Can Multiple Vaccines Overload the Immune System?

One hundred years ago, children were routinely vaccinated against one disease—smallpox. Forty years ago, it was 5 diseases—diphtheria, pertussis, tetanus, polio, and smallpox, necessitating as many as eight shots by 2 years of age. The routine childhood immunization schedule in 2010 calls for as many as 53 separate vaccine doses by 18 years of age. The good news is that through all of this, 16 different diseases are prevented; the bad news is that some people wonder if it is just too much.

The possibility of immune overload must be put into perspective. Every day, people are bombarded by antigens to which their immune systems must respond. This includes viruses and bacteria from the external environment as well as organisms from within, particularly those in the mouth, nasopharynx, and gut. Most people are not sick most of the time—this speaks to the robustness of the immune system's ability to meet these challenges.

Even the most vulnerable people—neonates—seem to do just fine. Within a matter of hours of birth, the initially sterile gastrointestinal tract becomes heavily colonized with a wide variety of bacteria, some of which are potentially harmful. Yet the specific secretory IgA responses that are stimulated by colonization are, by and large, adequate to prevent invasion. Even though children receive more vaccines today than they did 40 years ago, the number of separate immunologic challenges represented by the routine childhood schedule has drastically decreased. The main reason for this is the elimination of smallpox vaccine, which contained about 200 antigens, and the whole-cell pertussis vaccine, which contained about 3000 antigens. In this context, the vaccine schedule is “purer” than it used to be.

People fear that vaccines might weaken the immune system and thereby increase susceptibility to infectious agents not contained in the vaccines (so-called heterologous infections). Vaccines may cause temporary suppression of delayed-type hypersensitivity skin reactions or transiently alter certain lymphocyte function tests. MMR may decrease the immune response to VAR if the latter is administered within 30 days (and not given on the same day). However, the short-lived immunosuppression caused by certain vaccines does not result in an increased risk of heterologous infections. In fact, just the opposite has been seen. For example, a study from California showed a decrease in invasive heterologous infections in the few months following immunization.

Similarly, a study from Germany found that children who received the diphtheria, pertussis, tetanus, Hib, and polio vaccines in the first 3 months of life actually had fewer infections with vaccine-related as well as vaccine unrelated pathogens. A study in Denmark that included 2,900,463 person-years of follow-up found no causal association between any of the childhood vaccines and hospitalization for any of seven different infectious diseases unrelated to the vaccine-preventable diseases themselves. Other studies also refute the notion that vaccines increase susceptibility to infection.

Bacterial and viral infections, on the other hand, often do predispose children and adults to severe, invasive infections with other pathogens. For example, influenza infection clearly predisposes patients to pneumococcal and staphylococcal pneumonia. Similarly, varicella infection increases susceptibility to group A beta-hemolytic streptococcal infections including necrotizing fasciitis, toxic shock syndrome, and bacteremia. Thus if susceptibility to heterologous infection is the concern, vaccination makes more sense than no vaccination.

—Marshall, Gary S. "Addressing Concerns About Vaccines." The Vaccine Handbook: A Practical Guide for Clinicians. 3rd ed. New York: Professional Communications, Inc., 2010. 210-212. Print.

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