A study focused on the Greater Sydney area during the early epidemic stage of COVID-19 found an association between lower humidity and an increase in community transmission. Now a second study by the same team confirms the risk.
The research led by Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, and two researchers from our partner institution Fudan University School of Public Health in Shanghai, China, is the second peer-reviewed study of a relationship between weather conditions and COVID-19 in Australia.
"This second study adds to a growing body of evidence that humidity is a key factor in the spread of COVID-19," Ward said. Lower humidity can be defined as "dryer air." The study estimated that for a 1% decrease in relative humidity, COVID-19 cases might increase by 7%-8%.
The estimate is about a 2-fold increase in COVID-19 notifications for a 10 percent drop in relative humidity.
"Dry air appears to favour the spread of COVID-19, meaning time and place become important," he said. "Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks."
Ward said there are 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, adding that aerosols are smaller than droplets. "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.
"This suggests the need for people to wear a mask, both to prevent infectious aerosols escaping into the air in the case of an infectious individual, and exposure to infectious aerosols in the case of an uninfected individual," Ward said.
Key findings: additional evidence from the Sydney COVID-19 epidemic has confirmed cases to be associated with humidity; reduced humidity was found in several different regions of Sydney to be consistently linked to increased cases; the same link was not found for other weather factors – rainfall, temperature or wind; and climatic conditions conducive to the spread of COVID-19 present a challenge to public health.
Further studies on humidity for the remainder of the year are needed to determine how the humidity relationship works and the extent to which it drives COVID-19 case notification rates.
Abstract 1
There is growing evidence that climatic factors could influence the evolution of the current COVID‐19 pandemic. Here, we build on this evidence base, focusing on the southern hemisphere summer and autumn period. The relationship between climatic factors and COVID‐19 cases in New South Wales, Australia was investigated during both the exponential and declining phases of the epidemic in 2020, and in different regions. Increased relative humidity was associated with decreased cases in both epidemic phases, and a consistent negative relationship was found between relative humidity and cases. Overall, a decrease in relative humidity of 1% was associated with an increase in cases of 7–8%. Overall, we found no relationship with between cases and temperature, rainfall or wind speed. Information generated in this study confirms humidity as a driver of SARS‐CoV‐2 transmission.
Authors
Michael P Ward, Shuang Xiao, Zhijie Zhang
Abstract 2
Previous research has identified a relationship between climate and occurrence of SARS‐CoV and MERS‐CoV cases, information that can be used to reduce the risk of infection. Using COVID‐19 notification and postcode data from New South Wales, Australia during the exponential phase of the epidemic in 2020, we used time series analysis to investigate the relationship between 749 cases of locally acquired COVID‐19 and daily rainfall, 9 a.m. and 3 p.m. temperature, and 9 a.m. and 3 p.m. relative humidity. Lower 9 a.m. relative humidity (but not rainfall or temperature) was associated with increased case occurrence; a reduction in relative humidity of 1% was predicted to be associated with an increase of COVID‐19 cases by 6.11%. During periods of low relative humidity, the public health system should anticipate an increased number of COVID‐19 cases.
Authors
Michael P Ward, Shuang Xiao, Zhijie Zhang
[link url="https://www.sydney.edu.au/news-opinion/news/2020/08/18/low-humidity-increases-COVID-risk-another-reason-to-wear-a-mask.html"]University of Sydney material[/link]
[link url="https://onlinelibrary.wiley.com/doi/full/10.1111/tbed.13766"]Transboundary and Emerging Diseases abstract 1[/link]
[link url="https://onlinelibrary.wiley.com/doi/full/10.1111/tbed.13631"]Transboundary and Emerging Diseases abstract 2[/link]