Chlamydia, caused by infection with Chlamydia trachomatis, is the most common notifiable disease in the United States. It is among the most prevalent of all STDs, and since 1994, has comprised the largest proportion of all STDs reported to CDC (Table 1). Studies also demonstrate the high prevalence of chlamydial infections in the civilian, non-institutionalized US population, particularly among young women.1
Chlamydial infections in women are usually asymptomatic.2 Untreated infection can result in pelvic inflammatory disease (PID), which is a major cause of infertility, ectopic pregnancy, and chronic pelvic pain. Data from randomized controlled trials of chlamydia screening suggested that screening programs can lead to a reduction in the incidence of PID.3,4 As with other inflammatory STDs, chlamydial infection could facilitate the transmission of HIV infection.5 In addition, pregnant women infected with chlamydia can pass the infection to their infants during delivery, potentially resulting in ophthalmia neonatorum, which can lead to blindness, and pneumonia.6 Because of the large burden of disease and risks associated with infection, CDC recommends annual chlamydia screening for all sexually-active women younger than age 25 years and women ≥25 years at increased risk for infection (e.g., women with new or multiple sex partners).7
The Healthcare Effectiveness Data and Information Set (HEDIS) contains a measure which assesses chlamydia screening coverage of sexually-active young women who receive medical care through commercial or Medicaid managed care organizations. Among sexually-active women aged 16–24 years in commercial health maintenance organization (HMO) plans, chlamydia screening increased from 23.1% in 2001 to 48.9% in 2017. Among sexually-active women aged 16–24 years covered by Medicaid, screening rates increased from 40.4% in 2001 to 57.6% in 2017.8 Although chlamydia screening has expanded over the past two decades, many women who are at risk are still not being tested — reflecting, in part, the lack of awareness among some health-care providers and the limited resources available to support these screenings.
Interpreting Rates of Reported Cases of Chlamydia
Trends in rates of reported cases of chlamydia are influenced by changes in incidence of infection, as well as changes in diagnostic, screening, and reporting practices. As chlamydial infections are usually asymptomatic, the number of infections identified and reported can increase as more people are screened even when incidence is flat or decreasing. During 2000–2011, the expanded use of more sensitive diagnostic tests (e.g., nucleic acid amplification tests [NAATs]) likely increased the number of infections identified and reported independently of increases in incidence. Also, although chlamydia has been a nationally notifiable condition since 1995, it was not until 2000 that all 50 states and the District of Columbia required reporting of chlamydia cases. National case rates prior to 2000 reflect incomplete reporting. The increased use of electronic laboratory reporting over the last decade or so also likely increased the proportion of diagnosed cases reported. Consequently, an increasing chlamydia case rate over time may reflect increases in incidence of infection, screening coverage, and use of more sensitive tests, as well as more complete reporting. Likewise, decreases in chlamydia case rates may suggest decreases in incidence of infection or screening coverage.
Chlamydia — United States
In 2018, a total of 1,758,668 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 539.9 cases per 100,000 population. During 2000–2011, the rate of reported chlamydial infection increased from 251.4 to 453.4 cases per 100,000 population (Figure 1, Table 1). During 2011–2013, the rate of reported cases decreased to 443.5 cases per 100,000 population, followed by an increase in the rate of reported cases over each of the next five years. During 2017–2018, the rate increased 2.9%, from 524.6 to 539.9 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
In 2018, rates of reported cases of chlamydia were highest in the South (565.2 cases per 100,000 population, 1.9% increase from 2017), followed by the West (548.5, 5.6% increase from 2017), Midwest (524.0, 2.2% increase from 2017), and Northeast (492.1, 2.3% increase from 2017) (Table 3). During 2009–2018, rates of reported cases of chlamydia increased in all regions (Figure 2). During 2012–2013, rates decreased in the Northeast, Midwest, and South and remained stable in the West. Rates started increasing in 2013, and have increased in all regions over each of the last five years. During 2014–2018, the largest increase occurred in the West (436.4 to 548.5 cases per 100,000 population, 25.7% increase) (Figure 2, Table 3).
Chlamydia by State
In 2018, rates of reported cases of chlamydia by state ranged from 198.2 cases per 100,000 population in West Virginia to 832.5 cases per 100,000 population in Alaska (Figure 3, Table 2); the rate for the District of Columbia was 1,298.9 cases per 100,000 population (Table 3). During 2017–2018, rates of reported chlamydia cases increased in 40 states. The rate of reported chlamydia cases in 2018 was above the US total in 21 states (Table 2).
Chlamydia by Metropolitan Statistical Area
The overall rate of reported cases of chlamydia for the 50 most populous metropolitan statistical areas (MSAs) increased 3.0% during 2017–2018 (from 555.7 to 572.1 cases per 100,000 population) (Table 6). In 2018, 58.5% of chlamydia cases were reported by these MSAs. During 2017–2018, the rate of reported cases of chlamydia in these MSAs increased 1.3% among females (from 697.0 to 706.0 cases per 100,000 females) and 6.0% among males (from 406.7 to 430.9 cases per 100,000 males) (Tables 7 and 8).
