PHILADELPHIA—Uncontrolled studies and anecdotal reports have suggested that vaccines cause allergies, arthritis, multiple sclerosis, and other chronic diseases. All too often, these unconfirmed findings are trumpeted in the lay media and on the Internet, prompting some parents either to delay their children’s vaccinations or to withhold them entirely. By and large, however, there is really nothing to fear from vaccines, says Paul A. Offit, MD.

Dr. Offit, Chief of Infectious Diseases at Children’s Hospital of Philadelphia, recently coauthored a paper critiquing the theories set forth to explain the biological mechanisms that might link vaccines to chronic diseases.[1] “If you look closely at these mechanisms, they do not hold up,” he asserted in an interview.

ALLERGY AND AUTOIMMUNITY

The proposed mechanisms of how vaccines cause chronic disease fall into two categories. The first purportedly explains how vaccines can precipitate allergic disease; the second, how they can lead to autoimmune disease.

For example, some investigators believe that by preventing childhood infections, vaccines also prevent the development of a strong T helper (TH) 1 response. This arrested development would then perpetuate the bias toward a TH2 response that characterizes the immune system at birth; that, in turn, is thought to increase the likelihood of allergic disease.

This proposition stems from the “hygiene hypothesis,” which suggests that living in a highly sterile, infection-free environment keeps children from developing the TH1 response necessary to fend off allergic disease. Some epidemiologic studies support the hygiene hypothesis, showing greater rates of allergy in highly sanitized Western nations (where vaccination is prevalent) than in less developed countries with low vaccination rates and a relatively high risk of infection.

Other investigators argue that vaccination facilitates autoimmune disease via “molecular mimicry”—the presence of proteins on the surface of a wild-type or vaccine pathogen that are similar to those on the surface of human cells. This similarity may trigger an autoimmune response if the body mistakes its own cells for the pathogen.

FLAWED ARGUMENTS?

There are several inconsistencies in these hypotheses, and most epidemiologic and other data do not support the conclusion that vaccines cause chronic disease, Dr. Offit maintained (see box). Vaccines are unlikely to cause allergies through the eradication of childhood infections because they do not prevent most of those infections, he said. “In fact, they only prevent 11,” he pointed out, “and it just does not seem likely that humans would count on so few infections to eliminate the TH2 bias.”

Also, routine immunization programs do not exist for many viruses that commonly infect children—parainfluenza virus, rotavirus, and respiratory syncytial virus, for example. Those infections that are preventable through vaccination, such as pertussis, measles, mumps, and rubella, are highly contagious and easily transmitted regardless of the degree of hygiene in the home or public sanitation.

Interestingly, no increase in allergic disease has been observed among patients with worm infestation, idiopathic pulmonary fibrosis, or other conditions associated with a vigorous TH2 response. Furthermore, multiple sclerosis, type 1 diabetes mellitus, and other conditions that involve a strong TH1 response are common in regions with high rates of allergy.

The evidence linking vaccines to autoimmune disease is equally weak. For example, both the hepatitis B and influenza vaccines have been purported to cause multiple sclerosis through molecular mimicry. However, the former vaccine contains only one protein (hepatitis B surface antigen) and that protein does not resemble any human proteins. In addition, although the aggregation of hepatitis B surface antigen that occurs during natural hepatitis B infection far outweighs that given with the vaccine, the build-up during disease does not raise the risk of multiple sclerosis, Dr. Offit noted.

“Influenza vaccine appears to be a plausible candidate for molecular mimicry in the central nervous system,” he and his coauthor acknowledged. Influenza virus type A contains a protein that is similar to human myelin basic protein, and some studies have shown that natural infection with influenza virus exacerbates symptoms in patients with multiple sclerosis.[2,3] However, well-controlled studies have shown that influenza vaccine does not trigger multiple sclerosis exacerbations and may actually prevent them by guarding against primary influenza infection.

Similarly, the available evidence does not uphold the hypotheses that vaccination can cause diabetes or chronic arthritis. “To induce autoimmunity, you need the tremendous immune system stimulation that only occurs with natural infection and not with a weakened vaccine pathogen,” Dr. Offit stressed.

—Timothy Begany