What is the difference between an allergy and a side effect?
As with any medication, vaccines can have side effects such as fever, rash or local redness or swelling. This is not an allergy. An allergy is when the body reacts to a specific substance. An allergic reaction can be a rash, shortness of breath or swelling of the face, and these, almost immediately, or within an hour after the injection. As an example, the MMR (Measles, Mumps, Rubella) vaccine can cause a rash that occurs 7-10 days after the infection. This is not an allergic reaction. This is a side effect of the MMR vaccine itself.
What about egg allergy and vaccine allergy? The link between the two is that some vaccinations, including the MMR and Influenza vaccines are made using chicken egg or related substances. In theory, if one is allergic to eggs, then one may be allergic to egg-based vaccines. This is what we thought years ago and as a result, many egg-allergic children automatically did not receive the MMR vaccine because of this fear. So, to be sure, doctors used to perform an allergy test for the MMR vaccine itself. If, based on the allergy test, an egg-allergic child was not allergic to the MMR vaccine, he or she received the MMR vaccine without any problem. After years of this approach, it was realized that very, very few egg-allergic children were actually allergic to the MMR vaccine itself. As a result, current expert recommendations are to go ahead and vaccinate egg-allergic children with the MMR vaccine without any specific MMR allergy testing. So in reality, the relationship with egg allergy and the MMR vaccine is just in theory, as almost all egg-allergic children can receive the MMR, egg-based vaccine without a problem.
A similar situation relates to the flu vaccine. Recent studies have concluded that egg allergic individuals may be vaccinated against influenza without a prior influenza vaccine skin test.
Although a person can be allergic to any vaccine or any of its components or parts, this is quite rare. However there are some people that are allergic to the DT (Diphtheria, Tetanus) vaccine that is administered routinely during the first few years of life. The reason I focus on DT is that other newer vaccines, have been developed using the DT as a base; Such "DT-based vaccines" include the Meningitis vaccines. So a person, who has an allergy to DT, can theoretically be allergic to a DT based meningitis vaccine. Again, specific allergy testing for DT may be helpful.
On a final note, there are very few individuals who are truly allergic to a particular vaccine. However, if your child has had an allergic reaction to any vaccine in the past, you should discuss this with your health care professional before proceeding with any other vaccination. In such cases, a careful history of the timing and type of reaction and possibly allergy tests will help determine whether or not further vaccines can be given and under what circumstances.