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Table 1.  

Demographic Age-standardized average incidence, 1992–2002 Age-standardized incidence, 2018 Absolute increase in age-standardized incidence Age-standardized incidence risk ratio, 2018 to 1992–2002 baseline (95% CI) Increase in age-standardized incidence, %
Age group, y, not standardized
   0–4 0.03 0.01 −0.03 0.16 (0.02–1.19) −83.52
   5–14 0.02 0.01 −0.01 0.48 (0.15–1.54) −51.61
   15–24 0.07 0.19 0.12 2.80 (2.19–3.57) 179.80
   25–34 0.18 0.75 0.58 4.30 (3.79–4.88) 330.31
   35–44 0.38 1.97 1.59 5.15 (4.74–5.59) 414.89
   45–54 0.66 4.12 3.46 6.28 (5.91–6.69) 528.44
   55–64 1.02 6.52 5.50 6.39 (6.05–6.75) 539.14
   65–74 1.42 7.66 6.24 5.40 (5.11–5.70) 439.63
   75–84 1.57 8.52 6.96 5.44 (5.07–5.84) 444.13
   ≥85 1.49 9.69 8.20 6.50 (5.82–7.27) 550.35
   M 0.63 3.66 3.04 5.86 (5.67–6.05) 485.55
   F 0.35 1.86 1.50 5.29 (5.06–5.53) 429.22
   Native American or Alaska Native 0.26 1.27 1.01 4.93 (3.51–6.93) 392.94
   Asian or Pacific Islander 0.14 0.56 0.42 4.03 (3.19–5.10) 303.18
   Black or African American 0.47 5.21 4.74 11.04 (10.39–11.73) 1003.95
   White 0.37 1.99 1.61 5.30 (5.12–5.49) 430.15
Northeast 0.68 4.82 4.14 7.04 (6.70–7.40) 604.10
   New England 0.61 4.33 3.72 7.10 (6.40–7.87) 610.04
   Middle Atlantic 0.71 5.00 4.30 7.07 (6.69–7.48) 606.98
South 0.33 1.97 1.64 5.97 (5.67–6.29) 497.23
   South Atlantic 0.44 2.29 1.85 5.24 (4.91–5.59) 423.54
   East South Central 0.32 2.05 1.73 6.40 (5.63–7.27) 539.66
   West South Central 0.15 1.36 1.21 9.15 (8.10–10.34) 815.03
Midwest 0.67 4.10 3.43 6.13 (5.85–6.42) 513.06
   East North Central 0.77 5.01 4.24 6.48 (6.16–6.82) 548.02
   West North Central 0.42 2.04 1.62 4.81 (4.29–5.40) 381.38
West 0.29 0.99 0.70 3.39 (3.11–3.68) 238.50
   Mountain 0.43 1.07 0.64 2.47 (2.15–2.83) 146.55
   Pacific 0.23 0.95 0.72 4.13 (3.71–4.59) 312.91
United States 0.48 2.71 2.23 5.67 (5.52–5.83) 467.30

Table. Magnitude of increase in age-standardized incidence of Legionnaires’ disease, cases/100,000 population, from 1992–2002 (average) through 2018, United States

*Ethnicity was not analyzed because data were missing for 30.4% of cases.


Rising Incidence of Legionnaires’ Disease and Associated Epidemiologic Patterns, United States, 1992–2018

  • Authors: Albert E. Barskey, MPH; Gordana Derado, PhD; Chris Edens, PhD
  • CME / ABIM MOC Released: 2/17/2022
  • Valid for credit through: 2/17/2023, 11:59 PM EST
Start Activity

Target Audience and Goal Statement

This activity is intended for infectious disease specialists, primary care physicians, and other physicians who care for patients at risk for LD.

The goal of this activity is to assess the epidemiology of LD in the US over the past 3 decades.

