Vaccine will begin to provide its protective effect after about one or two weeks, and immunity may decrease, on average, after several months. Flu shots will not fully protect all persons who get them against the flu. They also will not protect against other illnesses that resemble the flu. The doctor or nurse giving the flu shot will discuss this with parents or guardians. Split-virus vaccines can also be used by adults.
Most people have no side effects from recent influenza vaccines.
Flu shots are given by injection, usually into a muscle of the upper arm. This may cause soreness for a day or two at the injection site and occasionally may also cause a fever or achiness for one or two days. In 1990-1 there may have been a small increase in GBS cases in vaccinated persons 18 to 64 years of age, but not in those 65 or older. This possible association with GBS was not as convincing as with the swine flu vaccine. Even if GBS was a true side effect, the very low estimated risk of getting GBS is less than that of getting severe influenza that could be prevented by vaccine.