Legio2020 - Temporal trends in legionellosis national notification data and the effect of COVID-19 in Switzerland, 2000- 2020

Legionnaires' disease (LD) is a severe form of pneumonia caused by Legionella spp. bacteria. The disease is notifiable to the Swiss Federal Office of Public Health (FOPH). Over the past ten years, annual reported incidence rates of LD in Switzerland are continuously increasing. The reason for this increase and the main sources of infection for LD remain unknown. In this research project, we are investigating Swiss notification data on LD from 2000 to 2020. The project focuses on temporal trends in notification rates and explores the impact of the COVID-19 pandemic on the reported case numbers. The pandemic of COVID-19 has lowered the number of reported cases for several notifiable diseases including LD. Different hypotheses for the cause of this decrease or the impact of the pandemic in general on LD case numbers are under consideration. These include the effect of the control measures on every step of the trajectory from exposure to case notification, such as different risk patterns due to changed population behavior (e.g. less travels, usage of spas) or changes in exposure levels such as water stagnation problems leading to increased Legionella proliferation in unused schools, offices, and other buildings. Other aspects, which might have changed and affected case numbers, are individual health-seeking behaviour, health care provision, case ascertainment and case reporting. ). Additionally, individual risk factors, as well as the syndromic occurrences of SARS-CoV-2 and Legionella spp. infections are strikingly similar. Following a detailed study on the Swiss notification data until 20161, we conducted a follow-up analysis of the notification data from 2000 until 2020 with a focus on the impact of the COVID-19 pandemic on reported cases. The data showed a continuous upward time trend of annual crude notification rate for legionellosis cases from 1.1/100,000 population (CI: 0.9 - 1.4) in 2000 to 5.6/100,000 population (CI: 5.1 - 6.1) in 2020. The highest notification rate was recorded in 2018 with 6.7/100,000 population (CI: 6.2 - 7.3). COVID-19 containment measures in 2020, such as travel restrictions and/or related behavioural changes, were associated with a temporary decline in cases of 35%. Overall, the quality of the notification data was good, yet there is potential for improvement using an electronic reporting system. Reporting of clinical data to the FOPH was more susceptible to interferences of the COVID-19 pandemic than data from laboratory reporting, which could be observed most clearly in the decline of clinical notification reports by 4.3 percentage points in 2020. As the case classification for Legionnaires' disease includes pneumonia symptoms, this decline could lead to an underestimation of Legionnaires' disease cases, yet the continuous reporting through the diagnostic laboratories suggested a robust surveillance system for legionellosis in Switzerland.