Means risk and relative risk of infection stratified by infection history and age group over the years tested.

The annual minimum and maximum values observed in the years are presented in parentheses. The relative risk was calculated compared with individuals from the same HMO that sought medical treatment in the year prior. The adjusted relative risk included as members only individuals who were diagnosed as influenza patients at least once along the entire period evaluated (2003–2012 in Clalit dataset and 1998–2010 in Maccabi dataset).