Influenza is caused by a virus and there are many types of influenza viruses. It spreads through respiratory droplets when an infected person coughs, sneezes or talks within 3 feet of another person. The flu vaccine contains 3 primary strains each year to protect against the strains predicted to circulate in the upcoming season, but the correct prediction is not always made. Getting the flu vaccine between late September and October provides around 5-6 months of immunity.