Introduction
The first cases of severe acute respiratory coronavirus 2 (SARS-CoV-2) were reported in Wuhan, China in December 2019 (Wu et al., 2020). Since that time, the disease caused by SARS-CoV-2 − novel coronavirus disease (COVID-19) − has been reported throughout the world and the resulting pandemic of more than 3 million infections and more than 200,000 deaths has caused a dramatic impact on the global economy and social structure.
The first case of COVID-19 was reported in Australia (Sydney, New South Wales [NSW]) on 21 January 2020 (NSW Government, 2020a). Preventing transmission is based on the practice of good hygiene, social distancing, limiting public gatherings and for those infected or a close contact of those infected, self-isolation (Australian Government Department of Health, 2020a). The Australian Government’s response to COVID-19 is staged − initial action, targeted action, stand-down and preparedness (Australian Government Department of Health, 2020a). In the initial stage, public awareness and health system capacity was increased; entry was refused to direct flights from Hubei Province, and health checks were initiated on people arriving from countries considered to be high risk. On 15 March, the response moved to targeted action and between 23 and 29 March restrictions were applied on non-essential gatherings and travel (Australian Government Department of Health, 2020b), an effective “shutdown”. As of the end of April, the number of notified cases was restricted to 6,753 and 91 deaths, with 563,641 tests conducted (Australian Government Department of Health, 2020c).
Although still an area of active research, the spread of SARS-CoV-2 between people appears to be predominantly via respiratory droplets and aerosols and fomites (Cai et al., 2020), with possibly fecal–oral spread being a route of transmission (Yeo et al., 2020). Due to the nature of the virus and likely mechanism of spread, environmental factors − especially temperature and relative humidity − likely influence coronavirus transmission (Casanova et al., 2010) via the relationship between virus survival in the environment and these factors. An inverse relationship between relative humidity and SARS cases (Cai et al., 2007; Tan et al., 2005), and a positive correlation with temperature (Tan et al., 2005), in China have previously been identified. Similarly, in a study of Middle East respiratory syndrome coronavirus (MERS-CoV) (Gardner et al., 2019) an inverse relationship was found been case occurrence in Saudi Arabia between 2015 and 2017 and humidity, however lower temperature was also associated with case occurrence. In another study based on MERS-CoV case occurrence between 2012 and 2018, high temperature and low relative humidity were identified as contributors to increased MERS-CoV cases (Altamimi and Ahmed, 2020). Thus, for both SARS and MERS-CoV, lower relative humidity appears to be associated with case occurrence, however the nature of the relationship between temperature and case occurrence is less clear. In one of the first studies of its type, based on the daily count of COVID-19 cases in 30 Chinese provinces Qi et al (2020) found significant negative associations between cases and average temperature and relative humidity. In addition, an interaction between temperature and humidity in Hubei province was identified. Every 1% increase in average relative humidity was predicted to decrease daily confirmed cases by 11% to 22%, when average temperature was in the range of 5−8°C. It was suggested that in China in the spring, there should be a focus on monitoring and prevention of COVID-19 in northern regions with low temperature and low relative humidity, because of the presence of suitable climatic conditions for SARS-CoV-2 transmission.
In the current study our aim was to further investigate the relationship between reports of COVID-19 cases during the early epidemic phase in NSW, Australia and temperature and relative humidity. Understanding this relationship in different parts of the world enables the development of better pandemic response plans and surveillance systems, and potentially highlights regions where additional public health inventions might be needed to control COVID-19.