New research suggests that Covid-19, the severe respiratory disease resulting from infection with the SARS-CoV-2 coronavirus, may become a “seasonal disease.”
An increase in positive cases in Australia has been associated with low humidity typically seen in the winter months – a 1 percent decrease in humidity could increase Covid-19 cases by 6 percent, according to the findings published in Transboundary and Emerging Diseases
"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 lead Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, in a statement.
The pandemic in China, Europe, and North America has occurred during the winter months. In order to determine how Covid-19 rates might be affected with changes in the season, researchers studied more than 700 cases of Covid-19, most of which occurred in the Greater Sydney area, between February 26 and March 31 – Australia’s late summer and early fall. Measurements on rainfall, temperature, and humidity were recorded and compared with postcodes of confirmed cases between January and March. Previous studies have noted that temperature and relative humidity have been reported as “important factors in the spread of respiratory diseases”. The findings confirm that relative humidity, but not temperature or rainfall, is associated with increased case rates.
"When it comes to climate, we found that lower humidity is the main driver here, rather than colder temperatures," said Ward. "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."
Low temperature and relative humidity appear to be suitable for the survival and transmission of some coronaviruses, though studies have found inconsistencies in how temperature may affect the spread. Generally, it appears that humidity affects the transmission of airborne viruses like SARS-CoV-2.
"When the humidity is lower, the air is drier and it makes the aerosols smaller," said Ward. "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."
The public health system should anticipate an increased number of Covid-19 cases during less humid months and should practice prudence during drier months, note the researchers.
"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," Professor Ward said. "Ongoing testing and surveillance remain critical as we enter the winter months, when conditions may favor coronavirus spread."
The researchers caution that the findings do not necessarily mean that the summer months in the northern hemispheres will see a decline in cases though they provide evidence that lower humidity is associated with an increase in Covid-19 cases. There is a chance that Covid-19 transmission could persist through the summer. In order to determine how this association is connected, further research is needed during the winter months and in the Southern Hemisphere.