Association between Syphilis Incidence and Dating App Use in Japan

Introduction The cause of the syphilis resurgence in Japan is still unknown. In this study, we hypothesized that the spread of mobile dating software for use on mobile phones might have contributed to it. We investigated possible contributing factors of the syphilis resurgence in Japan. Methods We retrieved the number of reported cases of syphilis, human immunodeficiency virus infection, carbapenem-resistant Enterobacteriaceae infection, invasive Streptococcus pneumoniae infection, general population, foreign national residents, international overnight guests, detachment-type sex trade shops, physician density, and smartphone penetration rate at the prefectural level in 2017. We also obtained the number of three major dating app users in each prefecture. Using them, we performed association analyses. Results The median of syphilis incidence per 100,000 prefectural population was 2.34 (range 0.72 to 12.90). The values of Spearman's rank correlation coefficients between syphilis incidence and app penetration rates were 0.59 (p < 0.001) for app 1, 0.57 (p < 0.001) for app 2, and 0.56 for app 3 (p < 0.001). The values of correlation coefficient between syphilis incidence and prefectural population (0.50, p < 0.001), foreign national residents per prefectural population (0.46, p = 0.001), and smartphone penetration rate (0.54, p < 0.001) were significant, while international overnight guests per prefectural population (0.19, p = 0.19), sex trade shops (0.24, p = 0.10), and physician density (0.10, p = 0.52) were not. In the multiple regression analyses, the contents of an optimized model included the following two factors: for app 1 penetration rate (0.39, p < 0.001) and the number of sex trade shops per prefectural population (0.18, p = 0.008) with the adjusted R-squared value 0.49 and F value 22.97 (p < 0.001). Conclusions Dating app penetration rate was significantly associated with syphilis incidence. The popularization of social media is a possible contributing factor in the syphilis resurgence in Japan. Information concerning the use of social media should be included in future studies on transmission and prevention of sexually transmitted infections.