Syphilis Risk in Young Women: Age-Period-Cohort Analysis of 21-Year Urban and Regional Trends

Background

In Japan, the incidence of syphilis experienced a significant epidemic in the late 1960s. Although the number of reported cases decreased in the 2000s, a resurgence began in 2011, escalating sharply after 2021. This increase may be attributable to age-related trends among high-risk populations and broader social factors. This study aimed to elucidate sex-specific time-series characteristics of syphilis incidence rates by region in Japan, utilizing an Age–Period–Cohort (APC) model.

Methods

Data from the National Epidemiological Surveillance of Infectious Diseases (NESID) were analyzed for individuals aged 15–70 years from 2001 to 2022. Incidence rates were calculated by dividing the annual number of reported cases within 5-year age groups in each prefecture by population estimates. A Bayesian APC model was employed to estimate age, period, and cohort effects for Tokyo and Osaka, as well as for other prefectures.

Results

The age effect peaked among males in their late 20s and among females in their early 20s, with females aged 15–19 years exhibiting positive values. The period effect demonstrated an increasing trend starting in the early 2010s, a decline during 2019–2020, followed by a resurgence in 2021, ultimately reaching its highest level in 2022. The recent sharp increase commenced earlier in Tokyo and Osaka compared to other prefectures. Regarding the cohort effect, among females in Tokyo and Osaka, the posterior mean became positive for the 1993–1997 birth cohort, with higher values observed in more recent cohorts.

Conclusions

The APC analysis indicated that the age effect varied between males and females, peaking among females in their late teens. The period effect began to rise earlier in Tokyo and Osaka, and the recent cohort-specific increase observed only among females in these prefectures may reflect underlying social changes. These social changes might have contributed to the recent epidemiological trends of syphilis.

Key messages

• APC analyses revealed urban hubs as drivers of syphilis surges, with period effects rising earlier in cities. This underscores the value of urban-centric surveillance for re-emerging STI outbreaks.

• Rising risk in young female cohorts, particularly in urban areas, may reflect shifting social determinants. These findings highlight the need for policies addressing emerging cohort vulnerabilities.