Generic protocol for ECDC studies of influenza vaccine effectiveness against confirmed infection using healthcare worker cohorts : version 2.0

Influenza is a disease of public health importance due to the substantial seasonal morbidity and mortality and the high pandemic potential of its aetiologic agents, influenza viruses. Diagnosis of influenza is mainly based on laboratory testing (i.e. molecular testing) of nasopharyngeal samples. Although patients generally prefer saliva testing, as it is less invasive, saliva specimens are used for diagnosis less often [1-3] due to lower sensitivity, probably due to lower viral load in this type of specimen. Influenza viruses are able to escape the human immune system through continuous evolution that can be caused by point mutations (antigenic drift) or recombination events (antigenic shift) [4]. For this reason, influenza vaccine components are re-evaluated each year and annual revaccination is recommended. Observed influenza vaccine effectiveness (IVE) varies from year to year, between population subgroups (e.g. age and risk groups) and according to the measured outcome (laboratory-confirmed influenza virus by (sub)type/clade or clinical outcome). IVE may vary between vaccine types and products, by time since vaccination and according to previous influenza infection and influenza vaccination history.