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Evidence for multiple cases of recurrent Legionella infection: a Danish national surveillance study
  1. Kelsie Cassell1,
  2. Charlotte Kjelsø2,
  3. Daniel M Weinberger1,
  4. Søren A Uldum3
  1. 1 Epidemiology of Microbial Diseases, Yale University School of Public Health, New Haven, Connecticut, USA
  2. 2 Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, Copenhagen, Denmark
  3. 3 Department of Bacteria, Parasites & Fungi, Statens Serum Institut, Copenhagen, Denmark
  1. Correspondence to Kelsie Cassell, Epidemiology of Microbial Diseases, Yale University School of Public Health, New Haven, CT 06510, USA; kelsie.cassell{at}


While case reports have documented recurrence of Legionnaires’ disease, the frequency of recurrent infections has not been systematically examined at a national level over multiple decades. Between 2000 and 2020 in Denmark, 21 individuals had repeat laboratory-identified Legionella infection, totalling 48 episodes of hospitalisation. The majority of these individuals had underlying comorbidities. In at least 3 of the 21 cases, a different Legionella serogroup was detected during the second episode of infection, which could indicate reinfection from a new source. These results emphasise that Legionella can, and does, reinfect high-risk individuals causing multiple hospitalisations.

  • respiratory infection
  • pneumonia

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  • Contributors KC conceived of the study, performed the data analysis, and prepared the initial manuscript. SAU ensured data access, accuracy of data and verified all analyses and interpretations. CK provided information on case comorbidities and hospitalisations. All authors contributed substantially to the study design, data interpretation and the final manuscript preparation and editing.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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