Results
The first COVID-19 case in which infection was locally acquired was notified on 26 February 2020.1 Between 26 February and 31 May, 1203 locally-acquired cases with a residence postcode were notified. Cases were reported from 11 PHUs (range 3−357; Fig 1A); 6 of these reported >100 cases, and climatic data was acquired from 27 weather observation stations within these PHUs.2
Based on correlation coefficients, 3pm temperature and relative humidity, and temperature and relative humidity range were excluded from univariate modelling (Fig 1B). Overall, only 9am temperature (range 8.05−26.6℃) and 9am relative humidity (35−100%) entered the final mode (Fig 1C). Mean temperature range was higher during the exponential epidemic phase than the descending epidemic phase, whereas the reverse occurred for relative humidity range (Table 1).
Overall, we observed a negative association between COVID-19 cases and relative humidity in both epidemic phases (Table 2), but no association with temperature. A 1% decrease in relative humidity was associated with a 7.7% (95% CI: 0.04−14.8%) and 6.8% (95% CI: 0.4%−12.2%) increase in the pooled estimate of daily counts of COVID-19 in the two epidemic phases, respectively. Heterogeneous effects across PHUs were obvious: a significant positive association between temperature and cases in South Eastern Sydney PHU during the descending epidemic phase (Figure 2A&C), whereas a significant negative association between humidity and cases in Sydney PHU during the exponential epidemic phase (Figure 2B&D), were noted. However, the association between humidity and cases was consistently negative in the epidemic phase- and location-specific analyses, except for South Western Sydney PHU in the exponential epidemic phase, and Western Sydney PHU in the descending epidemic phase (Table 2 & Fig 2). The overall exposure-response curves showed that the negative association between cases and relative humidity was more pronounced above 79% and 75% relative humidity in the exponential and descending epidemic phases, respectively (Appendix 1). The sensitivity analysis indicated that the associations between cases and relative humidity were robust (Appendix 2).