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. Both the same participants and also new participants will be recruited each year. The analysis will be mainly descriptive.

Inclusion Criteria:

• Members of health care or laboratory personnel in HUS having decided to take the seasonal influenza vaccination by the routine occupational health care service
• General good health as established by or volunteer's own statement
• Written informed consent
• Presumably able and willing to participate in the study during the starting influenza season

Exclusion Criteria:

• Any confirmed or suspected immunosuppressive or immunodeficient condition, including human immunodeficiency virus (HIV) infection.
• Administration (longer than 14 days) of immunosuppressants or other immune- modifying drugs within 6 months before the vaccination; oral corticosteroids in dosages of 20 mg/day or more prednisolone or equivalent are excluded; inhaled or topical steroids are allowed.
• Pregnancy or lactation
• Acute disease at the time of enrolment (defined as the presence of a moderate or severe illness with or without fever)
• Contraindication for influenza vaccination
• Any other criteria which, in the investigator's or dedicated study staff member's opinion, would compromise the ability of a subject to participate in the study, a subject's well-being, or the outcome of the study

Health care professionals affiliated by HUS