Discussion
We found that throughout the epidemic of COVID-19 in NSW, Australia − both during the exponential and descending phases of the epidemic − there was a consistent negative relationship between relative humidity and case occurrence: a 1% decrease in relative humidity was predicted to increase cases about 7−8%, with a more pronounced effect at a relative humidity < 75−79%. In almost all PHUs this negative relationship between relative humidity and cases was found.
Given that SARS-CoV-2 transmission is thought to be primarily via the respiratory route (Cai et al., 2020), and that coronaviruses are known to be susceptible in the environment (Casanova et al., 2010), the finding of an association with relative humidity is expected. This association might occur via the effect on respiratory aerosols and therefore infectious material remaining airborne for longer; or it could be a more direct effect on the survivability of the virus in the environment. The lack of a consistent association between temperature and COVID-19 cases in this and other studies (Qi et al., 2020) − as well as for SARS and MERS-CoV cases (Tan et al., 2005; Gardner et al., 2019; Altamimi and Ahmed, 2020) − suggests that it is the former that influences SARS-CoV-2 transmission. This raises an interesting question, and one with potentially profound importance for public health: could increasing relative humidity contribute to a reduction in SAR-CoV-2 infections when infectious individuals mix with susceptible individuals? In the current study − as with other studies conducted to date on SARS, SARS-CoV-2 and MERS-CoV − we used data collected from meteorological recording stations under the assumption that either cases were infected in an outdoors setting, or that ambient outdoors weather conditions are a proxy for the indoors environment (if that is where most infections occur). Measuring the indoors environment is not possible when retrospectively analyzing hundreds of disease cases that have occurred in an epidemic across an entire country or state. The conduct of controlled studies of the relationship between COVID-19 cases and factors such as relative humidity is challenging.
It is important to highlight that COVID-19 cases used in this study occurred predominantly during the autumn season in southern hemisphere. In contrast, most COVID-19 cases in northern hemisphere have been reported during the winter and spring seasons. Despite the seasons being diametrically opposed, the negative relationship between humidity and cases we observed in the Australian autumn is consistent with that observed in the Chinese winter (Qi et al., 2020). Combined with evidence from studies in the northern hemisphere, the influence of relative humidity on COVID-19 incidence was found to be always negative in different regions, suggesting that the relationship could be universal: humidity is more sensitive to COVID-19 transmission and periods of lower humidity might forecast spikes in SARS-CoV-2 transmission. In the absence of a vaccine, such observations allow the more timely, efficient and effective deployment of public health interventions.