Maximum protection develops by about 2 weeks after vaccination and lasts about 6 months. The vaccine is very effective in preventing the three strains of influenza for which the vaccine is made to prevent. In the elderly the vaccine is less effective in preventing infection but immunization reduces the risk of complications, hospitalization and death. Individuals who do contract the virus even after a vaccination will usually find that they experience very mild symptoms. Most often, people who think they contracted the flu after receiving their shot are confusing their symptoms with those of a cold, another virus or they have contracted a strain of influenza for which the vaccine was not designed.
Influenza vaccine is very safe, but like any medicine, side effects can occur.
Most people who get the vaccine have either no side effects or mild side effects such as soreness, redness or swelling where the shot was given. Some people may get a fever or muscle aches which start shortly after vaccination and may last 1-2 days. Life-threatening allergic reactions are extremely rare. If they do occur it is within a few minutes to a few hours after the shot. Your doctor or nurse will ask you to wait 20-30 minutes following your shot to watch for possible reactions.