The purpose of this observational study, which will be initiated as soon as the licensed H1N1 Pandemic Influenza Vaccine is used in a mass vaccination campaign, is to estimate the incidence of any medically-attended adverse events in all vaccinated subjects.

Inclusion Criteria:

Male and female subjects, per indication on license or per official vaccine recommendations, will be eligible for participation in this study if:

• They will be administered CELVAPAN (= Baxter´s H1N1 pandemic influenza vaccine)
• They are 2 months of age or older at the time of first vaccine administration
• Their medical history is available
• The investigator believes they will comply with the foreseen vaccination schedule and will reliably observe signs or symptoms of adverse events during the observational period
• They and/or their parent(s)/legal guardian(s) provide written informed consent, and assent where appropriate, prior to study entry according to national law

Exclusion Criteria:

Male and female subjects will be excluded from participation in this study if:

• They have already been administered another H1N1 pandemic vaccine
• They have any contraindication to vaccination (as per the Summary of Product Characteristics and/or relevant national immunization guidelines)

The enrollment of subjects in this study will follow national vaccination policy decisions in the UK.