Our retrospective study analyzed a total of 591 confirmed LD cases in HK from 2015–2021 and 53 LD-related deaths in HK from 2015–2020. The LD case fatality rate remained high, at over 10%. An abnormal increase in the number of confirmed LD cases occurred in February 2020, which was the month immediately following that of the first reported COVID-19 case in HK (January 2020). An increase in the LD notification number was observed from 2015 to 2020 (157%), and the annual increase of CFR of LD doubled (101.9%), from 2019 to 2020, after the COVID-19 outbreak. Overall, June to August had the largest number of confirmed LD cases (40%), which is in accordance with that of Ricketts et al,15 and the summer month of July accounted for the largest number of accumulated confirmed LD cases (15%) from 1 January 2015 to 30 November 2021. In addition to HK, North America, Europe, and Australia also experienced increases in the number of confirmed LD cases amid the COVID-19 pandemic.30 We also found that the number of confirmed LD cases and related deaths had a lag of approximately one month, from July to August, which is in accordance with Riddell et al who stated that the incubation period of Legionella spp. is 2–10 days.31 We also found that the LD-related death characteristics by gender and age are highly consistent with that of the confirmed LD cases in Leung et al.26
As reported in the previous study17, warm weather is an important factor in LD development (Falconi et al22 found July to be the most significant month). Regarding the number of monthly confirmed LD cases in HK from January 2015-November 2021, our study data quantitatively supplements that of Leung et al26 in terms of the number of confirmed LD cases in HK(Supplementary: Table S4). Furthermore, we are the first to report that the February 2020 spike was positively correlated with an abnormally large number of confirmed LD cases in HK in 2020; February 2020 was the month immediately following that of the first reported case of COVID-19 in HK (January 2020). In recent years, there has been increasing interest in studying the meteorological factors potentially associated with the increase in the number of confirmed LD cases. Unexpectedly, our study found that in HK, relative humidity (P > 0.05) was not significantly associated with the number of confirmed LD cases or related deaths; these results are in contrast with those of Ricketts et al, who found a significant association with humidity in England and Wales,15 but in accordance with those of Chen et al in Taiwan.16 Temperature and amount of rainfall, on the other hand, are related to the confirmed LD cases in HK, which is in accordance with studies in Scotland16, Netherlands17, Taiwan19, Denmark20, and Seoul Korea27.
Strong seasonal patterns were observed in the monthly data for maximum air temperature, average air temperature, relative humidity, and rainfall, though the two air temperature variables were highly correlated visually, which is in accordance with Ozeki et al.23 Our initial simple analysis using 83-month-average (83 MA) and relative risk found that the probability of contracting LD was inconclusive; for example, the relative risks associated with the monthly maximum air temperature above the 83 MA (26.9ºC, RR = 0.98, 95% CI = 0.88–1.07), the monthly average air temperature above the 83 MA (24.2ºC, RR = 1.03, 95% CI = 0.93–1.13), relative humidity and rainfall were inconclusive. Likewise, we found that the probabilities of dying from LD were increased by 72% and 62% when exposed to a monthly maximum air temperature above the 72 MA (26.7ºC, RR = 1.72, 95% CI = 1.06–2.79) and a monthly average air temperature above the 72 MA (24.05ºC, RR = 1.62, 95% CI = 1.014–2.595); however, the results of relative humidity and rainfall were inconclusive (RR = 0.65, 95% CI = 0.416–1.018; RR = 1.03, 95% CI = 0.658–1.620), respectively(Supplementary: Appendix H). Therefore, we further used more appropriate statistical models to analyze the raw number of confirmed LD cases (instead of transformed variable) as the response variable and raw weather variables (instead of transformed variables) as the explanatory variables to achieve conclusive results.32 Our study is the first to use negative binomial regression to model the number of confirmed LD cases and confirm that maximum air temperature is a robust risk factor positively related to the number of monthly confirmed LD cases (P = 0.081, RR = 1.3, 95% CI = 0.71–1.75). Rainfall was also positively related to the number of monthly confirmed LD cases with a relative risk marginally higher than one (P = 0.009, RR = 1.02, 95% CI = 1.0004–1.003). Furthermore, our Poisson regression model confirmed that maximum air temperature was positively related to the number of LD-related deaths (P = 0.034, RR = 1.71, 95% CI = 1.04–2.80).
From a public healthcare management policy perspective, our retrospective study suggests that the local government should inform the general public more about LD risk in HK. Frequent disinfection of water pipes in buildings during COVID-19 lockdowns should be implemented before reopening to prevent LD. Testing for LD and COVID-19 coinfection are also suggested to avoid missing a potential LD case in a COVID-19 patient. These are ongoing public health concerns. Rainfall exceeding 78.8 (in mm) is likely to increase the likelihood of contracting LD in HK, which is in accordance with Oda et al.18 We hypothesized that during the hot summer months (June-August)29 and rainy days in Hong Kong, more people tend to stay indoors longer with the air-conditioning turned on in the office or at home in a typical summer month. This could potentially result in people inhaling contaminated water aerosols released from the air-conditioning.33 As such, this could potentially increase the risk of contracting and dying from LD during the hot summer months. However, this hypothesis should be tested by future study. Weather related risk alerts should be disseminated from the HK Observatory Department to the general public and healthcare professionals when the weather risk threshold identified in our study is reached to prevent LD in HK; government should also educate people on regular air-conditioning maintenance during hot summers to minimize confirmed LD cases and deaths in HK.