Background. The association between chlamydia infection and pelvic inflammatory disease (PID) is a key parameter
for models evaluating the impact of chlamydia control programs. We quantified this association using a
retrospective population-based cohort.
Methods. We used administrative health data sets to construct a retrospective population-based cohort of
women and girls aged 12–24 years who were resident in Manitoba, Canada, between 1992 and 1996. We performed
survival analysis on a subcohort of individuals who were tested for chlamydia to estimate the risk of PID diagnosed in
a primary care, outpatient, or inpatient setting after ≥1 positive chlamydia test.
Results. A total of 73 883 individuals contributed 625 621 person years of follow-up. Those with a diagnosis of
chlamydia had an increased risk of PID over their reproductive lifetime compared with those who tested negative
(adjusted hazard ratio [AHR], 1.55; 95% confidence interval [CI], 1.43–1.70). This risk increased with each subsequent
infection: the AHR was 1.17 for first reinfection (95% CI, 1.06–1.30) and 1.35 for the second (95% CI,
1.04–1.75). The increased risk of PID from reinfection was highest in younger individuals (AHR, 4.55 (95% CI,
3.59–5.78) in individuals aged 12–15 years at the time of their second reinfection, compared with individuals
older than 30 years).
Conclusions. There is heterogeneity in the risk of PID after a chlamydia infection. Describing the progression to
PID in mathematical models as an average rate may be an oversimplification; more accurate estimates of the costeffectiveness
of screening may be obtained by using an individual-based measure of risk. Health inequalities may be
reduced by targeting health promotion interventions at sexually active girls younger than 16 years and those with a
history of chlamydia.