The only time you must wait is if two live vaccinations are not administered at the same time. After that, you must wait at least four weeks before administering the second live vaccination. The only exception to this rule is if the vaccine manufacturer states otherwise on its product label. Here are some examples of live vaccinations: BCG (Bacille Calmette-Guérin), MMR (Measles, Mumps, and Rubella), Varivax (measles vaccine). Two live vaccinations at once may either reduce or delay the immune response to each vaccine. This is important because the body needs time to respond to certain infections. If the immune system does not get enough time to mount a response, then the infection will not be cleared. If you receive multiple doses of a vaccine over a short period of time, the immune system may not have time to produce enough antibodies. For example, if you were to receive both the Tdap and the HepA vaccines at the same time, it would not be recommended. The Tdap vaccine contains tetanus and diphtheria toxoids, while the HepA vaccine contains antigens from three different strains of hepatitis A virus.