Mothers' intentions for adolescent sons' human papillomavirus vaccination in Japan: Psychosocial factors and relative influences across intention groups.

As human papillomavirus (HPV) vaccination for men remains non-routine in Japan, this study aimed to explore psychosocial factors associated with mothers' intentions to vaccinate their adolescent sons, and compared the strength of these correlates across intention groups. A web-based cross-sectional survey targeted mothers with boys aged 11-16 years in Japan. Participants were classified by intention into accepting (strongly/relatively agree), undecided (unknown), and refusing (strongly/relatively disagree) groups. Binary logistic regression compared the accepting versus undecided groups and accepting versus refusing groups. Among 301 mothers, 22.6% were accepting, 57.5% undecided, and 19.9% refusing. Normative beliefs exhibited the strongest association, with the highest odds ratio (OR), when comparing accepting versus undecided (OR = 12.63, 95% CI 6.03-26.47) and accepting versus refusing groups (OR = 26.72, 95% CI 9.69-73.74). This was followed by perceived importance of prevention, best-friend influence, anticipatory regret, and perceived susceptibility. Associations were generally stronger in comparisons between accepting and refusing groups than between accepting and undecided groups. The undecided group reported greater perceived logistical hassle. The refusing group was less likely to have completed children's routine childhood vaccinations before entering elementary school or eldest son's influenza vaccination and was less influenced by municipal notifications or doctors' recommendations. Tailored interventions are needed, including those enhancing subjective norms, perceived prevention value, and risk communication for undecided and refusing mothers; providing logistical support for undecided mothers; and implementing multi‑channel, trust‑building communication strategies for refusing mothers. Japan should prioritize boys' HPV vaccination through multifaceted strategies addressing these psychosocial barriers.