The effectiveness of influenza vaccination against medically-attended illnesses in Hong Kong across three years with different degrees of vaccine match, 2014-17.

•Overall vaccine effectiveness against medically-attended influenza was moderate in Hong Kong.•The estimated vaccine effectiveness was generally lower against influenza A(H3N2) than influenza A(H1N1) and influenza B.•Point estimates of vaccine effectiveness against influenza A(H3N2) were higher in young adults compared to other age groups.•The overall protection provided by seasonal influenza vaccination among different age groups and across different epidemics could be informed by routine monitoring in the community.