Samkange-Zeeb et al. BMC Public Health 2011, 11:727 http://www.biomedcentral.com/1471-2458/11/727 among year 9 than among year 11 pupils (p < 0.05). In another Swedish study by Höglund et al. 86% of the sur- veyed adolescents mentioned chlamydia as one of the STDs known to them in response to an open question [47]. In the two studies which reported on awareness among boys and girls separately, girls were observed to have higher awareness proportions than boys [38,42]. While the observation was not statistically significant in one of the studies, [27] this was not reported on in the other study [38]. Not many adolescents knew that chlamydia can be symptom-free: 40% and 56% in the 1986 and 1988 surveys by Andersson-Ellström et al. [37] and 46% in the study by Höglund et al. [47]. In one Swedish study where the level of knowledge in the same study population was assessed at age 16 and 18, a statistically significant increase in knowledge was observed over time [40]. Only the Finish study reported on the subjective rating of risk of contract- ing chlamydia. 55% of the adolescents surveyed reported ‘low perceived susceptibility’ [45] (Table 3). Awareness and knowledge of gonorrhoea Gonorrhoea was identified as an STD from a given list by 84% of adolescents in the survey by Tyden et al., [38] by 98% in the survey by Andersson-Ellström et al., [37] and by 53% in the survey by Garside et al. [42]. In the latter, the difference between year 9 and year 11 pupils was more pronounced among boys: 53% among year 9 and 60% among year 11 (p > 0.05). A statistically significant increase in knowledge over time was observed in a group of girls surveyed at age 16 and 18 [40]. Only 50% of the adolescents surveyed in the study by Höglund et al. men- tioned gonorrhoea in response to an open question on known STDs [47] (Table 3). Awareness of syphilis and herpes Awareness of syphilis was surveyed only in the study conducted in England where 45% of the participating adolescents correctly identified the disease from a given list as an STD. The proportion was slightly higher among year 11 compared to year 9 pupils and awareness was slightly higher among girls than among boys (p > 0.05) [42] (Table 3). In the Tyden et al. study, [38] 56% of the surveyed adolescents identified herpes as an STD from a given list. The proportion was 90% in the survey by Anders- son-Ellström et al. [37] and 59% in the Garside et al. study [42]. In the latter, considerable differences were observed between year 9 and year 11 pupils (p < 0.05), but not between girls and boys in the same school year. Herpes was mentioned as an STD by 64% of the adoles- cents surveyed in the study by Höglund et al. [47] (Table 3). Page 9 of 12 Awareness of STDs in general Five of the studies reviewed assessed the knowledge of participating adolescents on STDs in general. In the England study, all in all 59.7% of the participants knew that STDs in general can be symptom-free [42]. Among girls, knowledge was higher among year 11 than year 9 pupils, while the opposite was true for boys. The pro- portion of boys in year 9 who knew this fact (64.2%) was considerably higher than that of year 9 girls (53.8%) (Table 3). In two Swedish studies by Tyden et al. and by Andersson-Ellström et al., all surveyed adolescents knew that the use of condoms can protect against the contrac- tion of STDs in general [38,40]. In an earlier study by Andersson-Ellström et al., 20% of sexually active pupils surveyed in 1986 were aware that condoms protect against infection. The figure significantly went up to 43% in 1988, with boys having significantly higher awareness than girls in both years [22] (Table 3). In the same study, the proportion of girls who felt themselves to be at risk of contracting an STD in general went down from 32% in the 1986 survey to 24% in the 1988 survey. Among boys, the proportion increased from 16% in 1986 to 24% in 1988. These changes were not statisti- cally significant [37]. In the Finish study, 55% of the sur- veyed adolescents perceived themselves to be at low risk of contracting an STD [45]. Reported use of condoms Use of condoms by sexually active participants was assessed in three studies, all conducted in Sweden [38,46,47]. Reported use at sexual debut was lowest in the study published in 1991 (31%), [38] and higher in the other studies both published in 2009: 61% [47] and 65% [46] respectively (Table 3). In the earlier study, the pro- portion of girls reporting condom use was, at 50%, con- siderably higher than that of boys (40%) [38]. In the study by Gottvall et al., no difference in condom use was observed between girls and boys [46]. Condom use at recent coitus was reported on only in the earlier study [38]. It was observed that the decrease in the proportion of girls reporting using condoms was more pronounced than that of boys (26% vs. 40%) (Table 3). Discussion The highest awareness and knowledge were reported for HIV/AIDS. This is certainly linked to the fact that since the mid 1980s, extensive awareness campaigns on this topic have been conducted globally. The lowest propor- tions were reported for HPV, with awareness as low as 5.4% in one study [47]. With only about 1 in 8 respon- dents knowing that HPV is an STD, awareness was still very low in one of the two studies conducted after the introduction of the HPV vaccine [46]. A higher awareness