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Background: Seroprevalence surveys of Chlamydia trachomatis (CT) antibodies are promising for estimating age-specific CT cumulative incidence, however accurate estimates require improved understanding of antibody response to CT infection. Methods: We used GUMCAD, England’s national sexually transmitted infection (STI) surveillance system, to select sera taken from female STI clinic attendees on the day of or after a chlamydia diagnosis. Serum specimens were collected from laboratories and tested anonymously on an indirect and a double-antigen ELISA, both of which are based on the CT-specific Pgp3 antigen. We used cross-sectional and longitudinal descriptive analyses to explore the relationship between seropositivity and a) cumulative number of chlamydia diagnoses and b) time since most recent chlamydia diagnosis. Results: 919 samples were obtained from visits when chlamydia was diagnosed and 812 during subsequent follow-up visits. Pgp3 seropositivity using the indirect ELISA increased from 57.1% (95% confidence interval: 53.2–60.7) on the day of a first-recorded chlamydia diagnosis to 89.6% (95%CI: 79.3–95.0) on the day of a third or higher documented diagnosis. With the double-antigen ELISA, the increase was from 61.1% (95%CI: 53.2–60.7) to 97.0% (95%CI: 88.5–99.3). Seropositivity decreased with time since CT diagnosis on only the indirect assay, to 49.3% (95%CI: 40.9–57.7) two or more years after a first diagnosis and 51.9% (95%CI: 33.2–70.0) after a repeat diagnosis. Conclusion: Seropositivity increased with cumulative number of infections, and decreased over time after diagnosis on the indirect ELISA, but not on the double-antigen ELISA. This is the first study to demonstrate the combined impact of number of chlamydia diagnoses, time since diagnosis, and specific ELISA on Pgp3 seropositivity. Our findings are being used to inform models estimating age-specific chlamydia incidence over time using serial population-representative serum sample collections, to enable accurate public health monitoring of chlamydia. [ABSTRACT FROM AUTHOR]

Background: Opportunistic chlamydia screening of <25 year-olds was nationally-implemented in England in 2008 but its impact on chlamydia transmission is poorly understood. We undertook a population-based seroprevalence study to explore the impact of screening on cumulative incidence of chlamydia, as measured by C.trachomatis-specific antibody. Methods: Anonymised sera from participants in the nationally-representative Health Surveys for England (HSE) were tested for C.trachomatis antibodies using two novel Pgp3 enzyme-linked immunosorbent assays (ELISAs) as a marker of past infection. Determinants of being seropositive were explored using logistic regression among 16–44 year-old women and men in 2010 and 2012 (years when sexual behaviour questions were included in the survey) (n = 1,402 women; 1,119 men). Seroprevalence trends among 16–24 year-old women (n = 3,361) were investigated over ten time points from 1994–2012. Results: In HSE2010/2012, Pgp3 seroprevalence among 16–44 year-olds was 24.4% (95%CI 22.0–27.1) in women and 13.9% (11.8–16.2) in men. Seroprevalence increased with age (up to 33.5% [27.5–40.2] in 30–34 year-old women, 18.7% [13.4–25.6] in 35–39 year-old men); years since first sex; number of lifetime sexual partners; and younger age at first sex. 76.7% of seropositive 16–24 year-olds had never been diagnosed with chlamydia. Among 16–24 year-old women, a non-significant decline in seroprevalence was observed from 2008–2012 (prevalence ratio per year: 0.94 [0.84–1.05]). Conclusion: Our application of Pgp3 ELISAs demonstrates a high lifetime risk of chlamydia infection among women and a large proportion of undiagnosed infections. A decrease in age-specific cumulative incidence following national implementation of opportunistic chlamydia screening has not yet been demonstrated. We propose these assays be used to assess impact of chlamydia control programmes. [ABSTRACT FROM AUTHOR]

Seroepidemiology of chlamydia can offer study opportunities and insights into cumulative risk of exposure that may contribute to monitoring the frequency of, and control of, genital chlamydia–the most commonly diagnosed STI in England. We undertook retrospective anonymous population-based cross-sectional surveys using an indirect IgG ELISA for chlamydia Pgp3 antibody. Sera from 4,732 women aged 17–24 years were tested. Samples were taken at 3-yearly intervals between 1993 and 2002, a period during which other data suggest chlamydia transmission may have been increasing, and from each year between 2007 and 2010. Seroprevalence increased in 17–24 year olds over time between 1993 and 2002. Between 2007 and 2010, age-standardised seroprevalence among 17–24 year olds decreased from 20% (95% CI: 17–23) to 15% (95%CI 12–17) (p = 0.0001). The biggest drop was among 20 to 21 year olds, where seroprevalence decreased from 21% in 2007 to 9% in 2010 (p = 0.002). These seroprevalence data reflect some known features of the epidemiology of chlamydia infection, and show that exposure to antibody-inducing chlamydia infection has declined in recent years. This decline was concurrent with increasing rates of screening for asymptomatic chlamydia. Serology should be explored further as a tool for evaluation of chlamydia control, including chlamydia screening programmes. [ABSTRACT FROM AUTHOR]