Introduction
The global spread of severe acute respiratory coronavirus 2 (SARS-CoV-2), causing the novel coronavirus disease (COVID-19) pandemic, has been linked to climatic factors. This has a biological basis. The spread of SARS-CoV-2 among people is predominantly via respiratory droplets and aerosols, as well as fomites (Cai et al., 2020) and possibly fecal–oral (Yeo et al., 2020). Temperature and relative humidity can affect coronavirus transmission (Casanova et al., 2010) through virus survival (at lower temperatures coronaviruses survive longer) and the length of time infectious respiratory matter stays suspended in the air (at lower humidity more material stay suspended for longer) (Casanova et al., 2010; Chan et al., 2011; Guionie et al., 2013).
In previous observational research, a negative relationship between relative humidity and SARS cases has been found (Cai et al., 2007; Tan et al., 2005), and a similar negative relationship with Middle East respiratory syndrome coronavirus (MERS-CoV) cases has been described (Gardner et al., 2019; Altamimi and Ahmed, 2020). However, the relationship with temperature is inconsistent: a positive relationship has been described for SARS (Gardner et al., 2019) and MERS-CoV (Altamimi and Ahmed, 2020), but a negative relationship has also been observed for MERS-CoV (Gardner et al., 2019). Specifically for SARS-CoV-2, a negative relationship between COVID-19 cases in China and temperature and humidity has recently been described (Qi et al., 2020), and more recently in the state of New South Wales (NSW), Australia we described a significant negative association between COVID-19 cases during the initial exponential phase of the epidemic and relative humidity (Ward et al., 2020). In the current study we extend this research to examine the effect of a greater number of climatic factors on the occurrence of COVID-19 cases during both exponential and descending phases of the epidemic, and investigate whether there are regional and temporal differences in this relationship. This knowledge is needed to guide public health interventions to successfully control the spread of SARS-CoV-2.