Changes in knowledge of safe sex and sexual behaviour among male vocational high school students in Zhejiang Province, China: a 6-year cross-sectional comparison (2015-2021).

Background: Vocational high school students in China exhibit higher rates of sexual activity than their academic counterparts; however, existing studies predominantly focus on college populations, leaving a critical gap in longitudinal data tracking the evolution of sexual knowledge and behavioural patterns among vocational students. This study aimed to evaluate changes over time in sexual safety knowledge, STIs/HIV and reproductive health-related sexual behaviours among boys who have had sex in Chinese vocational high schools. Methods: In 2015 and 2021, an anonymous survey was conducted separately using the same questionnaire and survey method among students in Years 1-2 of the same vocational high school in a district of Zhejiang Province. The results of the two surveys were compared using univariate analyses, and multivariate analyses. Results: In 2015 and 2021, 180 and 90 male students, respectively, had ever had sex, representing rates of 17.44% (180/1,032) and 8.49%(90/1,060). The results of the multivariate analysis showed that, in 2021, those who knew that sexual contact was the main mode of HIV transmission(OR = 14.14, 95% CI: 7.16-27.92), knew that contracting an STI increased the likelihood of HIV infection (OR = 7.83, 95% CI: 4.30-14.29), and knew that condom use could reduce both STIs(OR = 33.51, 95% CI: 15.08-74.49) and HIV infection(OR = 16.77, 95% CI: 8.58-32.76) were significantly more prevalent than in 2015. Furthermore, students who had received sex and STI/HIV prevention education and rated it as "good" (OR = 5.62, 95% CI: 2.72-11.64), who believed they could confidently refuse sex without a condom(OR = 1.95, 95% CI: 1.14-3.32), and who reported consistent condom use(OR = 2.27, 95% CI: 1.21-4.25) were also more prevalent in 2021. Use of contraception methods such as condoms(OR = 2.76, 95% CI: 1.50-5.09), the pill(OR = 2.42, 95% CI: 1.35-4.35), extracorporeal ejaculation (OR = 1.77, 95% CI:1.02-3.09), and the safe period(OR = 2.22, 95% CI: 1.22-4.04) also increased significantly. However, the proportion of students who had their first sexual relationship with a non-regular partner decreased(OR = 0.47, 95% CI: 0.24-0.93). Conclusion: The participants' knowledge of safe sex and sexual behaviour improveed between the two surveys. However, these positive changes have not had a significant enough impact to prevent STIs/HIV transmission and reduce pregnancy. Further efforts are needed to ensure that these changes lead to effective STIs/HIV prevention and reduction among students.