The 'best friend effect': a promising tool to encourage HPV vaccination in Japan.

Background: In Japan, HPV vaccination rates has dramaticaly declined since 2013. Since mothers are the ones making the decision to vaccinate their daughters against HPV, we probed the mothers' intention to receive vaccinations for themselves and to vaccinate their daughters against HPV, and their reasoning.

Methods: An internet survey was conducted in March of 2021. Through the screening, 1576 participants were extracted from a survey panel and divided into 3 groups based on their daughter's birth fiscal year (Group 1: 1994 to 1999, Group 2: 2000 to 2003, Group3: 2004 to 2008). The chi-square test and residual analysis were used for the statistical analysis of comparison among the groups. Logistic regression analysis was used to identify independent variables with mothers intention to get their daughters vaccinated under specific situations.

Results: The percentage of respondents without anxiety regarding their daughter's general vaccination was significantly higher in Group 1 (p < 0.05). In the mothers of daughters born in or after 2000 when vaccination rates declined (Groups 2 and 3), a situation in which 'The daughter's best friends were vaccinated before her' made the mothers think positively about HPV vaccination, and to the same degree as a situation in which 'You received a notice from your local government recommending vaccination' (Group 2: 41.6% (214/514) and 40.5% (208/514), Group 3: 48.5% (257/530) and 47.0% (249/530)).

Conclusion: If mothers who have had their daughters vaccinated were to recommend HPV vaccination to their close friends, 'the best friend effect' should promote others to be vaccinated.