The vaccine does not give a person influenza, contrary to the misperception. Except for transient discomfort at the injection site, occasional low-grade fever and aches, the vaccine causes no symptoms. Because it is prepared in eggs, people allergic to egg protein should not receive it. In extremely rare instances, a neurological disorder, called Guillan-Barre syndrome, can follow vaccination. Influenza vaccine is effective because it causes a person to manufacture anti-virus antibodies in the absence of natural infection.
Full immunity occurs about four weeks after injection and lasts for about five months.
This is the reason scientists are under such pressure to predict the specific strains of virus that will cause disease and manufacture the appropriate vaccine. This alteration is virtually impossible to predict. Therefore, immunity to last year's flu is unlikely to be effective against this year's infection. Finally, some influenza viruses appear to be more virulent than others. For instance, the catastrophic flu epidemic of 1918-1919 was the worst.