Covid-19 could be a seasonal illness with a higher risk in winter as researchers have found a one percent decrease in humidity could increase the number of Covid-19 cases by six percent. The study, published in the journal ‘Transboundary and Emerging Diseases’ has found an association between lower humidity and an increase in locally acquired positive cases.
“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,” said study author Michael Ward, Professor at the University of Sydney in Australia.
According to the researchers, further studies – including during winter in the southern hemisphere – 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 the climate was different in Australia in late summer and early autumn,” Ward said. For the findings, the research team studied 749 locally acquired cases of Covid-19 – mostly in the Greater Sydney area of the state of New South Wales – between February 26 and March 31. 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 one percent was predicted to be associated with an increase of Covid-19 cases by six percent.
According to the researchers, there are biological reasons why humidity matters in 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,” Ward said. “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,” he explained. “This means we need to be careful coming into a dry winter. Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favor coronavirus spread,” Ward noted.