There are two different types of flu vaccines, trivalent and quadrivalent.
Trivalent vaccines protect against 3 strains of the flu, A/H3N2, A/H1N1, and influenza B. Trivalent vaccines are available in:
Quadivalent vaccines protect against 4 strains of the flu, A/H3N2, A/H1N1, and 2 strains of influenza B. Quadrivalent vaccines are available in:
The flu vaccine will protect you for one flu season.
Detail : http://www.flu.gov/prevention-vaccination/vaccination/
Thanks & Regards
Rajendra Tripathy.