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Chlamydia trachomatis infections are the most commonly reported notifiable disease in the United States. They are among the most prevalent of all STDs and, since 1994, have comprised the largest proportion of all STDs reported to CDC (Table 1). Recent studies also demonstrate the high prevalence of chlamydial infections in the general U.S. population. Among young adults (18-26 years of age) participating in the nationally-representative National Longitudinal Study of Adolescent Health from 2001 to 2002, chlamydia prevalence was 4.2%.1

In women, chlamydial infections, which are usually asymptomatic, may result in pelvic inflammatory disease (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 a reduction in the incidence of PID by as much as 60%.2 As with other inflammatory STDs, chlamydial infection can facilitate the transmission of 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. Due to the large burden of disease and risks associated with infection, CDC recommends screening all sexually active women younger than 26 years of age for chlamydia annually.4

The increase in reported chlamydial infections during the last 10 years reflects the expansion of chlamydia screening activities, use of increasingly sensitive diagnostic tests, an increased emphasis on case reporting from providers and laboratories, improvements in the information systems for reporting, and, possibly, true increases in disease. However, many women who are at risk are still not being tested, reflecting, in part, lack of awareness among some health care providers and limited resources available to support screening. Chlamydia screening and reporting are likely to continue to expand further in response to the Healthcare Effectiveness Data and Information Set (HEDIS) annual measure assessing chlamydia screening coverage of sexually active women 16 through 25 years of age who receive medical care through commercial or Medicaid managed care organizations.5 In 2007, 36.4% of women aged 16-20 years were screened in commercial care settings; in Medicaid populations, 48.8% of women aged 16-20 years were screened.6

To better monitor trends in disease burden in defined populations during the expansion of chlamydia screening activities, data on chlamydia positivity and prevalence among persons screened in a variety of settings are used. In most instances, test positivity serves as a reasonable approximation of prevalence.7

Chlamydia –  United States

In 2000, for the first time, all 50 states and the District of Columbia had regulations requiring the reporting of chlamydia cases.

In 2007, 1,108,374 chlamydial infections were reported to CDC from 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 370.2 cases per 100,000 population, an increase of 7.5% compared with the rate of 344.3 in 2006. The reported number of chlamydial infections was over three times the number of reported cases of gonorrhea (355,991 gonorrhea cases were reported in 2007) (Table 1).

From 1988 through 2007, the rate of reported chlamydial infection increased from 87.1 to 370.2 cases per 100,000 population (Figure 1, Table 1).

Chlamydia by Region

For the years 1998 to 2007, overall rates were similar in the Midwest, West, and South (Figure 2). Rates have consistently remained lowest in the Northeast. In 2007, reported cases continued to increase in all regions (Table 3).

Chlamydia by State

In 2007, chlamydia rates per 100,000 population by state ranged from 156.3 cases in New Hampshire to 745.1 cases in Mississippi (Figure 3, Table 2). Fifteen states, the District of Columbia, and Guam had chlamydia case rates higher than 400 cases per 100,000 population.

Chlamydia by Metropolitan Statistical Area (MSA)

In 2007, the chlamydia case rate per 100,000 population in the 50 most populous MSAs increased overall, among both women and men (Table 6). Among women, the 2007 case rate of 568.7 was a 7.7% increase over the 2006 case rate of 528.1 (Table 7). The 2007 case rate among men (211.8 per 100,000 population) increased 11.9% from the 2006 case rate (189.2) (Table 8). In 2007, 57.3% of chlamydia cases were reported by these MSAs.

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 2007, 597 (19.0%) of 3,140 counties had rates greater than 400.0 cases per 100,000 population. Rates per 100,000 population were 300.0 or less in 2,228 counties (71.0%) and between 300.1 and 400.0 in 315 counties (10.0%). Fifty-three counties and independent cities reported 40% of all chlamydia cases in 2007 (Table 9). Case rates ranged from 247.9 (Miami-Dade County, Florida) to 1,265.0 (St. Louis (City), Missouri) per 100,000 population.

Chlamydia by Sex

In 2007, the overall rate of reported chlamydial infection among women in all 50 states and the District of Columbia (543.6 cases per 100,000 females) was almost three times higher than the rate among men (190.0 cases per 100,000 males), likely reflecting a greater 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 being diagnosed or reported as having chlamydia. However, with the advent of highly sensitive nucleic acid amplification tests (NAATs) that can be performed on urine, symptomatic and asymptomatic men are increasingly being diagnosed with chlamydial infection. From 2003 through 2007, the chlamydial infection rate in men increased by 42.9% (from 133.0 to 190.0 cases per 100,000 males) compared with a 17.3% increase in women during the same period (from 463.6 to 543.6 cases per 100,000 females).

Chlamydia by Age

Among women, the highest age-specific rates of reported chlamydia in 2007 were among those 15 to 19 years of age (3,004.7 cases per 100,000 females) and 20 to 24 years of age (2,948.8 cases per 100,000 females) (Figure 5, Table 10). When compared to 2003, case rates per 100,000 women have increased in these two age groups by 12.4% and 17.3%, respectively. These increased rates in women may, in part, reflect increased screening in this group. Age-specific rates among men, while substantially lower than the rates among women, were highest in the 20- to 24-year-old age group (932.9 cases per 100,000 males) (Figure 5, Table 10). Chlamydia case rates among men have increased in most age groups since 2003.

