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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
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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: Methods



US jurisdictions (the 50 states plus New York, NY, and Washington, DC) report cases of legionellosis (referred to as LD)[1] to CDC through the National Notifiable Diseases Surveillance System (NNDSS). We included data from 1992 (the earliest year of electronically available data) through 2018. Although 2019 data are available, completeness of the data reported by more than one third of US jurisdictions is uncertain because of the coronavirus disease pandemic[12]. LD was not reportable in Connecticut during 1992–1996 or in Oregon or West Virginia during 1992–2002; we excluded cases and populations from these jurisdictions and years from analyses.

During the study period, the LD case definition changed (in 1997 and 2006); we included cases meeting the case classification criteria for reportable conditions in use at the time the cases occurred[13–15]. All 3 case definitions defined a confirmed case of LD as a clinically compatible illness with isolation of any Legionella organism from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluid; detection of L. pneumophila serogroup 1 antigen in urine using validated reagents; or a ≥4-fold rise in specific serum antibody titer to L. pneumophila serogroup 1 using validated reagents[13–15]. The 1996 case definition included the detection of L. pneumophila serogroup 1 in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody testing, and it required the ≥4-fold rise in antibody titer to reach ≥128. The 1990 case definition included probable cases, defined as a clinically compatible illness with demonstration of a reciprocal antibody titer ≥256 from a single convalescent-phase serum specimen.

Available patient data included age, sex, race, ethnicity, jurisdiction of residence, and date of earliest reported event in case history (event date). We did not analyze ethnicity because data were missing for 30.4% of cases. Cases were associated with the event date rather than the date reported to the health department or CDC. Event dates consisted of onset date (78%), diagnosis date (9%), laboratory result date (6%), date first reported to any public health authority (3%), and date reported to the state health department or CDC (3%); 1% of cases were missing date type.

Jurisdictions were grouped by US Census Bureau regions and divisions (Figure 1). To quantify seasonality, we calculated the annual maximum-to-minimum monthly case ratio by dividing the maximum number of monthly cases by the minimum number of monthly cases within a calendar year. For most analyses, we aggregated data within 2 time periods (baseline years [1992–2002] and increase years [2003–2018]) and then compared them. We selected 2002, the last year before annual cases numbered >2,000, as a breakpoint for our analyses to aid in comparisons with previously published work[9–11]. To quantify the magnitude of increase, we compared the age-standardized incidence in 2018 with the age-standardized average incidence for 1992–2002 (Appendix). We used bridged-race postcensal population estimates to calculate incidence[16]. Incidence was age-standardized by using the 2005 US standard population as the reference population.


Figure 1. US Census Bureau regions and divisions. Regions: Northeast: Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York City, New York State, Pennsylvania, Rhode Island, Vermont; Midwest: Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Wisconsin; South: Alabama, Arkansas, Delaware, District of Columbia, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, West Virginia; West: Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming. Divisions: New England: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont; Middle Atlantic: New Jersey, New York City, New York State, Pennsylvania; East North Central: Illinois, Indiana, Michigan, Ohio, Wisconsin; West North Central: Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota; South Atlantic: Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia; East South Central: Alabama, Kentucky, Mississippi, Tennessee; West South Central: Arkansas, Louisiana, Oklahoma, Texas; Mountain: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming; Pacific: Alaska, California, Hawaii, Oregon, Washington.

We performed statistical analyses by using SAS (version 9.4; SAS Institute, We performed joinpoint regression analysis, also known as change point regression or segmented regression (Joinpoint software version, https:// on the age-standardized incidence and mean and median age over time to identify the optimal year when population parameters changed (Appendix).