HIV, syphilis, and gonorrhea co-infection and associated factors among adolescents and young adults aged 15–24 years in Ningbo, China, 2005–2024

Background Sexually transmitted infections (STIs), including HIV, syphilis, and gonorrhea, have exhibited a rising trend among adolescents and young adults aged 15–24 years in China, presenting a major public health concern. This age group faces heightened risks due to behaviors such as unprotected sex, having multiple partners, and engaging in online dating, as well as limited access to healthcare services. Monitoring long-term trends and factors associated with STI co-infections is essential for developing effective prevention and intervention strategies. Methods Historical data on newly reported syphilis and gonorrhea cases in Ningbo from 1 January 2005 to 31 December 2024 were retrieved from the China Disease Prevention and Control Information System. Cases with final review dates between 2005 and 2024 that were reported in Ningbo were selected and matched with concurrent case data from the Comprehensive HIV/AIDS Prevention and Control Information System to investigate factors influencing co-infections with two or more STIs (specifically syphilis, gonorrhea, or HIV) among individuals aged 15–24 years. Chi-squared tests for trend were employed to assess temporal patterns in HIV, syphilis, and gonorrhea epidemics, while multivariate logistic regression models were used to evaluate factors associated with STI co-infections. Results From 2005 to 2024, the total reported cases of HIV, syphilis, and gonorrhea among adolescents and young adults aged 15–24 years in Ningbo were 1,576 (6.0%), 14,623 (55.4%), and 10,204 (38.6%), respectively. The reported epidemics of HIV, syphilis, and gonorrhea in this demographic exhibited a significant upward trajectory over the period (linear regression analysis, p < 0.05). Over the 20-year span, the reported HIV incidence rate demonstrated an overall increasing trend (trend χ 2 = 11.735, p < 0.001). The reported prevalence of HIV, syphilis, and gonorrhea co-infections in this population was 4.9% (1,304/26,403). Significant predictors of STI co-infections included the reporting year (2015–2024 vs. 2005–2014, aOR = 11.637, 95% CI: 9.740–13.905), sex, occupation, household registration, and sample source ( p < 0.05). Conclusion Targeted interventions should prioritize male individuals, those in high-risk situations (such as those engaged in housework, unemployed, incarcerated, working in commercial services, students, or medical personnel), and those with non-local (out-of-province) household registration through enhanced STI health education, behavioral interventions, and expanded provider-initiated HIV testing and counseling (PITC) to curb the rising trends in HIV and STI epidemics.