How often does a severe allergic reaction occur? what percent of people have anaphlaxis?

1.5 to 15%: Depends on what you read and the type of agent causing anaphylaxis, the reported incidences ranged from 1.5% to 15% although i think the incidence is under 0.1% annually. The severity was not detailed in the reports but a survey done about 10 years ago noted that there were 1500 deaths/yr from anaphylaxis in the us- some cases of unexplained deaths might not be included.