Unmasked: The Impact of the COVID-19 Pandemic on Reported Cases of Legionnaires’ Disease

BACKGROUND: An increasing public health concern has been the rising incidence of Legionnaires’ disease (LD). The rate of reported cases has increased nearly 9 times since 2000. The emergence of SARS-CoV-2 (COVID-19) in December 2019 constituted a major epidemiological event and has the potential to influence reported cases of LD. Both are respiratory diseases sharing similar clinical presentations and risk groups, which may have confounded the diagnosis of LD — The aim of this study was to investigate the impact of the COVID-19 pandemic on reported cases of LD in 2020 compared to the previous three years (2017-2019). METHODS: We conducted a retrospective descriptive study. A two-sample t-test was used to compare nationally reported cases of LD in 2020 (COVID-19 intervention year) to 2017-2019 (pre-intervention). Syphilis was used as a non-respiratory bacterial disease control. We analyzed data from the Centers for Disease Control and Prevention's (CDC) National Notifiable Disease Surveillance System (NNDSS). RESULTS: At the conclusion of 2020 there were 4,245 reported cases of LD. Average monthly cases decreased >50% in the post-intervention period compared to the pre-intervention period (353.75 from 730 respectively; p< 0.005). We did not observe significant changes in reported cases of syphilis post-intervention. CONCLUSIONS: Reported cases of LD decreased in 2020. Infection control precautions intended to reduce COVID-19 likely contributed to this reduction. Other factors may include delayed reporting, underdiagnosis and low building occupancy. Relaxing precautions and increasing building occupancy may portend spikes in cases in the “post-pandemic” period. Our findings demonstrate the need to better understand the consequences of major epidemiological events on other reportable diseases.