C. trachomatis infection is the most commonly reported 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 general U.S. population. Based on estimates from national surveys conducted from 1999–2008, chlamydia prevalence is 6.8% among sexually active females aged 14–19 years.1
Chlamydial infections in women, which are usually asymptomatic, can result in PID, which is a major cause of infertility, ectopic pregnancy, and chronic pelvic pain. Data from a randomized controlled trial of chlamydia screening in a managed care setting suggested that screening programs can lead to as much as a 60% reduction in the incidence of PID.2 As with other inflammatory STDs, chlamydial infection can facilitate the transmission of human immunodeficiency virus (HIV) infection.3 In addition, pregnant women infected with chlamydia can pass the infection to their infants during delivery, potentially resulting in neonatal ophthalmia and pneumonia. Because of the large burden of disease and risks associated with infection, CDC recommends that all sexually active women younger than age 26 years receive annual chlamydia screening.4
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.5 Among sexually-active women aged 16-24 years in commercial plans, chlamydia screening increased from 23.1% in 2001 to 43.1% in 2010. During the same time period, the screening rate among sexually-active women aged 16-24 years covered by Medicaid increased from 40.4% to 57.5%.6 Although chlamydia screening is expanding, 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.
The increase in reported chlamydial infections during the last 20 years reflects the expansion of chlamydia screening activities, the use of increasingly sensitive diagnostic tests, an increased emphasis on case reporting from providers and laboratories, and improvements in the information systems used for reporting. To supplement case report data, chlamydia positivity and prevalence among people screened in a variety of settings are monitored.
In 2011, a total of 1,412,791 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 457.6 cases per 100,000 population, which is an increase of 8.0% compared with the rate of 423.6 in 2010. During 1991–2011, the rate of reported chlamydial infection increased from 179.7 to 457.6 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
During 2002–2011, chlamydia rates increased in all regions (Figure 2). In 2011, rates were highest in the South (505.3 per 100,000 population), followed by the Midwest (445.7), the West (424.9), and the Northeast (415.8) (Table 3).
Chlamydia by State
Chlamydia by Metropolitan Statistical Area
In 2011, the chlamydia rate per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) increased (Table 6). In 2011, 57.2% of chlamydia cases were reported by these MSAs. Among women, the 2011 rate of 674.8 cases per 100,000 females was a 6.2% increase over the 2010 rate of 635.2 cases per 100,000 females (Table 7). Among men, the 2011 rate (287.6 per 100,000 males) increased 10.1% from the 2010 rate (261.3 cases per 100,000 males) (Table 8).
Chlamydia by County
Counties in the United States with the highest chlamydia case rates per 100,000 population were located primarily in the Southeast and West, including Alaska (Figure 4). In 2011, 877 (27.9%) of 3,142 counties had rates higher than 400.0 cases per 100,000 population. Seventy counties and independent cities reported 44% of all chlamydia cases in 2011 (Table 9).
Chlamydia by Sex
In 2011, the overall rate of reported chlamydial infection among women in all 50 states and the District of Columbia (648.9 cases per 100,000 females) was over two and a half times the rate among men (256.9 cases per 100,000 males), likely reflecting a larger number of women screened for this infection (Figure 1, Tables 4 and 5). The lower rates among men also suggest that many of the sex partners of women with chlamydia are not receiving a diagnosis of chlamydia or being reported as having chlamydia.
However, with the advent of highly sensitive nucleic acid amplification tests (NAATs) that can be performed on urine, chlamydial infection is increasingly being diagnosed in symptomatic and asymptomatic men. During 2007–2011, the reported chlamydial infection rate among men increased 36.2% (from 188.6 to 256.9 cases per 100,000 males) compared with a 20.2% increase among women during the same period (from 539.8 to 648.9 cases per 100,000 females).
Chlamydia by Age
Among women, the highest age-specific rates of reported chlamydia in 2011 were among those aged 15–19 years (3,416.5 cases per 100,000 females) and 20–24 years (3,722.5 cases per 100,000 females) (Figure 5, Table 10). Within these age ranges, reported rates were highest among women aged 18 years (4,760.0 cases per 100,000 females), aged 19 years (5,012.4 cases per 100,000 females), and aged 20 years (4,883.2 cases per 100,000 females) (Table 12). Age-specific rates among men, although substantially lower than the rates among women, were highest in those aged 20–24 years (1,343.3 cases per 100,000 males) (Figure 5, Table 10).
Chlamydia by Race/Ethnicity
In 2011, chlamydia rates were highest among black men and women (Figure 6, Table 11B). The rate of chlamydia among blacks was more than seven times the rate among whites (1,194.4 and 159.0 cases per 100,000 population, respectively). The rate among American Indians/Alaska Natives (648.3 cases per 100,000) was 4.1 times the rate among whites. The rate among Hispanics (383.6 cases per 100,000) was 2.4 times the rate among whites.
