Do Children Get Too Many Immunizations?
The Answer is No.

Mark Crislip, M.D.
Stephen Barrett, M.D.

Some vaccination opponents try to look respectable by stating that they are not against vaccination but believe that fewer vaccines should be given at once and that the shot schedule should be spaced out. On a recent episode of "Good Morning America," for example, actors Jenny McCarthy and Jim Carrey—with help from hostess Diane Sawyer—promoted the idea that vaccines have a "cumulative effect" and that giving several at once could provoke adverse interactions [1]. Pediatrician Jay Gordon, M.D., whom McCarthy consulated about her son, also advocates delaying certain vaccines [2]. This article explains why the "too many immunizations" claim does not have the slightest basis in reality.

Vaccines deliver either small amounts of antigen (substances that provoke antibody responses) or genetically weakened germs that multiply more slowly and for a shorter period of time than their disease-producing counterparts. As a result, rather than being exposed to full-blown infections over a 7 to 14 day illness, the body "sees" just enough antigen to develop protective antibodies. For example, whereas a hepatitis B infection exposes the body to 1,100 micrograms of antigen per hour for a week, the hepatitis vaccine series provides a total of 30 micrograms [3]. Thus if any alleged ill effects of a vaccine were due to too many antigens, too much antigen, or too frequent antigens, the diseases would still cause far more trouble than the vaccines.

Immune System Capacity Is Huge

The currently recommended schedule—posted at the U.S. Centers for Disease Control and Prevention Web site calls for 5 live or weakened organisms and 21 different antigens by age 6. A few more are added from ages 7 to 18. The goal is to protect children as early as possible from diseases that are very dangerous to young children.

The capacity of the immune system to respond to antigens is vast and far greater than most people realize. Experts estimate that humans can generate about 10 billion different antibodies [4] and that, due to exposures to germs and other foreign material, people make between 1 million and 100 million different antibodies during our lifetime [5]. The vaccine schedule produces a total of about 30 antibodies. It is also estimated that (a) each infant has the theoretical capacity to respond to about 10,000 vaccines at any one time and (b) if the 11 routinely recommended vaccines were administered together, the immune system would need to use only about 0.1% of its capacity to process them [6].

Life Provides Far More Microorganism Exposure Than Vaccines

We have, on and in us, about 100 billion bacteria, which is 10 to 100 times more bacteria than there are cells that make us. That is just our normal flora. These represent about 1,000 separate species of bacteria. Humans are born bacteria-free and acquire a complex and enormous normal bacterial flora in months. In the first year of life, babies ares exposed, for the first time, to all the bacteria of their parents and siblings and some from the family pet and the environment. The resultant antigen exposure is thousands of times greater than the exposure from the vaccine schedule.

The number of bacteria in your own ecosystem, of course, pales into insignificance compared to the bacteria in the soil, at about a million species per gram of soil, plus those in the water, on pets, in the air, etc., etc. Estimates run as high as a billion different bacterial species in the world. Viruses, yeasts, molds, parasites, and mites may provide millions more. These microorganisms are kept at bay by the immune system. Each species of bacteria has multiple sites that can elicit an antibody response. It is not simply one antibody for each organism. How many develop against a microorganism depends on the complexity of the organism. It is not unusual to make dozens of antibodies against one bacterial strain. If we respond with only 3 antibodies to each bacterial species in our normal flora and 3 antibodies each to only 100,000 of the 10+ billion environmental organisms. that would be 300,000 antibodies. Making that many antibodies by age 18 would average 46 per day. The standard childhood vaccine schedule provokes a total of about 150 [7]. The leading infectious disease textbook identifies roughly 1,300 bacteria that can cause disease. This translates into more than 13,000 potential antigens, nearly 100 times as many as the recommended vaccine schedule.

Even though the number of shots has risen in recent years, the actual load on the immune system has gone down. That's because today's vaccines are "smarter" and better engineered than the shots from a few decades ago. For example, before 1991, the whooping cough (pertussis) vaccine had 3,000 different antigens. Today's whooping cough shot has no more than five particles—just as effective, but much better designed to be easy on your immune system [8].

In May 2010, the journal Pediatrics published a study that compared more than 40 variables related to mental and neurological function among a large group of children to see whether delaying vaccination provided any benefit. After finding that no statistically significant differences favored the less-vaccinated children, the researchers concluded: "Timely vaccination during infancy has no adverse effect on neuropsychological outcomes 7 to 10 years later. These data may reassure parents who are concerned that children receive too many vaccines too soon." [9]

Delay Can Harm

Delaying vaccines increases the time during which children are susceptible to certain diseases, some of which are still fairly common. Chickenpox, whooping cough, influenza, and pneumococcus still cause hospitalizations and deaths in previously healthy children. Spacing out or separating vaccines will also require more doctor visits, which will increase overall costs as well as the number of times the children must experience any associated discomfort and stress.

The Bottom Line

No matter how you slice it, the vaccine schedule represents a miniscule exposure to antigens and organisms compared to what people encounter as part of life. Worrying about too many vaccines is like worrying about a thimble of water getting you wet when you are swimming in an ocean.

For Additional Information


  1. Brady J., Dahle S. Celeb couple to lead 'Green Vaccine' rally: Experts say no link between vaccinations and autism. ABC News, June 4, 2008.
  2. Dr. Jay Gordon. The Skeptic's Dictionary, Jan 30, 2014.
  3. Mandal G and others. Principles and Practice of Infectious Diseases. New York: Churchill, Livingstone, 2004.
  4. Fanning J and others. Development of the immunoglobulin repertoire. Clinical Immunology and Immunopathology 79:1-14, 1996.
  5. Harris DT. Genetic basis of antibody diversity. Medical microbiology and immunology class noted, University of Arizona Web site, accessed Dec 13, 2008.
  6. Offit P and others. Addressing parents’ concerns: Do multiple vaccines overwhelm or weaken the infant’s immune system? Pediatrics 109:124-129, 2003.
  7. Offit PA. Bell LM. Too many vaccines? What you should know. Vaccine Education Center, Children's Hospital of Philadelphia, Fall 2008.
  8. Composition of nine DTaP vaccines evaluated in efficacy trials. Clinical Microbiology Reviews, April 2005, p 356.
  9. Smith MJ, Woods CR. On-time vaccine receipt in the first year does not adversely affect neuropsychological outcomes. Pediatrics DOI: 10.1542/peds.2009-2489, May 24, 2010.

Dr. Crislip is an infectious disease specialist who practices in Portland, Oregon and contributes to the Science Based Medicine Blog and Rubor Dolor Calor Tumor, a blog about infectious diseases. He is also responsible for the Quackcast podcast (an occasional review of supplements, complementary and alternative medicine) and the Persiflagers Infectious Diseases Podcast (a bimonthly review of infectious diseases).

This article was revised on Seotember 11, 2016.

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