Background: Chlamydia screening programmes have been implemented in several countries, but the effects of screening on incidence, prevalence and reproductive sequelae remain unclear. In England, despite increases in testing with the roll-out of the National Chlamydia Screening Programme (2003-2008), prevalence estimated in population-based surveys was similar in 1999-2001 and 2010-12 – although the precision of the estimates was limited.
Methods: We used newly-published annual figures for chlamydia test coverage and diagnoses in England before, during and after the scale-up of national screening, with recently-developed statistical methods which account for symptomatic chlamydia testing and asymptomatic screening, to infer changes in prevalence and average duration of infections in each year.
Findings: The data provided numbers of tests and diagnoses in 15-19 and 20-24-year-old men and women in England each year from 2000 to 2015, allowing annual – rather than ten-yearly – estimates of prevalence change. We inferred reductions in prevalence and average infection duration in both sexes once screening was fully implemented. From 2008 to 2010, inferred prevalence reductions were 0.7(0.3,1.4) and 0.8(0.4-1.3) percentage points in 15-24-year-old men and women, respectively (posterior median; 95% credible interval), and average duration of infection fell by 74(17-247) days in men and 30(22-40) days in women. Progress has recently reversed with declining testing.
Interpretation: Our analysis provides the first evidence for a reduction in chlamydia prevalence in England concurrent with large-scale population testing. It also shows a consistent decline in the average duration of infections: a measure of screening effectiveness that is unaffected by behavioural changes.