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.