“So we don’t think it had anything to do with the lockdown,” said report co-author Professor Michael Ward, an epidemiologist and chair of veterinary public health at the University of Sydney’s School of Veterinary Science.
“Every time we look at it, we find humidity is linked with cases. And we cannot really explain that any other way. I cannot come up with any other explanations.”
The same trend has been found in a study of China’s COVID-19 outbreak, and during outbreaks of the related coronaviruses that cause SARS and MERS.
Two leading experts in virology and infection control said the link between humidity and transmission was possible, but that it was far from proven and the study had limitations.
“It is provocative,” said Professor Tony Cunningham, director of the Centre for Virus Research at the Westmead Institute, “but not definitive.”
The researchers gathered postcode data from Sydney’s 1203 local infections between February and May and correlated it with weather readings from the closest weather station.
The study has flaws: the researchers used people’s residential addresses, not where they caught the virus. And it looked at outside humidity, but most virus spread occurs inside.
Nevertheless, a study in China found the same trend: for each 1 per cent increase in relative humidity on a cold day, new COVID-19 infections fell between 11 and 22 per cent.
A huge study of 166 countries found evidence for the same link, although at a much lower level.
And the same link was found in the SARS and MERS epidemics, Professor Ward said.
SARS-CoV-2, the virus that causes COVID-19, is spread by droplets when infected people cough and sneeze.
In dry air, these droplets shrink, which allows them to “get further down the respiratory tract”, Professor Cunningham said.
“Some of the data is a bit iffy, and some of it a bit old. But it certainly supports the concept,” he said.
Professor Cunningham was co-lead on a report from the Rapid Research Information Forum, which is led by the Australian Academy of Science, that reviewed evidence on the effect of weather on COVID-19 for the federal government. It concluded there was some evidence dry air might make it spread faster, but no solid proof.
Associate Professor Philip Russo, president of the Australasian College for Infection Prevention and Control, said it was possible drier air meant the virus survived longer when big droplets fell on surfaces.
“Humidity and temperature also influences behaviour regarding crowding, indoor versus outdoor activity, ventilation etc, which are also all factors in transmission,” Professor Russo said.
At any rate, said Professor Ward, humidity was a hard risk factor to avoid: you cannot control the weather. A forecast for health authorities warning them of high-transmission days might be helpful, he said.
But ultimately, the same steps remain important in dry or humid air: stay 1.5 metres away from people, wear a mask and wash your hands.
Liam is The Age and Sydney Morning Herald’s science reporter