The strains included in the vaccine are chosen based on the strains of influenza most likely to cause disease each year. Your body develops immunity to all three strains. You cannot get influenza disease from the vaccine because it contains only killed virus. Protection against flu develops 2-4 weeks after receiving the vaccine. Protection begins to wear off after several months and is gone by a year.
Flu shots are most effective at preventing severe influenza with complications.
People who live with or provide care to those at high-risk including doctors, nurses, therapists, etc. Anyone who wishes to reduce their risk of influenza and its complications. The vaccine virus is grown in eggs and a trace of egg protein may remain in the final vaccine. People with severe egg allergy should be vaccinated only if their health care provider believes it is necessary and if the vaccine is given under close observation by a health care provider. Most people have little or no reaction to the vaccine.