The pertussis vaccine is a different product today than it was some years ago. Policies regarding the administration of injected and oral poliomyelitis vaccines have been changed. We are now in the process of eliminating thimerosal as a preservative and replacing it with other agents. Questions have been raised regarding a possible relationship between the measles–mumps–rubella vaccine and the significant increase in the incidence of autism; such an association remains unproven, but more research is needed. We are administering more vaccines than ever before, and concern has been expressed about possible overburdening of the immune system in some infants; this needs to be evaluated.

As responsible and caring people of science, we must keep our minds open and, from time to time, recheck beliefs and policies we have taken for granted.