COVID-19 outbreaks may be seasonal, scientists say

A one per cent decrease in atmospheric humidity may increase the number of COVID-19 cases by six per cent, according to a study which draws a link between the local climate and transmission of the novel coronavirus.

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The study, published in the journal Transboundary and Emerging Diseases assessed the relationship between climate and COVID-19 in the southern hemisphere, and claims that the disease may be seasonal in nature.

"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 study co-author Michael Ward, who is an epidemiologist from the University of Sydney in Australia.

However, the scientists wrote in a statement that further studies, including ones in winter in the southern hemisphere, are needed to validate the finding and explain how this relationship works.

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They said the extent to which humidity drives COVID-19 case notification rates is also unknown.

Citing earlier studies, the researchers said the link between climate and the occurrence of SARS-CoV cases in Hong Kong and China, during the 2002-03 pandemic, and MERS-CoV cases in Saudi Arabia is known.

They added that a recent study on the COVID-19 outbreak in China also 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," Ward said.

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According to the researchers, lower humidity is the main driver rather than colder temperatures.

"It means we may see an increased risk in winter here, when we have a drop in humidity," Ward said.

"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," he cautioned.

Ward said there are biological reasons why humidity matters in the transmission of airborne viruses.

When the humidity is lower, he said the air is drier, making 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," Ward said.

"When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker," he explained.

In the current study, Ward and his team studied 749 locally acquired cases of COVID-19 -- mostly in the Greater Sydney area of the state of New South Wales in Australia -- between February 26 and March 31.

The scientists matched the patients' postal codes with the nearest weather observation station and studied the rainfall, temperature, and humidity for the period January to March 2020.

In their analysis, the scientists found lower humidity was associated with an increased case notifications with a reduction in relative humidity of one per cent predicted to be associated with an increase of COVID-19 cases by six per cent.

"This means we need to be careful coming into a dry winter," Ward said, adding that the average humidity in Sydney is lowest 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," he added.

According to Ward, continued testing and surveillance remain critical as we enter the winter months, when conditions may favour coronavirus spread.