We detected a substantial burden of chlamydia infections with a twofold higher prevalence in girls than in boys and with the infections beginning to be acquired soon after sexual initiation. The girls started to have sexual intercourse at younger age, had older partners, more frequently were in steady relationships, and reported higher numbers of lifetime partners than the boys. The boys claimed more substance use related to last intercourse and overall, had same-aged or younger partners, and remained better condom users. Accordingly, girls and boys had differing independent risk factors for chlamydia infection.

Prevalence

A chlamydia prevalence of 5.7% was significantly higher than detected in two high school studies in South Norway; 2.0%, and Luxembourg; 1.9%, and in a population based Dutch study in age group 15–19 years; 2.9% [6, 7, 9]. It is more comparable to 5.2% detected in a high school study in urban Philadelphia, USA [8]. The high prevalence is in line with the high incidence rates observed in surveillance data, and was to be expected as adolescents living in high prevalence STI areas have significantly increased odds of having a current STI given the available pool of infected partners [16]. A twofold higher prevalence in girls is similar to the results in the above mentioned studies [6–9]. However, 7.3% is probably a minimum estimate in the female participants as C. trachomatis was detected in FVU samples that are 10% less sensitive than self-collected vaginal swabs [17].

Socio-demographic characteristics

Sami/Sami-Norwegian girls having twice the prevalence of ethnic Norwegian girls is in line with a surveillance-based study from 1993 that observed a 6 times higher chlamydia incidence in a Sami municipality in Finnmark compared to the national average [18]. The Sami/Sami-Norwegian girls more frequently lived outside the family home and reported higher numbers of lifetime sexual partners than the Norwegian girls.

One-third of all participants lived in villages without high schools and had left home to attain further education, and these participants had twice the odds of infection compared to those living at home. To our knowledge, this has not been assessed in previous studies. Lack of parental control and detachment from the norms of their community of origin may explain the observed differences.

Maternal educational level ≥ college was associated with a twofold higher prevalence in girls, but not in boys. Daughters of higher educated mothers reported more substance use overall and in connection with last intercourse than those with less educated mothers. In contrast, maternal education ≥ college level was shown to protect against STIs in a longitudinal study in the USA [19]. The opposing results may reflect cultural differences regarding sexual norms with higher educated mothers in the Nordic countries leaving their daughters more freedom than their American counterparts.

Sexual behaviour

Condom use at first intercourse was a significantly better predictor of condom use at last intercourse in boys than in girls and can partly explain why condom use at sexual debut was highly protective against chlamydia only in the boys’ multivariable model. The poorer predictability of condom use at last intercourse in girls and the finding that more boys than girls used condoms at last intercourse may indicate that girls switch to hormonal contraceptives. Adolescent girls may also lack power to negotiate safe sex with their mostly older partners [5]. Condom use at last intercourse with last partner may be associated with use at previous sexual encounters and thereby explain the protective effect against chlamydia observed in all participants. Most studies show that condom use is associated with reduced chlamydia risk in both women and men [20].

As observed in other studies, number of sexual partners past 6 months was strongly associated with chlamydia infection in girls [7, 21]. The lack of association in boys could be due to boys frequently over-reporting their number of sexual partners [22].

A higher number of gender-specific C. trachomatis genotypes had previously been detected in girls than in boys in this study population [14]. Based on the genotyping results and most girls reporting older last sexual partners, we concluded that the girls were linked to off-school sexual networks with a different genotype reservoir than same-age boys. As chlamydia infections in surveillance data peak in males aged 20–24 years, we assumed that the older male partners would have higher chlamydia rates than our high school boys. Accordingly, we expected that having an older partner would increase the odds of infection in girls [12]. Due to less than one-fourth of girls having last sexual partner same age or younger, the increased infection risk in adolescent girls usually associated with age disparities may have been obscured [23]. To our knowledge, this is the first study to apply high-resolution genotyping as biological support for participants’ self-reported sexual behaviour in a population based study. Only 12% of the boys reported last sexual partner ≥1 year older, but this increased the odds of chlamydia threefold in boys and is similar to the results observed in a recent study [24]. The increase in odds disappeared when adjusting for number of partners past 6 months, indicating that adolescent boys who attract older women may have more opportunities for sex and hence are more sexually active than peers with younger or same-aged partners.

An increased infection risk associated with sexual partners met at a private party, bar or disco could reflect high-risk sexual behaviours and higher chlamydia prevalence among individuals who frequent these venues [23].

Young age at first sexual intercourse is a commonly reported risk factor for chlamydia in adolescents [25]. The Nordic countries traditionally have a higher acceptance of both female and adolescent sexuality than most other Western industrialized countries and are often regarded as representing liberated cultures [4, 26]. More than 40% of the sexually active girls in our study reporting sexual debut at ≤14 years may indicate that sexual activity in adolescent girls is accepted in these communities and could explain why early sexual debut did not appear as a risk factor in girls. This is supported by a recent study showing that early coital debut was independently associated with living in Northern Norway [27]. In the boys’ crude analysis, early first intercourse was only borderline significant.

The following factors were assumed to be important for the unusually high participation rate in our study: the school-based setting, a test result notification time of only 1–2 days, and class-wise data collection by the same professional study staff.

This is one of few population based studies on prevalent chlamydia infections and associated sexual behaviours in Europe covering both girls and boys aged 15–20 years. We showed that girls and boys accumulate different experiences early in their sexual careers which contribute to the differing chlamydia risk. It confirmed traditional factors commonly associated with chlamydia (female gender, multiple sex partners, older partners, no condom use), but also detected less studied demographic characteristics (residence outside the family home, maternal education) and risk factors (meeting venues for sexual partners).

Limitations

The study is limited by the cross-sectional design that precludes establishing causality, the self-reported behavioural data, and the lack of statistical power with only 41 chlamydia cases in girls and 18 in boys. Although the use of a web-based questionnaire is likely to have reduced social desirability bias [28], sensitive information on sexual behaviour and substance use were self-reported and could be prone to such bias. Finally, our findings may be applicable mainly to the Nordic countries as sexual behaviour has been shown to vary between different cultures and countries [29].

Conclusions and recommendations

In conclusion, girls this age may be the most cost-effective targets for preventive measures and screening due to a high burden of infections and our finding that young girls often make poor choices regarding their sexual health. However, young boys should also be targeted to make them partners in STI control early on. Gender-specific approaches to control chlamydia infections at this particular age may be the best alternative.