1) First of all, many infective agents cause flu-like symptoms, including
rhinovirus, influenza virus, adenovirus, coronovirus etc. Flu vaccine is
only effective at preventing flu caused by influenze virus, which is
associated with high morbidity & mortality.
2) Many subtypes of influenza virus are present, and a vaccine only covers
several of them. For instance, this year's vaccine covers two subtypes.
The decision of which subtype to cover is the job of epidemiologists based
on the study of