Overdiagnosis of vaccine allergy is a major public health problem http://buff.ly/1tI9y44
"True" allergic, or immediate hypersensitivity, reactions to vaccines range from 1 per 50,000 doses for diphtheria-tetanus-pertussis (DTP) to 1 per 500,000 to 1,000,000 doses for most other vaccines.
In the US, the incidence of immediate hypersensitivity reactions to vaccines is 1-2 per 1 million vaccinations. A a true vaccine reaction occurs within the first 2 hours after the shot in 99 percent of the cases.
Some vaccines which include gelatin (e.g., Japanese encephalitis) have a higher rates of serious allergic reactions.
This summary from the journal Pediatrics includes a detailed algorithm for evaluation and treatment of patients with suspected vaccine hypersensitivity and 3 practice cases
An Algorithm for Treatment of Patients With Hypersensitivity Reactions After Vaccines. Robert A. Wood, MD, Melvin Berger, MD, PhD, Stephen C. Dreskin, MD, PhD, Rosanna Setse, MD, MPH, Renata J.M. Engler, MD, Cornelia L. Dekker, MD, Neal A. Halsey, MD, Pediatrics, Vol. 122 No. 3 September 2008, pp. e771-e777.
Image source: OpenClipArt.org, public domain.