Chlamydia by Race/Ethnicity

In 2007, chlamydia rates increased for all racial and ethnic groups except American Indian/Alaska Natives. (Figure 6, Table 11B). The rate of chlamydia among blacks was over eight times higher than that of whites (1,398.7 and 162.3 cases per 100,000, respectively). The rates among American Indian/Alaska Natives (732.9) and Hispanics (473.2) were also higher than that of whites (4.5 and 2.9 times higher, respectively). In 2007, the chlamydia case rate per 100,000 population among Asian/Pacific Islanders was 139.5.

Chlamydia by Reporting Source

The majority of chlamydia cases reported in 2007 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 2007, among women, only 11.9% of chlamydia cases were reported through an STD clinic (98,382 of 825,660 total cases). In contrast, among men, 33.1% of chlamydia cases were reported through an STD clinic in 2007 (92,906 of 280,337 total cases).

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 14- to 39-year old population and provides an important measure of chlamydia disease burden. From 1999 to 2002, the overall prevalence of chlamydia infection was 2.2% and was similar between males and females (2.0% and 2.5%, respectively).8 Prevalence was higher among non-Hispanic blacks than non-Hispanic whites in all age groups (Figure 8).

Chlamydia Prevalence Monitoring Project

Chlamydia screening and prevalence monitoring activities were initiated in Health and Human Services (HHS) Region X (Alaska, Idaho, Oregon, Washington) in 1988 as a CDC-supported demonstration project. In 1993, chlamydia screening services for women were expanded to three additional HHS regions (III, VII, and VIII) and, in 1995, to the remaining HHS regions (I, II, IV, V, VI, and IX). In some regions, federally-funded chlamydia screening supplements local-and state-funded screening programs. Screening criteria and practices vary by region and state.

Family Planning Clinics

In 2007, the median state-specific chlamydia test positivity among 15- to 24-year-old women who were screened during visits to selected family planning clinics in all 50 states, Puerto Rico, and the Virgin Islands was 6.9% (range: 2.9% to 16.8%) (Figures 9 and 10). Since 1997, the median chlamydia positivity rate has slightly increased over time. This increase is likely due primarily to increasing usage of more sensitive test technology. See Appendix (Chlamydia, Gonorrhea, and Syphilis Prevalence Monitoring) for details.

Chlamydia test positivity among 15-24-year-old women screened in family planning clinics fluctuated in all 10 HHS regions between 2003 and 2007 (Figure 11). Positivity has remained fairly stable in four regions (I, III, V, X). From 2003 to 2006, slight decreases in positivity occurred in one region (II), followed by a small increase in 2007. In the remaining five regions (IV, VI, VII, VIII, IX), positivity rates increased slightly over the five-year time frame from 2003 to 2007. The positivity rates presented in Figure 11 are not adjusted for changes in laboratory test methods and associated increases in test sensitivity. Utilization of more sensitive tests has been shown to impact positivity rates.9 Usage of NAAT technology in family planning clinics to screen women aged 15-24 years for chlamydia is widespread (Figure 12). In four regions, NAATs were used nearly exclusively from 2003 to 2007 (I, V, VII, VIII). By 2007, five additional regions used NAATs over 50% of the time (II, III, IV, IX, X). Only one region (VI) reported a low NAAT-usage rate in 2007; however, usage increased from 2003 to 2007.

Chlamydia Among Special Populations

Additional information on chlamydia screening programs for women of reproductive age and chlamydia among adolescents, minority populations, and in corrections facilities can be found in the Special Focus Profiles.

Chlamydia Summary

Both prevalence and reported cases of genital Chlamydia trachomatis infections remain high across age groups, racial/ethnic groups, geographic locales, and both sexes. The burden of chlamydia appears higher among women, especially those of younger age (15 to 19 and 20 to 24 years of age), but this may be a reflection of persons recommended for screening. Racial differences also persist; case rates among blacks continue to be substantially higher than rates among other racial/ethnic groups.

1 Miller WC, Ford CA, Morris M, Handcock MD, Schmitz JL, Hobbs MM, Cohen MS, Mullan Harris K, Udry JR. Prevalence of chlamydial and gonococcal infections among young adults in the United States. JAMA 2004;291(18): 2229–36.

2 Scholes D, Stergachis A, Heidrich FE, Andrilla H, Holmes KK, Stamm WE. Prevention of pelvic inflammatory disease by screening for cervical chlamydial infection. N Engl J Med 1996;34(21):1362–66.

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, 2006. MMWR, 2006;55(No.RR-11):38.

5 National Committee for Quality Assurance (NCQA). HEDIS 2000: Technical Specifications, Washington, DC, 1999, pp. 68–70, 285–286.

6 National Committee for Quality Assurance (NCQA). The state of healthcare quality 2008, Washington, DC, 2008, pp.39.

7 Dicker LW, Mosure DJ, Levine WC. Chlamydia positivity versus prevalence: whats the difference? Sexually Transmitted Diseases 1998;25:251–3.

8 Datta SD, Sternberg M, Johnson RE, Berman S, Papp JR, McQuillan G, Weinstock H. Gonorrhea and chlamydia in the United States among persons 14 to 39 years of age, 1999 to 2002. Ann Intern Med 2007;147(2):89–96.

9 Dicker LW, Mosure DJ, Levine WC, et al. Impact of switching laboratory tests on reported trends in Chlamydia trachomatis infections. Am J Epidemiol 2000;51:430–5.

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