Chlamydia by County
In 2018, 628 (20.0%) of 3,142 counties had rates of reported chlamydia at or higher than 560 cases per 100,000 population (Figure 4). Seventy counties and independent cities reported 44% of all chlamydia cases in 2018 (Table 9). Of the 70 counties and independent cities reporting the highest number of chlamydia cases, 47 (67.1%) were located in the South and West (Table 9).
Chlamydia by Sex
In 2018, 1,145,063 cases of chlamydia were reported among females for a rate of 692.7 cases per 100,000 females (Table 4). After increasing each year during 2000–2011, the rate of reported chlamydia cases among females decreased during 2011–2013, followed by an increase in the rate of reported cases over each of the next five years (Figure 1, Table 4). The total rate increase during 2014–2018 among females was 11.4%.
Among males, 610,447 cases of chlamydia were reported in 2018 for a rate of 380.6 cases per 100,000 males (Table 5). The rate of reported cases among males increased each year during 2000–2018, with the exception of 2012–2013, when rates remained stable (Figure 1). During 2017–2018 alone, the rate among males increased 5.7%; during 2014–2018, rates of reported cases among males increased 37.8% (Tables 4 and 5). This pronounced increase among males could be attributed to either increased transmission or improved case identification (e.g., through intensified extra-genital screening efforts) among gay, bisexual, and other men who have sex with men (MSM). This cannot be assessed, however, as most jurisdictions do not routinely report sex of sex partners or anatomic site of infection for cases of chlamydia.
Despite this considerable increase in males, the rate of reported chlamydia cases among females was still about two times the rate among males in 2018, likely reflecting a larger number of females screened for this infection (Figure 1, Tables 4 and 5). The lower rate among males also suggests that many of the sex partners of females with chlamydia are not receiving a diagnosis of chlamydia or being reported as having chlamydia.
Chlamydia by Age
The rates of reported cases of chlamydia were highest among adolescents and young adults aged 15–24 years during 2014–2018 (Table 10). In 2018, the age-specific rate of reported cases of chlamydia among 15–19 year olds was 2,110.6 cases per 100,000 population and the rate among 20–24 year olds was 2,899.2 cases per 100,000 population (Table 10).
In 2018, 97.4% of all reported chlamydia cases in females were among those aged 15–44 years (Table 10). The highest age-specific rates of reported cases of chlamydia in 2018 were among those aged 15–19 years (3,306.8 cases per 100,000 females) and 20–24 years (4,064.6 cases per 100,000 females) (Figure 5, Table 10). Within these age groups, rates were highest among women aged 19 years (5,485.8 cases per 100,000 females) and 20 years (5,309.8 cases per 100,000 females) (Table 12A).
Increases have been observed in recent years in rates of reported cases of chlamydia among all age groups in females aged 15–44 years (Figure 6). The rate among 15–19 year old females increased 1.3% during 2017–2018, with a total increase of 12.1% during 2014–2018 (2,949.3 to 3,306.8 cases per 100,000 females) (Table 10). The rate among 20–24 year old females increased 0.8% during 2017–2018, with a total increase of 11.9% during 2014–2018 (3,632.7 to 4,064.6 cases per 100,000 females) (Table 10).
In 2018, 94.0% of all reported chlamydia cases in males were among those aged 15–44 years (Table 10). The age-specific rates of reported cases of chlamydia among males, although substantially lower than rates among females, were highest in those aged 20–24 years (1,784.5 cases per 100,000 males) (Figure 5, Table 10). Similar to trends in females, increases have been observed in rates of reported cases of chlamydia among all age groups in males aged 15–44 years, although for males these increases have been more pronounced (Figure 7). The rate among 15–19 year olds increased 3.7% during 2017–2018, with a total increase of 32.8% during 2014–2018 (from 722.4 to 959.0 cases per 100,000 males). The rate among 20–24 year old males increased 3.3% during 2017–2018, with a total increase of 31.1% during 2014–2018 (1,361.3 to 1,784.5 cases per 100,000 males) (Table 10).
Chlamydia by Race/Hispanic Ethnicity
In 2018, rates of reported cases of chlamydia were highest among Black, American Indian/Alaska Native (AI/AN), and Native Hawaiian/Other Pacific Islander (NHOPI) persons (Figure S, Table 11B). Overall, the rate of reported cases of chlamydia among Blacks was 5.6 times the rate among Whites (1,192.5 and 212.1 cases per 100,000 population, respectively). The rate among AI/ANs (784.8 cases per 100,000 population) was 3.7 times the rate among Whites. The rate among NHOPIs (700.8 cases per 100,000 population) was 3.3 times the rate among Whites. The rate among Hispanics (392.5 cases per 100,000 population) was 1.9 times the rate among Whites. The rate among Asians (132.1 cases per 100,000 population) was 0.6 times the rate among Whites.
During 2014–2018, rates of reported chlamydia cases increased among all racial/Hispanic ethnicity groups, with AI/ANs increasing 7.0%, Hispanics 8.1%, Blacks 9.0%, Whites 17.6%, NHOPIs 20.1%, Asians 29.3%, and Multirace 59.7% (Figure 8). During 2017–2018, rates increased among some racial/Hispanic ethnicity groups (Whites: 0.3%, AI/ANs: 1.1%, Blacks: 2.5%, Asians: 5.7%, and Multirace: 16.4%), and decreased among others (NHOPIs: -0.4% and Hispanics: -0.4%).
