Viral infections like influenza or flu are seasonal, and the common cold is more likely in winter. Now, scientists wonder if the novel coronavirus spreads more easily under specific climatic conditions.
A new study by researchers from the University of Sydney and the Fudan University School of Public Health in Shanghai suggests that low humidity increases the risk of spreading the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Climatic factors of COVID-19 spread
The global spread of SARS-CoV-2, the virus that causes the coronavirus disease (COVID-19), has been tied to climatic factors, which have a biological basis. Generally, the spread of SARS-CoV-2 among people is through respiratory droplets and aerosols, and possibly fecal-oral routes.
Further, previous studies have shown that temperature and relative humidity can affect the spread of SARS-CoV-2 through virus survival, wherein they thrive longer in lower temperatures, and the length of time infectious respiratory matter stays suspended in the air. In lower humidity, more virus material stays suspended for longer.
Coronaviruses can survive for long periods on surfaces and in the air. When an infected person sneezes, coughs, or talks, they can produce infectious respiratory droplets and aerosols. Since droplets are larger, they land on surfaces more quickly.
On the other hand, since aerosols are smaller and lighter, they hand in the air for long periods, which explains why the transmission of SARS-CoV-2 is more likely when the air is drier, and the humidity is lower.
What the study is about
The researchers aimed to determine how temperature affects SARS-CoV-2 transmission, specifically looking at humidity.
The study, which was published in the journal Transboundary and Emerging Diseases, the team has found the number of locally acquired cases of SARS-CoV-2 in Sydney increased as the air became drier and the humidity level dropped.
To arrive at the study findings, the team monitored the daily numbers of reported SARS-CoV-2 cases from New South Wales (NSW) Health. The team also noted that since the cases were reported by postcode, it was easier to identify the source of each one, allowing the team to compile the daily numbers of cases from February to May, matching them to the nearest weather recording stations.
From there, the researchers downloaded meteorological data and utilized the time-series analysis method to project cases based on weather recorded up to 14 days prior.
The team found that they needed only the relative humidity to predict cases of SARS-CoV-2, which is a measure of the water vapor in the air. When there is lower humidity, the air is drier. They discovered that for every 1 percent decrease in relative humidity, there was a 7 to 8-percent increase in cases.
The lockdown between the study period was a substantial contributor in containing the pandemic in the area. Still, the researchers found the same link between dry air and cases, whether or not a lockdown was in force.
However, the team did not see any link between SARS-CoV-2 cases and rainfall, temperature, or wind speed.
“It is important to highlight that SARS-CoV-2 cases used in this study occurred predominantly during the autumn season in southern hemisphere. In contrast, most SARS-CoV-2 cases in northern hemisphere have been reported during the winter and spring seasons,” the researchers added.
Adding to previous findings
Previous studies have also tied humidity and SARS-CoV-2 transmission. For instance, the research team cited a study conducted in China, wherein they found that both drier air and lower temperatures were associated with more SARS-CoV-2 cases.
In May, the research team conducted a study in Sydney, focusing on the early stages of the outbreak. However, in the study, the team did not analyze the findings by area.
“The fact we were able to identify relative humidity as an important factor in both the Chinese winter and Australian summer, using the same research methods, gave us confidence this is a real phenomenon. Our latest study strengthens this hypothesis even further,” Michael Ward, Chair of Veterinary Public Health and Food Safety, University of Sydney and study co-author, explained.
“Of course, laboratory research on SARS-CoV-2, the coronavirus that causes COVID-19, is still in its infancy. But there has been research on closely related coronaviruses, including those that cause sudden acute respiratory syndrome (SARS) and the Middle East respiratory syndrome (MERS),” he added.
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