From birth and in early infancy and childhood, humans are exposed to countless numbers of foreign antigens and infectious agents in the everyday environment. Responding to these stimuli helps the immune system to develop and mature. Newborn babies are exposed to millions of microbes before they even leave the hospital, far more than those contained in three or four vaccines. Compared with exposure in the natural environment, vaccines provide specific stimulation to a small number of antigens. Responding to these specific antigens uses only a tiny proportion of the capacity of an infant’s immune system.
If an infant’s immune system could be exhausted by multiple vaccines, one would expect vaccinated children to be at a higher risk of serious infections. Studies to investigate whether vaccines increase susceptibility to serious infections have shown no evidence of such an effect, with infection rates generally being lower in vaccinated children.
Extensive studies have been carried out on the possibility of vaccines overloading a child’s immune system. None have shown that the quantity of immune-alerting antigens causes a problem.
Children develop the ability to produce humoral and cellular responses to most vaccinations between two and six months of age. By six months of age, the child’s immune system can theoretically sufficiently respond to 10,000 at one time. In other words, 0.1% of a child’s immune system is “focused on” fighting the vaccination, leaving the remaining 99.9% to fend off other pathogens and infections (even this is a conservative estimate as B- and T- cells are constantly being replenished, meaning the vaccine will never “use up” 0.1% of the immune system’s capabilities).