Background: Although understanding chlamydia incidence assists prevention and control, analyses based on diagnosed infections may distort the findings. Therefore, we determined incidence and examined risks in a birth cohort based on self-reports and serology.
Methods: Self-reported chlamydia and behavior data were collected from a cohort born in New Zealand in 1972/3 on several occasions to age 38 years. Sera drawn at ages 26, 32, and 38 years were tested for antibodies to Chlamydia trachomatis Pgp3 antigen using a recently developed assay, more sensitive in women (82.9%) than men (54.4%). Chlamydia incidence by age period (first coitus to age 26, 26–32, and 32–38 years) was calculated combining self-reports and serostatus and risk factors investigated by Poisson regression.
Results: By age 38 years, 32.7% of women and 20.9% of men had seroconverted or self-reported a diagnosis. The highest incidence rate was to age 26, 32.7 and 18.4 years per 1000 person-years for women and men, respectively. Incidence rates increased substantially with increasing number of sexual partners. After adjusting age period incidence rates for partner numbers, a relationship with age was not detected until 32 to 38 years, and then only for women.
Conclusions: Chlamydia was common in this cohort by age 38, despite the moderate incidence rates by age period. The strongest risk factor for incident infection was the number of sexual partners. Age, up to 32 years, was not an independent factor after accounting for partner numbers, and then only for women. Behavior is more important than age when considering prevention strategies.
A third of women born in Dunedin, New Zealand had evidence of chlamydia by age 38 years, with risk more strongly determined by behavior than age to the early 30s.
From the *Department of Preventive and Social Medicine, University of Otago, Dunedin; †Sexual health Clinic, Waikato Hospital, Hamilton, New Zealand; ‡School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom; §National Institute for Health Research Health Protection Research Unit in Evaluation of Interventions in partnership with Public Health England, University of Bristol, Bristol; and ¶Jefferiss Trust Laboratories, Wright-Fleming Institute, Imperial College London, London, United Kingdom
M.M.C. and N.P.D. are joint senior authors.
Conflicts of interest: none declared.
Sources of Support: The Dunedin Multidisciplinary Health and Development Research Unit is supported by the Health Research Council of New Zealand and the New Zealand Ministry of Business, Innovation and Employment. The authors are grateful to Richie Poulton, Director of the Research Unit, and Unit Research staff. The authors are indebted to Dr Phil A. Silva, the founder of the Dunedin Study, and to the Study members and their families and friends for their long-term and continued involvement. The research was also partly supported by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Evaluation of Interventions at the University of Bristol in partnership with Public Health England (PHE) and the NIHR BRC at Imperial College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, the Department of Health or PHE.
Correspondence: Antoinette Righarts, PhD, Department of Preventive and Social Medicine, University of Otago, P. O. Box 56, Dunedin 9054, New Zealand. E-mail: email@example.com.
Received for publication July 28, 2016, and accepted January 31, 2017.
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