 This is an investigator-initiated 10 year long open cohort observational study, aiming to increase the understanding of the humoral and cellular immunological mechanisms of vaccination against influenza, including effects of repeated vaccination, the duration of protection, immunity against circulating viruses, as well as factors affecting the immunological responses and immunity against other vaccine preventable infectious diseases among health care personnel. Influenza vaccination is given according to the normal routine of Hospital District of Helsinki and Uusimaa HUS occupational health care. Blood samples will be collected before vaccination, 5 weeks after vaccination and 6 months after vaccination. The analysis will be mainly descriptive. 