I took a look at Marnica's link.
That's a big problem with vaccines and adjuvants (aluminum, usually, but in other parts of thew world, increasingly, squalene or other lipid-based adjuvants). The adjuvants are there to stimulate an immune response to the antigen, but the body doesn't know it's only supposed to create antibodies to the antigen. It creates antibodies to the other ingredients, too. And some of those ingredients are things that are common in our diets, such as gelatin and eggs. The flu vaccines in the U.S., which contain egg ingredients, do not contain an adjuvant, but they are often given with other vaccines that do contain adjuvants. Flu vaccines in other parts of the world do contain adjuvants.