Background:
In Japan, the human papillomavirus (HPV) vaccination rate has dropped to nearly zero since the suspension of proactive government recommendations in 2013. Following the termination of vaccination suspension in 2021 and subsequent proactive vaccination recommendation in 2022, it is crucial to promote catch-up vaccinations for those who missed their initial opportunity.

Objective:
This study aims to evaluate the effect of video-based informational intervention and explore factors associated with the uptake of catch-up HPV vaccinations among unvaccinated young adult women in Japan.

Methods:
In this randomized, parallel, single-blinded, internet-based trial, we recruited women aged 18‐26 years unvaccinated for HPV through a web-based research panel. The participants were randomly assigned (1:1) to receive either an educational leaflet containing information on the HPV vaccine and a short narrative video (intervention) or the leaflet alone (control). The primary outcome was the difference in proportion between both groups regarding the uptake of the free catch-up vaccinations at the follow-up survey after 3 months. No deviations from the registered protocol occurred during the study.

Results:
We enrolled 4065 women in the trial and randomly assigned them to either the intervention (2274 women) or the control (2331 women) group. Of these, we excluded 2595 women (63.8%) who did not respond to the follow-up survey, resulting in 1017 and 993 women in the intervention and control groups, respectively, for the final analysis. At the 3-month follow-up, 11.3% (228/2010) of the participants received at least one catch-up vaccine dose. The intervention and control groups had 10.5% (107/1017) and 12.2% (121/993) uptake, respectively. The difference in proportions between both groups was −1.7% (95% CI −4.5 to 1.2%; P=.26), and the adjusted difference was −1.6% (95% CI −4.3 to 1%; P=.23). In the subgroup analysis, the intervention group had a lower proportion of catch-up vaccination among sexually experienced women who had previous sexual intercourse experience (difference in proportion −5%, 95% CI −10% to −1%; P=.03) and those who had undergone a Pap test within the past 2 years (difference in proportion −11%, 95% CI −20% to −1%; P=.03). In addition, in the overall sample, factors positively associated with catch-up vaccination included higher educational background (difference in proportion: 7%, 95% CI 4%-10%; P<.001) and having undergone a Pap test within the past 2 years (difference in proportion 5%, 95% CI 0%-9%, P=.06).

Conclusions:
Our study demonstrated that video-based interventions did not have a substantial impact on the uptake of catch-up HPV vaccinations among young adults. However, our subgroup analyses suggested that the effectiveness of interventions may vary depending on individual characteristics. It is therefore desirable to explore and tailor more effective strategies based on the background and needs of specific populations in real-world settings.