Division of Infectious Diseases, Children's Hospital
of Philadelphia, University of Pennsylvania School of Medicine, and the
Wistar Institute of Anatomy and Biology, Philadelphia, PA 19104, USA.
offit@email.chop.edu
Anecdotal case reports and uncontrolled
observational studies in the medical literature claim that vaccines
cause chronic diseases such as asthma, multiple sclerosis, chronic
arthritis, and diabetes. Several biological mechanisms have been
proposed to explain how vaccines might cause allergic or autoimmune
diseases. For example, allergic diseases might be caused by prevention
of early childhood infections (the "hygiene hypothesis"), causing a
prolongation of immunoglobulin E-promoting T-helper cell type 2-type
responses. However, vaccines do not prevent most common childhood
infections, and large well-controlled epidemiologic studies do not
support the hypothesis that vaccines cause allergies. Autoimmune
diseases might occur after immunization because proteins on microbial
pathogens are similar to human proteins ("molecular mimicry") and could
induce immune responses that damage human cells. However, wild-type
viruses and bacteria are much better adapted to growth in humans than
vaccines and much more likely to stimulate potentially damaging
self-reactive lymphocytes. Consistent with critical differences between
natural infection and immunization, well-controlled epidemiologic
studies do not support the hypothesis that vaccines cause autoimmunity.
Flaws in proposed biological mechanisms that explain how vaccines might
cause chronic diseases are consistent with the findings of many
well-controlled large epidemiologic studies that fail to show a causal
relationship.