Warm and humid weather may combat the spread of the coronavirus disease (Covid-19), a new study suggests, arguing that Asian countries experiencing monsoon may see a slowdown in the transmission of the deadly infection that has claimed 10 lives in India and almost 20,000 around the world.

Analysing Covid-19 infection data from across the world until March 22, two researchers from the Massachusetts Institute of Technology (MIT) show that the cases have a correlation with two parameters: temperature and absolute humidity, which is a measure of water vapour per square metre of area (more humid countries will have higher absolute humidity).

The study finds that 90% of the cases were reported from countries with a range of temperature between 3 and 17 degree C and absolute humidity between 4 and 9g/m3.

“The relationship between temperature, humidity and spread is evolving with time. The association between temperature and the spread of coronavirus is looking weak as we have so many new cases coming from warm US states such as Florida and Louisiana and warm countries such as Brazil, India, Malaysia. Absolute humidity might be an important factor as my paper suggests but it needs to be confirmed with laboratory measurements,” said Yusuf Jameel, one of the paper’s authors.

Indian authorities don’t measure absolute humidity. The average relative humidity in Delhi this month is 47%, in Mumbai, 60%. Relative humidity is a ratio of water vapour in the air to the maximum amount of vapour it can hold, at a given temperature.

Covid-19 is caused by the Sars-CoV-2 virus, which is closely related to the SARS-Cov virus that caused the Severe Acute Respiratory Syndrome outbreak in 2003 and was shown to lose its ability to survive or infect at higher temperatures.

Two previous studies – one from China and another by researchers from Spain, Portugal and Finland – have argued that Covid-19 spread is likely to be constrained by climate and that the virus favours cool and dry climates.

But the MIT study is the first one to suggest that temperature alone may not make a dent on the spread of Covid-19 and that humidity also plays a role. Because humidity is a factor, the rise in temperature and approaching summer may not help many regions in the US and Europe because they remain dry.

“Our analysis shows that the chances of reduced spreading due to environmental factors would be limited across most of northern Europe and North America (USA and Canada) in summer,” the researchers said.

The MIT analysis showed that for each 10 day period between January 22 and March 21, the maximum number of new cases was reported from regions with mean temperature between 4 to 10C and absolute humidity between 3 and 9 g/m3.

Between March 11 and 19, a surge was observed in countries with temperature greater than 18C – this coincides with the initial rush of cases in India – but was still much lower than the jump in cases in countries between 8 and 12C.

“We believe that the 10,000 cases in regions above 18C in the last week is unlikely due to rapid transmission of the virus in the last few days,” the study says. “Based on the current data on the spread of 2019-nCoV, we hypothesize that the lower number of cases in tropical countries might be due to warm humid conditions, under which the spread of the virus might be slower as has been observed for other viruses,” the study added.

What does this hold for India? Jameel said there is a possibility that with monsoon the spread might slow down a bit but that it is too early for the government to rely on such findings. “The government should take appropriate steps assuming that the spread is not going to slow down in the coming days,” he added.