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.