Sexual Behaviors among Individuals Aged 20-49 in Japan: Initial Findings from a Quasi-Representative National Survey, 2022

Different time scales used for sexual partner surveys pose a challenge in modelling dynamics of sexually transmitted infections

Mathematical models for sexually transmitted infections (STIs) are parameterised by empirical data on sexual behaviour (e.g. the number of partners over a given period) obtained from surveys. However, the time window for reporting sexual partnerships may vary between surveys and how data for different windows can be translated from one to another remains an open question. To highlight this issue, we compared the distributions of the number of sexual partners over one year and four weeks from the British National Surveys of Sexual Attitudes and Lifestyles. The results show that simple linear rescaling did not render the one-year and four-week partner distributions aligned. Parameterising STI models using survey-based sexual encounter rates without considering the implication of the reporting window used can lead to misleading results.

Human papillomavirus vaccine impact on invasive cervical cancer in Japan: Preliminary results from cancer statistics and the MINT study

The first prophylactic vaccine against human papillomavirus (HPV) 16 and HPV18 was licensed in Japan in 2009. HPV vaccine effectiveness against high-grade cervical lesions has been demonstrated among young Japanese women, but evidence of its effects on invasive cervical cancer (ICC) is lacking. Using data from two different cancer registries, we compared recent trends of new ICC cases by age group using Poisson regression analysis. We also analyzed time trends in HPV16/18 prevalence among 1414 Japanese women aged <40 years newly diagnosed with ICC in the past decade. Based on the population-based cancer registry, the incidence of ICC among young women aged 20-29 years showed a significant decline from 3.6 to 2.8 per 100 000 women-years during 2016-2019, but no similar decline was observed for older age groups (p < 0.01). Similarly, using data from the gynecological cancer registry of the Japan Society of Obstetrics and Gynecology, the annual number of ICCs among women aged 20-29 years also decreased from 256 cases to 135 cases during 2011-2020 (p < 0.0001). Furthermore, a declining trend in HPV16/18 prevalence in ICC was observed only among women aged 20-29 years during 2017-2022 (90.5%-64.7%, p = 0.05; Cochran-Armitage trend test). This is the first report to suggest population-level effects of HPV vaccination on ICC in Japan. Although the declining trend in HPV16/18 prevalence among young women with ICC supports a causal linkage between vaccination and results from cancer registries, further studies are warranted to confirm that our findings are attributable to vaccination.