The novel strain of coronavirus SARS-CoV-2 may thrive in less humid climates - meaning New Zealand could face a possible resurgence of the disease during the winter months, according to new Australian research.
The study discovered a link between lower humidity and an increase in locally-acquired, positive COVID-19 cases, indicating that the likes of Australia and New Zealand could face a heightened risk of the disease throughout the winter. Researchers discovered a 1 percent decrease in humidity could increase the number of COVID-19 cases by 6 percent.
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 the Fudan University School of Public Health in Shanghai, China, is the first peer-reviewed study of a relationship between climate and COVID-19 in the southern hemisphere. It was conducted during the early epidemic stage of the virus, which was eventually declared a pandemic by the World Health Organization on March 11.
"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," Ward said in a statement issued by the University of Sydney on Tuesday.
"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."
"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."
Previous studies have identified a link between climate and the occurrence of COVID-19 cases in Hong Kong and China, as well as MERS-CoV cases in Saudi Arabia. Recent research also discovered an association between SARS-CoV-2 transmission, daily temperature and relative humidity in mainland China.
Although Ward noted that colder temperatures do not appear to be as relevant during the summer months, cooler weather could possibly be a factor in winter. He acknowledged that the study was limited to cases contracted in the summer, mostly in and around Sydney.
As a result, the team has acknowledged that further research, including during winter, is needed to fully determine how this relationship between climate and COVID-19 operates, and the extent to which it drives the virus.
Why humidity matters
Ward said there are biological reasons why humidity is a significant factor in the transmission of airborne viruses.
"When the humidity is lower, the air is drier and it makes the aerosols smaller," he said. "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."
Method
The team analysed 749 locally-acquired cases of COVID-19 - mostly in the Greater Sydney area of NSW - 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 from January to March 2020.
The study found lower humidity was associated with an increase in new cases. A 1 percent reduction in relative humidity may be associated with a 6 percent increase in COVID-19 cases, the team predicted.
"This means we need to be careful coming into a dry winter," Ward said, adding that the average humidity in Sydney is at its 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.
"Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favour coronavirus spread."
Cases of COVID-19 have officially surpassed the 6 million mark, the global total currently standing at more than 6.2 million. The United States, Brazil and Russia are suffering from the highest case totals respectively, with nearly 1.8 million infections and more than 100,000 deaths in the US alone. Worldwide, more than 373,000 deaths have been recorded.
On Monday, New Zealand marked its tenth consecutive day of no new cases, the country's overall confirmed and probable case total remaining at 1504. The death toll is sitting at 22, with only one active case remaining.
The findings by Ward and his team were published on Tuesday in the Transboundary and Emerging Diseases journal.
