Study time-table describing surveillance for viral respiratory infections, influenza vaccination and follow-up after vaccination period. *Surveillance for upper respiratory viruses occurred during two subsequent years, from March to August: 2007- surveillance 1; and 2008 – surveillance 2. ** Influenza vaccine was offered between April and July 2008, during the surveillance 2 period. # Follow-up after vaccination: period of six months after the vaccine. n = number of patients included in each period.