COVID-19 may be a seasonal illness that thrives in low humidity

Researchers at the University of Sydney have found evidence to suggest that COVID-19 is a seasonal illness that becomes more of a threat in low humidity. The study revealed a link between lower humidity and an increase in locally acquired COVID-19 cases. 

The research, which was conducted in Sydney during an early stage of the epidemic, is the first peer-reviewed investigation into a potential connection between COVID-19 and climate in the southern hemisphere.

Study lead author Professor Michael Ward is an epidemiologist in the Sydney School of Veterinary Science.

“COVID-19 is likely to be a seasonal disease that recurs in periods of lower humidity. We need to be thinking if it’s winter time, it could be COVID-19 time,” said Professor Ward.

In previous studies, experts established that climate influenced the transmission rate of the first SARS coronavirus outbreak in Hong Kong and China, as well as the MERS coronavirus outbreak in Saudi Arabia. 

More recently, researchers identified an association between daily temperature, relative humidity, and COVID-19 transmission.

“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,” said Professor Ward.

“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.”

Professor Ward explained why humidity has on influence on the transmission of airborne viruses.

“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.”

The current study was focused on 749 locally acquired cases of COVID-19 between February 26 and March 31. The majority of the cases originated in the Greater Sydney area of New South Wales.

The researchers used the postal codes of the patients and analyzed information from the nearest weather observation station. They looked at rainfall, temperature, and humidity data for January, February, and March.

The analysis showed that there were more case notifications during periods of lower humidity. In particular, a one percent reduction in humidity was associated with a six percent increase in the number of COVID-19 infections.

“This means we need to be careful coming into a dry winter,” said Professor Ward, noting that Sydney’s lowest average humidity is in August.

“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 favor coronavirus spread.”

The study is published in the journal Transboundary and Emerging Diseases.

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By Chrissy Sexton, Earth.com Staff Writer