Vaccines have had a dramatic effect on the prevalence of communicable diseases, but, in selected individuals, the injection presents a risk of anaphylaxis. Fortunately, most people have no allergic reactions to vaccines. In egg-allergic individuals, care must be taken before administering specific vaccines; the algorithm provided in this article gives specific recommendations for skin testing and desensitization. This algorithm is not needed for individuals receiving the measles-mumps-rubella vaccine because the risk of anaphylaxis is extremely low, even in those with known egg-protein sensitivity. Some individuals have gelatin sensitivity, which may cause anaphylaxis. Selected vaccines contain antibiotic drugs, so it is important to note if an individual has any known drug sensitivity, especially to neomycin, polymyxin B, or amphotericin B. Lastly, vaccine preservatives may cause reactions, but this occurs very infrequently.