COVID-19 Could Be a Seasonal Illness – Recurring in Periods of Lower Humidity

Winter COVID Season

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A research study carried out in Sydney throughout the early epidemic phase of COVID-19 has actually discovered an association in between lower humidity and a boost in locally gotten favorable cases. Researchers found a 1 percent reduction in humidity might increase the variety of COVID-19 cases by 6 percent.

“We need to be thinking if it’s wintertime, it could be COVID-19 time.” — Professor Michael Ward

The research study led by Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, and 2 scientists from our partner organization Fudan University School of Public Health in Shanghai, China, is the very first peer-reviewed research study of a relationship in between environment and COVID-19 in the southern hemisphere.

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

The research study is released on May 21, 2020, in Transboundary and Emerging Diseases.

Further research studies — consisting of throughout winter season in the southern hemisphere — are required to figure out how this relationship works and the level to which it drives COVID-19 case alert rates.

Previous research study has actually recognized a link in between environment and incident of SARS-CoV cases in Hong Kong and China, and MERS-CoV cases in Saudi Arabia, and a current research study on the COVID-19 break out in China discovered an association in between transmission and day-to-day temperature level 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,” Professor Ward stated.

“When it comes to climate, we found that lower humidity is the main driver here, rather than colder temperatures,” Professor Ward stated. “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.”

Why humidity matters

Professor Ward stated there are biological reasons humidity matters in transmission of air-borne infections.

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


Professor Ward and his group studied 749 in your area got cases of COVID-19 — mainly in the Greater Sydney location of the state of New South Wales — in between February 26 and March 31. The group matched the clients’ postal codes with the closest weather condition observation station and studied the rains, temperature level, and humidity for the duration January to March 2020.

The research study discovered lower humidity was related to an increased case notices; a decrease in relative humidity of 1 percent was forecasted to be related to a boost of COVID-19 cases by 6 percent.

“This means we need to be careful coming into a dry winter,” Professor Ward stated, including that the typical humidity in Sydney is most affordable 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,” Professor Ward stated. “Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favor coronavirus spread.”

Further research study

Professor Ward stated the research study was restricted to cases contracted in the summer season mainly around Sydney, so additional research study is required in the months to come and additional afield. In winter season, cooler temperature levels might likewise be an element.

Reference: “The role of climate during the COVID‐19 epidemic in New South Wales, Australia” by Michael P. Ward, Shuang Xiao and Zhijie Zhang, 21 May 2020, Transboundary and Emerging Diseases.
DOI: 10.1111/tbed.13631

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