Chlamydia is the most common reportable bacterial sexually transmitted infection (STI) in the United States, with more than 1.5 million cases reported in 2015. Since many persons with chlamydial infection may have minimal or no symptoms, the actual number of annual infections is significantly higher than the reported cases. The number of reported chlamydia cases have significantly increased since the early years of reporting that began in the 1980’s (Figure 1), which may reflect an increase in the number of true infections, enhanced screening with more sensitive diagnostic tests, or a combination of both. Chlamydial rates of reported cases have consistently been higher in women than in men (Figure 2), with the highest rates (reported cases per 100,000 population) among among females 15 to 24 years of age (Figure 3). In the United States, racial and ethnic minorities are disproportionately affected by chlamydia, particularly blacks (Figure 4). Factors contributing to these inequities may include differential access to quality health care, social and economic conditions, and higher prevalence of disease in sexual networks, and differences in immunogenetic determinants that influence the immune response to chlamydia. The South has consistently had the highest rate of reported cases, although the difference between the rate in the South and other regions is small (Figure 5). The three states with the highest rates are Alaska, Louisiana, and North Carolina; of note, the rate in Washington DC is higher than any state (Figure 6).
Based on National Health and Nutrition Examination Surveys (NHANES), chlamydia prevalence in the U.S. is estimated to be 1.5%. Chlamydia prevalence is highest among adolescents and young adults, as well as among racial and ethnic minorities. Test positivity is often used as a proxy of chlamydia prevalence in a population. During 2007 to 2012, chlamydia test positivity among males and females aged 14 to 39 was 1.7%. Among sexually active females aged 14 to 24 years (the population targeted for routine screening), chlamydia prevalence was 4.7%; black females had, by far, the highest prevalence (Figure 7).
Risk factors associated with acquisition of chlamydial infection include new or multiple sex partners, a history of STIs, presence of another STI, and lack of barrier contraception. The presence of columnar epithelial cells on the ectocervix, referred to as ectopy, is a condition that may increase susceptibility to chlamydial infection; oral contraceptive use contributes to ectopy. Adolescents and young adults are at increased risk for chlamydial infection for a combination of biological, behavioral, and cultural reasons, including difficulty accessing preventive health care services for STIs.
Chlamydia is the most common non-viral STDs and the most significant contributor to cost, with total lifetime direct medical costs estimated at $516.7 million.[5,7,8,9] Although diagnosis and management of chlamydia is costly, untreated genital chlamydia infections can result in major complications for women, including pelvic inflammatory disease, chronic pelvic pain, fallopian tube scarring, and infertility.[10,11] In addition, studies have shown that rectal chlamydia infection in men who have sex with men significantly increases the risk of HIV acquisition. Screening for rectal chlamydia in men who have sex with men can be a cost-effective intervention for HIV prevention.