A study conducted in NSW during the early epidemic stage of COVID-19 has found an association between lower humidity and an increase in locally acquired positive cases.
Researchers at the University of Sydney discovered that a 1 per cent decrease in humidity could increase the number of COVID-19 cases by 6 per cent.
The research led by Professor Michael Ward, an epidemiologist at the University of Sydney, and two researchers from Fudan University in Shanghai, China, is the first peer-reviewed study of a relationship between climate and COVID-19 in the southern hemisphere.
“COVID-19 is likely to be a seasonal disease that recurs in periods of lower humidity,” explained Prof. Ward.
“We need to be thinking if it’s winter time, it could be COVID-19 time.”
Further studies – including those conducted during winter – are needed to determine how this relationship works and the extent to which it drives COVID-19 case notification rates.
Previous research has identified a link between climate and occurrence of SARS-CoV cases in Hong Kong and China and MERS-CoV cases in Saudi Arabia, and a recent study on the COVID-19 outbreak in China found an association between transmission and daily temperature and relative 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,” Prof. Ward said.
“When it comes to climate, 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. So, vigilance must be maintained.”
Prof. Ward said there were biological reasons why humidity matters in transmission of airborne viruses.
“When the humidity is lower, the air is drier and it makes the aerosols smaller,” he said.
“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.”
Prof. Ward and his team studied 749 locally acquired cases of COVID-19 – mostly in Sydney – between 26 February and 31 March.
The team matched the patients’ postcodes with the nearest weather observation station and studied the rainfall, temperature and humidity for the period January to March 2020.
The study found lower humidity was associated with increased case notifications; a reduction in relative humidity of 1 per cent was predicted to be associated with an increase of COVID-19 cases by 6 per cent.
“This means we need to be careful coming into a dry winter,” Prof. Ward said.
“Even though the cases of COVID-19 have gone down in Australia, we still need to be vigilant and public health systems need to be aware of potentially increased risk when we are in a period of low humidity.
“Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favour coronavirus spread.”
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