Background There have been substantial increases in chlamydia screening among young adults in England since the introduction of the National Chlamydia Screening Programme in 2003. The impact of the programme on the incidence of chlamydia in practise is unknown. We used stored sera to investigate trends in seroprevalence for chlamydia.

Methods Unlinked, anonymous, residual serum specimens were obtained from: (1) an approximate population-based collection, the Health Protection Agency Seroepidemiology Unit (SEU), consisting of sera submitted to laboratories in England for routine investigations (n = 4,732); (2) a higher STI-risk collection, the Unlinked Anonymous Survey of Genitourinary Medicine Clinic Attendees (GUMAnon), consisting of sera from women tested for syphilis in two GUM clinics (n = 5,431). Specimens from women aged 17–24, between 1993 and 2010 (SEU) and women aged < 20–34, between 1998 and 2009 (GUMAnon) were tested using an indirect IgG ELISA for chlamydia Pgp3 antibody.

Results In the SEU samples, seroprevalence amongst 17–24 year-olds increased between 1993 and 2002 (from 17% to 21%), and decreased between 2007 and 2010 (20% to 15%). The biggest decrease was among 20–21 year-olds (21% to 9%). In the GUMAnon samples, seroprevalence was consistently higher and declined in < 20 year olds between 2002 and 2009 (48% to 38%); no notable changes were seen in older ages. Seroprevalence was generally higher among older age groups within each collection.

Conclusions Pgp3 seroprevalence reflected known epidemiology of chlamydia infection with regard to increases between 1993–2002, by age and by risk. Given this, the decline in seroprevalence in recent years, particularly in younger age groups, suggests that the increased chlamydia screening during these years is changing the epidemiology of Pgp3 antibody-inducing chlamydia infection. Further exploration of Pgp3 seroprevalence as a tool for evaluation of chlamydia screening programmes is warranted.