Introduction Genital infection with Chlamydia trachomatis (CT) is the most commonly-diagnosed bacterial sexually transmitted infection in England. The National Chlamydia Screening Programme (NCSP) was implemented nationwide in 2008, offering opportunistic CT testing to people under 25. Not all chlamydia infections result in a lasting antibody response, however, monitoring age-specific seroprevalence of antibodies against CT over time may offer insights into the impact of this intervention. We explored trends in seroprevalence from 2007 up to 2015.
Methods Samples were obtained from the PHE Seroepidemiology Unit, which collects unlinked, anonymous, residual sera submitted to laboratories in England for routine investigations. Samples known to come from GUM clinics were excluded. Sera from 2007–2015 from women aged 15–30 (n=9,798) were tested using an indirect IgG ELISA for chlamydia Pgp3 antibody. Women in 2007 had limited exposure to the NCSP, increasing over time. Age-standardised seroprevalence was calculated for 17–24 year-olds using 2015 population data. Samples were classified by the number of years individuals were eligible for the screening programme, based on year of birth.
Results Age-standardised seroprevalence among 17–24 year-olds varied, being highest at 20.3% (95% CI 17.2–23.4) in 2007 and lowest at 15.5% (95% CI 10.0–20.9) in 2015, although no clear trend was seen. Although incomplete data were available for those with ‘limited’ and ‘high’ exposure to the NCSP, age-specific seroprevalence did not vary by exposure to NCSP.
Conclusion There was no evidence that age-specific seroprevalence varied by exposure to the NSCP. Interpretation of this is complicated by the potential effects of antibody prevalence waning over time, and being affected by factors such as treatment and re-infection. Other limitations include a high number (86.2%) of specimens from ‘unknown’ source which could have been from GUM clinics. Multi-parameter evidence synthesis models are being developed to explore the use of these data to estimate incidence.