During 2007–2011, rates among blacks increased 17.6% (from 1,015.7 to 1,194.4 cases per 100,000). Among whites, rates increased 34.6% (from 118.1 to 159.0 cases per 100,000).
Chlamydia by Reporting Source
Most chlamydia cases reported in 2011 were from venues outside of STD clinics (Table A2). Over time, the proportion of cases reported from non-STD clinic sites has continued to increase (Figure 7). In 2011, among women, only 7.7% of chlamydia cases were reported through an STD clinic (Figure 8). Most cases among women were reported from private physicians/health maintenance organizations (HMOs) (37.7%). Among men, 23.6% of chlamydia cases were reported from an STD clinic in 2011 and 26.4% were reported from private physicians/HMOs.
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES) is a nationally representative survey of the U.S. civilian, non-institutionalized population aged 14–39 years that provides an important measure of chlamydia disease burden. From 1999–2000 to 2007–08, there was an estimated 40% reduction (95% Confidence Interval [CI]: 8%, 61%) in prevalence among persons aged 14–39 years (Figure 10).7 During 2005–2008, the overall prevalence of chlamydia among persons aged 14–39 years was 1.5% (95% CI: 1.2%, 1.9%). Prevalence was highest among non-Hispanic blacks (5.9%, 95% CI: 4.5%, 7.7%) (Figure 11).
Chlamydia Positivity in Selected Populations
Chlamydia screening and monitoring activities have been conducted in all ten U.S. Department of Health and Human Services (HHS) regions since 1995. In some regions, federal funds may support local- and state-funded screening programs. Screening criteria and practices vary by region and state. See Definitions of HHS Regions in the Appendix for details.
In 2011, the median state-specific chlamydia test positivity among women aged 15–24 years who were tested during visits to selected family planning clinics in all 50 states, Puerto Rico, and the Virgin Islands was 8.3% (range: 3.8% to 15.9%) (Figures 12 and 13). Chlamydia test positivity among women aged 15–24 years screened in family planning clinics increased in most HHS regions during 2007–2011 (Figure 14).
The positivity trend data in Figure 12 and Figure 14 are not adjusted for changes in laboratory test methods and associated increases in test sensitivity. Use of NAATs in family planning clinics to screen women aged 15–24 years for chlamydia has increased over time, with all ten HHS regions using NAATs nearly exclusively in 2011 (Figure 15). Additionally, positivity trends are influenced by changes in the population of women accessing care,8 clinic screening criteria, and clinic participation in the screening program. In a regression analysis accounting for individual-level and clinic-level factors, chlamydia positivity among women aged 14–25 years who were screened in family planning clinics remained stable from 2004–2008.9
In 2005, the STD Surveillance Network (SSuN) was established to improve the capacity of national, state, and local STD programs to detect, monitor, and respond to trends in STDs. In 2011, a total of 42 STD clinics at 12 sites collected enhanced behavioral information on patients who presented for care to these clinics. More detailed information about SSuN methodology can be found in the STD Surveillance Network section of the Appendix, Interpreting STD Surveillance Data.
In 2011, the proportion of STD clinic patients testing positive for chlamydia varied by age, sex, and sexual behavior. Adolescent men who have sex with women (MSW) had the highest prevalence (34.8%). Among MSW and women, prevalence among those tested decreased with age. The variation in prevalence by age was not as pronounced for men who have sex with men (MSM) (Figure 9).
Chlamydia Among Special Populations
More information on chlamydia screening programs for women of reproductive age and chlamydia among adolescents, minority populations, and people in corrections facilities is presented in the Special Focus Profiles.
Both test positivity and the number of reported cases of C. trachomatis infections remain high among most age groups, racial/ethnic groups, geographic areas, and both sexes. The reported number of chlamydia cases is higher among women, especially those of younger age (15–19 and 20–24 years), but this finding could be a reflection of screening recommendations. Racial differences also persist; reported case rates and prevalence estimates among blacks continue to be substantially higher than among other racial/ethnic groups.
1 Centers for Disease Control and Prevention. CDC Grand Rounds: Chlamydia prevention: challenges and strategies for reducing disease burden and sequelae. MMWR Morb Mortal Wkly Rep. 2011;60(12):370-3.
3 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;75:3-17.
4 Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2010; No.59(RR-12):1-110. Erratum in: MMWR Recomm Rep. 2011;60(1):18.
7 Datta SD, Torrone E, Kruszon-Moran D, Berman S, Johnson R, Satterwhite CL, Papp J, Weinstock H. Chlamydia trachomatis trends in the United States among persons 14 to 39 years of age, 1999-2008.Sex Transm Dis. 2012 Feb;39(2):92-6.
9 Satterwhite CL, Grier L, Patzer R, Weinstock H, Howards PP, Kleinbaum D. Chlamydia positivity trends among women attending family planning clinics: United States, 2004-2008.Sex Transm Dis. 2011 Nov;38(11):989-94.