Understanding the burden of Chlamydia trachomatis in populations is based primarily on studies of prevalence of current infection measured using sensitive and specific nucleic acid amplification tests on genital or urinary specimens. Such studies have produced widely varying results, but most have emphasised the high prevalence in young sexually active women, with fewer studies in men.1 Studies have frequently been based on clinic populations, which suffer from problems of selection bias (towards those at higher risk) and limited demographic and behavioural data. The few studies based on random and representative population samples have generally demonstrated lower prevalence and increased risk linked to increasing numbers of sexual partners.2 Studies of chlamydia incidence in population samples are few and far between. Data from national surveillance programmes consistently show rising rates of diagnosed infection in most developed countries over the last decade. But this cannot measure true incidence of infection because they are biased by the introduction and wider spread use of new sensitive tests leading to increased diagnosis of asymptomatic infection. All these studies measure current infection. They cannot account for past exposure or measure cumulative risk of infection over time that screening programmes aim to minimise.

In the last issue, Lyytikainen et al reported on Chlamydia trachomatis antibody seroprevalence in a stratified probability sample of 8000 sera from a large Finnish population serum bank.3 Participants were women under 29 years, …