Changes in chlamydia prevalence over time: how to observe the unobserved
Authors: Nicola Low, Joost H Smid
Journal: The Lancet Public Health (2018)

The ways that widespread testing and treatment of sexually transmitted Chlamydia trachomatis infections affect the prevalence and incidence of infection at the population level are not easy to understand, and the difficulties are well documented.1Low N Screening programmes for chlamydial infection: when will we ever learn?.BMJ. 2007; 334: 725-728Crossref PubMed Google Scholar First, chlamydia is an infection, so the dynamics of transmission between infectious and susceptible individuals are given by a non-linear function. Second, chlamydia screening recommendations were introduced without randomised controlled trial evidence of the effects of screening on prevalence. Third, most chlamydia infections are asymptomatic, so no reliable signal in surveillance data exists that distinguishes between changes in incidence and testing. Fourth, yearly population-based prevalence surveys with adequate coverage and precision are financially unfeasible. To help interpret the real-world effects of widespread chlamydia testing, England has more comprehensive and complete surveillance data2Chandra NL Soldan K Dangerfield C et al.Filling in the gaps: estimating numbers of chlamydia tests and diagnoses by age group and sex before and during the implementation of the English National Screening Programme, 2000 to 2012.Euro Surveill. 2017; 22: 30453Crossref Scopus (15) Google Scholar than most countries. And, by chance rather than design, the British National Surveys of Sexual Attitudes and Lifestyles (Natsal) measured C trachomatis prevalence in 1999–2001 (Natsal-2; just before the National Chlamydia Screening Programme in England, [NCSP] started its roll out) and 2010–12 (Natsal-3; just after the NCSP achieved countrywide coverage).3Sonnenberg P Clifton S Beddows S et al.Prevalence, risk factors, and uptake of interventions for sexually transmitted infections in Britain: findings from the National Surveys of Sexual Attitudes and Lifestyles (Natsal).Lancet. 2013; 382: 1795-1806Summary Full Text Full Text PDF PubMed Scopus (263) Google Scholar The essence of the dilemma is that point prevalence estimates in the two surveys were nearly identical—chlamydia prevalence in women aged 18–24 years was 3·1% (95% CI 1·8–5·2) in Natsal-2 and 3·2% (2·2–4·6) in Natsal-3—and CIs were compatible with a decrease or increase in prevalence. But complex transmission dynamic mathematical modelling studies had predicted that, with the levels of investment and test coverage that the NCSP achieved, chlamydia prevalence should have dropped measurably and sustainably by now.4Department of HealthYoung people's sexual health: the National Chlamydia Screening Programme. National Audit Office, LondonNov 12, 2009https://www.nao.org.uk/wp-content/uploads/2009/11/0809963.pdfDate accessed: May 8, 2018Google Scholar In a model-based analysis, Joanna Lewis and Peter White5Lewis J White PJ Changes in chlamydia prevalence and duration of infection inferred from testing and diagnosis rates in England: a modelling study using surveillance data, 2000–15.Lancet Public Health. 2018; (published online May 15.)http://dx.doi.org/10.1016/S2468-2667(18)30071-9PubMed Google Scholar aim to clarify what might have happened between, and since, Natsal-2 and Natsal-3. The advantages of their approach are that they use national data on chlamydia testing and diagnosis and they developed a mathematical model that represents the relationships between uninfected and infected groups of people, with a minimal level of complexity. The code for the model is available online, so anyone can reproduce the findings and explore the model further. The model does not make predictions but allows a post-hoc interpretation of what might have resulted in the inferred changes in prevalence year by year. Lewis and White5Lewis J White PJ Changes in chlamydia prevalence and duration of infection inferred from testing and diagnosis rates in England: a modelling study using surveillance data, 2000–15.Lancet Public Health. 2018; (published online May 15.)http://dx.doi.org/10.1016/S2468-2667(18)30071-9PubMed Google Scholar propose that increasing chlamydia test coverage in the NCSP to more than 20% of men and more than 40% of women by 2010 resulted in a reduction in chlamydia prevalence, which occurred between the Natsal-2 and Natsal-3 data collection periods. The model-estimated rebound in prevalence, from 2011 to 2012, coincided with a decrease in chlamydia testing. The model probably does not include some essential processes. Sex differences in prevalence and incidence highlight important issues. Between 2000 and 2007, prevalence inferred by the model increased in men and decreased in women (based on the authors' preferred estimates of testing). The data presentation, as year-to-year changes, can be difficult to interpret. Using the published code to plot absolute chlamydia incidence rates, with the maximum testing and diagnosis estimates, the model output estimates that between 2000 and 2007, chlamydia incidence rates increased by a factor of 2·04 (95% credible interval 1·92–2·14) in men aged 15–19 years and by a factor of 1·84 (1·76–1·91) in men aged 20–24 years (figure). Lewis and White5Lewis J White PJ Changes in chlamydia prevalence and duration of infection inferred from testing and diagnosis rates in England: a modelling study using surveillance data, 2000–15.Lancet Public Health. 