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Study shows climate link between coronavirus cases and humidity


Sydney researcher warns that ‘winter time could be Covid-19 time’

Australia’s health services should prepare for an increase in Covid-19 infections during winter, following a study that has linked transmission of the virus with lower humidity, a University of Sydney academic has said.

Michael Ward, an epidemiologist with the school of veterinary science, led a study that found a 1 per cent decrease in humidity could increase the number of Covid-19 cases by 6 per cent.

The results are published online in the journal Transboundary and Emerging Diseases.

Ward worked with researchers from the school of public health at Fudan University in Shanghai, China, to look at the impact of climate on transmission of the virus.

They analysed more than 700 cases in Sydney that occurred between 26 February and 31 March—late summer and early autumn in Australia. They also mapped regional weather patterns, including rainfall, temperature and humidity.

“The pandemic in China, Europe and North America happened in winter so we were interested to see if the association between Covid-19 cases and climate was different in Australia in late summer and early autumn,” Ward said in a university statement.

The study found that lower humidity was associated with a 6 per cent increase in Covid-19 case notifications across Sydney.

“This means we need to be careful coming into a dry winter,” Ward said.

“We found that lower humidity is the main driver here, rather than colder temperatures…It means we may see an increased risk in winter here, when we have a drop in humidity. But in the northern hemisphere, in areas with lower humidity or during periods when humidity drops, there might be a risk even during the summer months.”

When the humidity is lower, “the air is drier, and it makes the aerosols smaller”.

“When you sneeze and cough, those smaller infectious aerosols can stay suspended in the air for longer. That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.”

Ward said that despite a recent decrease in the number of Covid-19 cases in Australia, the country’s public health systems should be aware of the risks associated with lower humidity during winter.

“Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favour coronavirus spread.”