Chlamydia trachomatis Pgp3 Antibody Population Seroprevalence before and during an Era of Widespread Opportunistic Chlamydia Screening in England (1994-2012)

Abstract

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.

Fig 1. Pgp3 seroprevalence by age group (1a & 1b) and by years since first heterosexual sex (1c & 1d) (sexually-experienced 16- to 44-year-olds, HSE2010 & HSE2012).

Solid lines show point estimates; dashed lines show 95% confidence intervals. N shows unweighted denominators. 95% confidence intervals are not shown in Fig 1a for 16–17 year-olds or in Fig 1b for those within 0–1 years of first heterosexual sex as no individuals in this group were Pgp3 seropositive.

Fig 2. Pgp3 seroprevalence by reported numbers of lifetime sexual partners (among 16- to 44-year olds, HSE2010 & HSE2012).

Fig 3. Pgp3 seroprevalence by year (16- to 24-year-old women, HSE 1994 to 2012).

Solid lines show point estimates; dashed lines show 95% confidence intervals. Unweighted denominators: 1994–1996, n = 1,555; 2001–2002, n = 1097; 2008–2012, n = 709.

Fig 4. Pgp3 seroprevalence by birth cohort and year of age (16- to 24-year-old women, HSE 1994 to 2012).

Unweighted denominators: High screening exposure (born 1992–1996), n = 185; Partial screening exposure (born 1984–1991), n = 853; Limited screening exposure (born 1976–1983), n = 1349; Limited screening exposure (born 1966–1975), n = 929. See online appendix (S1 File) for details of denominators by year of age.