2018; (published online May 15.)http://dx.doi.org/10.1016/S2468-2667(18)30071-9PubMed Google Scholar suggest that male sexual risk-taking behaviour increased during that period, but this explanation is not entirely convincing because the substantial changes that would be needed to result in such large changes in incidence should affect both men and women. Data from Natsal6Mercer CH Tanton C Prah P et al.Changes in sexual attitudes and lifestyles in Britain through the life course and over time: findings from the National Surveys of Sexual Attitudes and Lifestyles (Natsal).Lancet. 2013; 382: 1781-1794Summary Full Text Full Text PDF PubMed Scopus (528) Google Scholar and from the USA over the same period,7Chandra A Mosher WD Copen C Sionean C Sexual behavior, sexual attraction, and sexual identity in the United States: data from the 2006–2008 National Survey of Family Growth.Natl Health Stat Report. 2011; 36: 1-36PubMed Google Scholar did not find marked or sex-specific changes in behaviour. Increased test coverage but poorer targeting over time of people with chlamydia could also explain a negligible decrease in prevalence. The model assumes the same screening rate in uninfected and infected people, although people at higher risk of chlamydia infection are more likely to be tested.3Sonnenberg P Clifton S Beddows S et al.Prevalence, risk factors, and uptake of interventions for sexually transmitted infections in Britain: findings from the National Surveys of Sexual Attitudes and Lifestyles (Natsal).Lancet. 2013; 382: 1795-1806Summary Full Text Full Text PDF PubMed Scopus (263) Google Scholar, 8Riha J Mercer CH Soldan K French CE Macintosh M Who is being tested by the English National Chlamydia Screening Programme? A comparison with national probability survey data.Sex Transm Infect. 2011; 87: 306-311Crossref PubMed Scopus (32) Google Scholar, 9Woodhall SC Soldan K Sonnenberg P et al.Is chlamydia screening and testing in Britain reaching young adults at risk of infection? Findings from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3).Sex Transm Infect. 2016; 92: 218-227Crossref PubMed Scopus (30) Google Scholar NCSP test coverage targets after full implementation resulted in more tests, but the diagnosis rate did not change, suggesting that tests were done disproportionately amongst uninfected people.10Simms I Talebi A Riha J et al.The English National Chlamydia Screening Programme: variations in positivity in 2007/2008.Sex Transm Dis. 2009; 36: 522-527Crossref PubMed Scopus (28) Google Scholar Thus, the model outputs are consistent with Natsal-2 and Natsal-3 prevalence estimates but, to balance the increase in screening rates, the model estimates large increases in incidence in men and high absolute incidence in women. At the public health level, the model output suggests that large increases in chlamydia testing were compatible with a modest temporary reduction in chlamydia prevalence, well below expectations. Whether these changes represent good value for money can be debated.4Department of HealthYoung people's sexual health: the National Chlamydia Screening Programme. National Audit Office, LondonNov 12, 2009https://www.nao.org.uk/wp-content/uploads/2009/11/0809963.pdfDate accessed: May 8, 2018Google Scholar Duration of infection might, as Lewis and White suggest, be a better indicator of programme performance than prevalence. However, adopting this measure depends on the reliability of model-estimated prevalence and incidence. Lewis and White's study5Lewis J White PJ Changes in chlamydia prevalence and duration of infection inferred from testing and diagnosis rates in England: a modelling study using surveillance data, 2000–15.Lancet Public Health. 2018; (published online May 15.)http://dx.doi.org/10.1016/S2468-2667(18)30071-9PubMed Google Scholar highlights important research priorities. Empirical estimates of C trachomatis incidence in the presence of widespread testing would be very valuable. Further mathematical modelling studies should investigate reasons for the gap between model-predicted and observed reductions in chlamydia prevalence, including the effects of differential testing according to infection status. And, a fourth Natsal is needed to provide the next estimates of C trachomatis prevalence and of sexual behaviour in Britain. National chlamydia control strategies need to be informed by sound evidence of effectiveness and optimal chlamydia case management. The search for robust indicators to monitor the dynamics of C trachomatis during screening interventions continues. We declare no competing interests. Changes in chlamydia prevalence and duration of infection estimated from testing and diagnosis rates in England: a model-based analysis using surveillance data, 2000–15Our 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, which is a measure of screening effectiveness that is unaffected by behavioural changes. Full-Text PDF Open AccessEstimating chlamydia prevalence: more difficult than modelling suggests – Authors' replyLow and Smid's Comment1 on our Article2 analysing chlamydia testing data from England stated: "The model assumes the same screening rate in uninfected and infected people, although people at higher risk of chlamydia infection are more likely to be tested." The authors' use of "screening" interchangeably with "testing" is ambiguous. In our model,2,3 testing in the absence of symptoms (ie, screening) by those with asymptomatic infection and those who are uninfected is indeed at the same rate. However, the rate of (diagnostic) testing by those with symptomatic infection is much higher. Full-Text PDF Open Access