More information on chlamydia rates among race/Hispanic ethnicity groups can be found in the Special Focus Profiles, STDs in Racial and Ethnic Minorities.
Chlamydia by Reporting Source
In 2018, 5.2% of chlamydia cases were reported from STD clinics, 80.2% were reported from venues outside of STD clinics, and 14.6% had an unknown reporting source (Table A2). In 2018, among females, only 3.5% of chlamydia cases were reported through an STD clinic, with a large proportion of cases (30.9%) being reported from a private physician/HMO (Figure 9 and Table A2). Over time, the proportion of male cases reported from STD clinic sites has decreased substantially, from 30.1% in 2009 to 8.6% in 2018 (Figure 10). Nearly one quarter (24.0%) of male cases were reported from a private physician/HMO.
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES; see Section A2.4 in the Appendix) is a nationally representative survey of the US civilian, non-institutionalized population that provides an important measure of chlamydia disease burden in respondents aged 14–39 years. During 2013–2016, the overall prevalence of chlamydia among persons aged 14–39 years was 1.7% (95% Confidence Interval [CI]: 1.3–2.1) (Figure 11). Among sexually-active females aged 14–24 years, the population targeted for screening, prevalence was 4.3% (95% CI: 2.7–5.8), with the highest prevalence among Mexican American females (10.0%, 95% CI: 4.0–15.9) (Figure 12).
Chlamydia Positivity in Selected Populations
The STD Surveillance Network (SSuN) is an ongoing collaboration of state, county, and city health departments conducting sentinel and enhanced surveillance activities. These include collecting enhanced clinical and behavioral information among all patients attending selected STD clinics, among women aged 15–44 years in selected reproductive health clinics, and conducting enhanced patient and provider investigations on a representative sample of gonorrhea cases diagnosed and reported from all reporting sources in their jurisdiction (See Section A2.2 of the Appendix).
In 2018, the proportion of STD clinic patients testing positive for chlamydia varied by sex and sex of sex partners, as well as age. Women and men who have sex with women only (MSW) aged ≤19 years had the highest positivity; positivity was nearly equal at 30.6% and 30.5%, respectively (Figure 13). Positivity among all those tested decreased with age, though the decrease in positivity by age was not as pronounced for MSM (Figure 13). The overall positivity, represented by the average of the mean value by the contributing SSuN jurisdictions, was 16.9% for MSM, 13.8% for MSW, and 10.9% for women.
Chlamydia Among Special Populations
More information on chlamydia among females of reproductive age, adolescents and young adults, MSM, and racial and ethnic minorities can be found in the Special Focus Profiles.
Chlamydia continues to be the most commonly reported nationally notifiable disease, with 1,758,668 cases reported in 2018 and increasing rates of reported cases over each of the last five years. While the rate of reported chlamydia cases increased just 2.9% during 2017–2018, the rate increased 19.4% during 2014–2018. The Southern region of the US reported the highest rate of chlamydial infection in 2018; the West reported the largest rate increase during 2017–2018. In 2018, similar to prior years, the rate of reported cases of chlamydia in females was nearly two times the rate in males. However, during 2014–2018, the rate in males increased 37.8%, whereas the rate in females increased only 11.4%. Potential reasons for this considerable increase in male cases could be due to a true increase in infections or to improved screening coverage in males, especially increased extra-genital screening in MSM, or both.
The facilities reporting chlamydial infections have changed over the last 10 years with most (80.2%) chlamydia cases in 2018 reported from venues outside of STD clinics. The proportion of males being diagnosed with chlamydia in STD clinics decreased 71.4% from 30.1% in 2009 to 8.6% in 2018. In females, approximately one-third of chlamydia cases were reported in 2018 from a private physician/HMO, while only 3.5% were reported from STD clinics. Racial differences also persist. Reported case rates among Blacks continue to be substantially higher than among all other racial/Hispanic ethnicity groups, although rates have increased substantially among all racial/Hispanic ethnicity groups over the last five years. Ultimately, both test positivity and the number of reported cases of C. trachomatis infections remain high among younger age groups, and most racial/Hispanic ethnicity groups, geographic areas, and both sexes.
4. Oakeschott P, Kerry S, Aghaizu A, et al. Randomised controlled trial of screening for Chlamydia trachomatis to prevent pelvic inflammatory disease: The POPI (prevention of pelvic infection) trial. BMJ. 2010;340:c1642.
5. Fleming DT, Wasserheit JN. From epidemiological synergy to public health policy and practice: The contribution of other sexually transmitted diseases to sexual transmission of HIV infection. Sex Transm Infect. 1999;75(1):3–17.
8. National Committee for Quality Assurance. The State of Healthcare Quality: Chlamydia Screening in Women (CHL). Available at: https://www.ncqa.org/hedis/measures/chlamydia-screening-in-women/external icon. Accessed July 24, 2019.