an a cumulative increase in cases by 0.83, while in the higher-temperature group, every 1°C increase in the minimum temperature led to a fall in the cumulative number of cases by 0.86.

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There may be a best temperature for the viral transmission and the virus’s sensitivity to high temperature could prevent it from spreading in warmer countries during the summer, concluded the study, after analysing the cases from around the world from January 20 to February 4, against meteorological data for January from China and the capitals of the affected countries.

The study from Sun Yat-sen University in Guangzhou in Guangdong province was published on February 22. It is yet to be peer reviewed.

Another study from Harvard T.H. Chan School of Public Health, however, points to sustained Sars-CoV-2 transmission in diverse climate conditions, from cold and dry provinces to tropical locations even within China. Kerala, the state with the first cases in India and where further transmissions were reported this week, has humid weather and a maximum temperature of around 32 degrees Celsius.

“Weather alone, such as an increase of temperature and humidity in spring and summer in the northern hemisphere, will not necessarily lead to decline in case counts without implementing extensive public health interventions,” said the Harvard study, which is also awaiting scientific review. “If Sars-CoV-2 behaves like other betacoronaviruses, it may transmit more efficiently in lower temperature than in the summer heat, but the size of the change is expected to be modest, and not enough to stop transmission on its own,” said Marc Lipsitch, professor of epidemiology at the Harvard School of Public Health, and one of the co-authors of the study.

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“It’s a false hope to say it will disappear like the flu [in the summer] … we can’t make that assumption. And there is no evidence,” said Mike Ryan, executive director of World Health Organistaion Health Emergencies Programme, in a statement.

“We cannot depend on temperature alone. Rising temperatures lower the survivability of viruses on surfaces, but this is a modified virus that has started infecting humans. We don’t know how these changes will affect the survivability of the virus with changes in temperature or humidity,” said Dr Lalit Kant, former head of epidemiology at Indian Council of Medical Research, and communicable disease advisor with Public Health Foundation of India. “Screening, contact tracing, testing, isolating cases, quarantining contacts and social are the way to control infection,” he added.

Massive containment efforts, and not rising temperatures, helped end the 2003 outbreak of Sars-Cov, the virus closest to Sars-CoV-2. “Sars (severe acute respiratory syndrome) did not die of natural causes. It was killed by intense public health interventions in mainland China, Hong Kong, Vietnam, Thailand, Canada, and elsewhere... but in Toronto, Sars resurged after the initial wave and precautions were discontinued. The resurgence confirms that it was control measures that stopped transmission the first time,” said Lipsitch.

With little evidence on how this new virus will behave, epidemiologists are falling back on the behaviour of coronaviruses, particularly Sars-CoV, with which it shares the most similarity among the six other human coronaviruses, including Sars-Cov and seasonal coronaviruses OC43, HKU1, 229E, and NL63, which cause the common cold.

“Seasonal flu outbreaks show new viruses don’t follow seasonality associated with the common cold and flu viruses as people have no immunity against novel viruses, which makes infection in the first wave more potent. This makes it is difficult to predict behaviour. As infections reach a critical mass, people build herd immunity and the symptoms get milder. In countries like India, along with the seasonal peaks, we get flu and common cold cases throughout the year,” said an epidemiologist with the health ministry on condition of anonymity.