One in three New Zealand women and one in five New Zealand men have had the Sexually Transmitted Infection (STI) chlamydia by age 38, a new study reveals.
It's also believed the rate of infection could be even higher in young people, according to the latest findings from the Dunedin Multidisciplinary Health and Development Study.
The findings show chlamydia, which can affect fertility in women, is more common in New Zealand than previously thought.
Lead author Dr Antoinette Righarts from the Dunedin School of Medicine says the prevalence could even be higher in younger people, as the study focused on those born between April 1972 and March 1973.
"This high cumulative exposure we found at age 38 was mainly due to infections occurring when this cohort were teenagers and young adults, which was before New Zealand - and other high-income countries - experienced a marked increase in chlamydia in the late 1990s."
Dr Righarts says tests for chlamydia are better at detecting past infection in women than in men, which is one reason for the higher rates found among women.
"The true gender difference might not be as marked as we found," she says.
Co-author Dr Paddy Horner from the University of Bristol says the high rates are worrying for their potential impact on women's reproductive health.
"Recent estimates in the United Kingdom indicate 17 percent of infections in women progress to pelvic inflammatory disease, with 0.5 percent of women becoming infertile and 0.2 percent having an ectopic pregnancy as a consequence of irreversible damage to the fallopian tubes."
Otago University honorary research associate professor Nigel Dickson says the data collected on several occasions allowed researchers to see the rates of infection at various ages.
"We showed that overall the chlamydia rates in the cohort were lower after age 26, but this decline was mainly because fewer individuals had multiple sexual partners as they got older."
Sexual health specialist Dr Jane Morgan says the findings highlight that primary prevention with condoms and early detection through testing needs to be aimed at the wide age group of men and women whose behaviour puts them at increased risk, and not just young people.
The findings from the study, also known as The Dunedin Study, have been published in the journal Sexually Transmitted Diseases.