Discussion
There have been few published investigations on the relationship between climate and COVID-19, and to our knowledge none in the Southern Hemisphere or specifically Australia. We found a negative relationship between the evolving COVID-19 epidemic in NSW, Australia and relative humidity, but not rainfall nor temperature; a 1% decrease in 9am relative humidity could increase the number of COVID-19 cases by 6.11%.
Temperature and relative humidity are known to be important factors in the spread of respiratory diseases. Several epidemiological and laboratory studies have found that temperature and relative humidity affect the spread of coronavirus-related diseases, with both low temperature and relative humidity being suitable for the survival and transmission of coronavirus (Casanova et al., 2010; Chan et al., 2011; Guionie et al., 2013). Some statistical modelling studies have found that higher relative humidity could lead to a reduction in COVID-19 cases (Qi et al., 2020; Ma et al., 2020), which is consistent with our study findings. However, we did not find an association between temperature and COVID-19 cases. Despite some studies suggesting that lower temperature could increase the incidence of COVID-19 (Qi et al., 2020), other studies have not identified such a relationship (Yao et al., 2020; Zhu and Xie, 2020). These inconsistencies might suggest that the impact of temperature on SARS-CoV-2 transmission is more complex and heterogeneous across the landscape and in different countries − or even hemispheres. Temperature ranged from 16 to 24°C (9am) and 16 to 34 °C (3pm) in our study area during the epidemic period, which is very different from most of the recent studies from China and elsewhere in the northern hemisphere winter. The timing of the COVID-19 pandemic − beginning in January − and the differences between climate (especially temperature) between the northern and southern hemispheres could explain a lack of association in our study with temperature and might also partially explain the relatively limited transmission of SARS-CoV-2 in Australia to date (265 cases per million). However, despite the temperatures experienced in the southern hemisphere summer, SARS-CoV-2 can be transmitted. Our study suggests that even with higher temperature, COVID-19 could persist through the coming northern summer and ongoing surveillance and prevention will be needed. In addition, our finding that lower relative humidity is associated with cases together with other published studies suggests that any area − regardless of season or location − can be at risk of SARS-CoV-2 transmission, and that this risk increases at lower humidity.
In conclusion, under the conditions of high temperature in the southern hemisphere summer, our study provides evidence that lower relative humidity is associated with COVID-19 cases. It also suggests that all countries need to maintain vigilance for COVID-19, even during the summer months.