Analysis of public concerns about influenza vaccinations by mining a massive online question dataset in Japan.

•The largest Japanese Q&A bulletin board service data including more than 16 million questions was analyzed.•We noted the emergence of similar questions in the same period every year.•Concerns regarding effectiveness abruptly increased during the epidemic period.•Pregnant or breastfeeding women increasingly asked questions about feasibility from October.•Providing accurate and timely information is essential to promote vaccinations.