title: Estimating Local Chlamydia Incidence and Prevalence Using Surveillance Data.
abstract: Understanding patterns of chlamydia prevalence is important for addressing inequalities and planning cost-effective control programs. Population-based surveys are costly; the best data for England come from the Natsal national surveys, which are only available once per decade, and are nationally representative but not powered to compare prevalence in different localities. Prevalence estimates at finer spatial and temporal scales are required.
  
  
  
  We present a method for estimating local prevalence by modeling the infection, testing, and treatment processes. Prior probability distributions for parameters describing natural history and treatment-seeking behavior are informed by the literature or calibrated using national prevalence estimates. By combining them with surveillance data on numbers of chlamydia tests and diagnoses, we obtain estimates of local screening rates, incidence, and prevalence. We illustrate the method by application to data from England.
  
  
  
  Our estimates of national prevalence by age group agree with the Natsal-3 survey. They could be improved by additional information on the number of diagnosed cases that were asymptomatic. There is substantial local-level variation in prevalence, with more infection in deprived areas. Incidence in each sex is strongly correlated with prevalence in the other. Importantly, we find that positivity (the proportion of tests which were positive) does not provide a reliable proxy for prevalence.
  
  
  
  This approach provides local chlamydia prevalence estimates from surveillance data, which could inform analyses to identify and understand local prevalence patterns and assess local programs. Estimates could be more accurate if surveillance systems recorded additional information, including on symptoms. See video abstract at, http://links.lww.com/EDE/B211.
authors.affiliation: From the aNIHR Health Protection Research Unit in Modelling Methodology and MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom; and bModelling and Economics Unit, National Infection Service, Public Health England, London, United Kingdom.
authors.forename: Joanna
authors.initials: J
authors.lastname: Lewis
authors.forename: Peter J
authors.initials: PJ
authors.lastname: White
mesh_terms: D000293:Adolescent; D017677:Age Distribution; D002690:Chlamydia Infections / Q000453:epidemiology* / Q000517:prevention & control; D003140:Communicable Disease Control / Q000458:organization & administration*; D004739:England / Q000453:epidemiology; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D011159:Population Surveillance; D011237:Predictive Value of Tests; D015995:Prevalence; D018570:Risk Assessment; D017678:Sex Distribution; D012749:Sexually Transmitted Diseases / Q000453:epidemiology* / Q000517:prevention & control; D055815:Young Adult