Upon completion of this activity, participants will:

  1. Analyze trends in the incidence of Legionnaires' disease (LD) according to age
  2. Assess trends in the incidence of LD according to sex
  3. Evaluate trends in the incidence of LD according to race
  4. Distinguish the geographic regions and seasons associated with the highest rates of LD


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  • Albert E. Barskey, MPH

    Centers for Disease Control and Prevention
    Atlanta, Georgia

  • Gordana Derado, PhD

    Centers for Disease Control and Prevention
    Atlanta, Georgia

  • Chris Edens, PhD

    Centers for Disease Control and Prevention
    Atlanta, Georgia

CME Author

  • Charles P. Vega, MD

    Health Sciences Clinical Professor of Family Medicine
    University of California, Irvine School of Medicine


    Disclosure: Charles P. Vega, MD, has disclosed the following relevant financial relationships:
    Served as an advisor or consultant for: GlaxoSmithKline; Johnson & Johnson Pharmaceutical Research & Development, L.L.C.


  • Jill Russell, BA

    Emerging Infectious Diseases


    Disclosure: Jill Russell, BA, has disclosed no relevant financial relationships.

CME Reviewer

  • Leigh A. Schmidt, MSN, RN, CMSRN, CNE, CHCP

    Associate Director, Accreditation and Compliance
    Medscape, LLC


    Disclosure: Leigh A. Schmidt, MSN, RN, CMSRN, CNE, CHCP, has disclosed no relevant financial relationships.

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Rising Incidence of Legionnaires’ Disease and Associated Epidemiologic Patterns, United States, 1992–2018

Authors: Albert E. Barskey, MPH; Gordana Derado, PhD; Chris Edens, PhDFaculty and Disclosures

CME / ABIM MOC Released: 2/17/2022

Valid for credit through: 2/17/2023, 11:59 PM EST


Abstract and Introduction


Reported Legionnaires’ disease (LD) cases began increasing in the United States in 2003 after relatively stable numbers for ≥10 years; reasons for the rise are unclear. We compared epidemiologic patterns associated with cases reported to the Centers for Disease Control and Prevention before and during the rise. The age-standardized average incidence was 0.48 cases/100,000 population during 1992–2002 compared with 2.71 cases/100,000 in 2018. Reported LD incidence increased in nearly every demographic, but increases tended to be larger in demographic groups with higher incidence. During both periods, the largest number of cases occurred among White persons, but the highest incidence was in Black or African American persons. Incidence and increases in incidence were generally largest in the East North Central, Middle Atlantic, and New England divisions. Seasonality was more pronounced during 2003–2018, especially in the Northeast and Midwest. Rising incidence was most notably associated with increasing racial disparities, geographic focus, and seasonality.


Legionnaires’ disease (LD) is a severe pneumonia caused by Legionella spp. bacteria. Approximately 95% of patients require hospitalization, and 10% die[1]. Risk factors include older age (>50 years), smoking, a weakened immune system, and chronic lung conditions[2]. Pontiac fever (a self-limited, influenza-like illness) and extrapulmonary legionellosis (Legionella infection with a primary focus outside the lungs) are other less common legionellosis syndromes[1].

Legionella is found in most freshwater environments in low numbers. The bacteria can proliferate in built environments, particularly when the water is warm (25°C–45°C), stagnant, and lacking residual disinfectant. Some devices, such as cooling towers, hot tubs, showers, and decorative fountains, can aerosolize water and have frequently been associated with LD outbreaks[3]. LD can be acquired when aerosolized water containing Legionella bacteria is inhaled. A properly designed and implemented water management program (WMP) can reduce the risk for Legionella growth and transmission in buildings with complex water systems[3–5]. WMPs were first recommended in 2015[4].

L. pneumophila was discovered in 1977 and recognized as the etiologic agent in an outbreak of severe pneumonia the previous year[6,7]. LD cases reported to the Centers for Disease Control and Prevention (CDC) steadily increased from 235 in 1976 to 1,370 in 1990[8]. Reported cases in the United States remained relatively stable during 1990–2002 but began increasing steadily in 2003[9–11]; however, the reasons are unclear. To explore factors that might have contributed to the increase, we compared epidemiologic patterns associated with the baseline years before the increase (1992–2002) and those associated with the years of increase (2003–2018).