This case-control study of healthcare workers at Capital & Coast District Health Board will investigate whether seasonal influenza vaccination has any protective effect against pandemic influenza A H1N1 infection. The vaccination status of 111 cases who presented with an influenza-like illness and in whom influenza A H1N1 infection was virologically confirmed (positive PCR on nasopharyngeal swabs) will be compared with 467 controls who presented with an influenza-like illness and were virologically negative for influenza A H1N1. The analysis will be controlled for confounding variables including age, sex, deprivation index, domiciliary isolation, ethnicity, occupation including patient contact, overseas travel, comorbidity and pregnancy. The data will be obtained from information prospectively recorded within the CCDHB Occupational Health Service and the CCDHB and Hutt Valley Health Board IBA Patient System databases. The study findings will be relevant to the development of future influenza vaccination strategies for both pandemic and seasonal influenza infection.