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).