Incidence and reinfection rates of genital chlamydial infection among women aged 16-24 years attending general practice, family planning and genitourinary medicine clinics in England: a prospective cohort study by the Chlamydia Recall Study Advisory Group

Background: In England, screening for genital chlamydial infection has begun; however, screening frequency for women is not yet determined. We aimed to measure chlamydia incidence and re-infection rates among young women to suggest screening intervals. Methods: An 18-month prospective cohort study of women aged 16-24 years recruited from general practices (GPs), family planning (FP) and genitourinary medicine (GUM): baseline-negative women followed for incidence and baseline-positive women for re-infection; urine tested every six months via nucleic acid amplification; and behavioural data collected. Extra test and questionnaire completed three months after initial positive test. Factors associated with infection and re- infection investigated using Cox regression stratified by healthcare setting of recruitment. Results: Chlamydia incidence was 4.9 (2.7-8.8) per 100 person-years (py) among women recruited from GPs, 6.4 (4.2-9.8) from FP, 10.6 (7.4-15.2) from GUM. Incidence was associated with young age, history of chlamydial infection, and new sex partner acquisition. If recently acquiring new partners, condom use at last sex was independently associated with lower incidence. Chlamydia re-infection was 29.9 (19.7-45.4) per 100/py from GPs, 22.3 (15.6-31.8) from FP, and 21.1 (14.3-30.9) from GUM. Factors independently associated with higher re-infection rates were acquiring new partners and failure to treat all partners. Conclusions: Sexual behaviours determined incidence and re-infection, regardless of healthcare setting. Our results suggest annual screening of women aged 16-24 years who are chlamydia-negative, or sooner if partner change occurs. Re-screening chlamydia- positive women within six months of baseline infection may be sensible, especially if partner change occurs or all partners are not treated.