Almost 50% of children have not received all the recommended vaccines at some point before their second birthday, according to a study of 300,000 children that was recently published in JAMA Pediatrics. While some cases of skipped vaccines may be due to missed opportunities or difficulty accessing healthcare services, one in eight children in this study were undervaccinated due to parental choice to either delay or refuse vaccinations.

Here are some questions I hear about alternative vaccine schedules:

Why do parents veer from the recommended vaccination schedule?

Parents ask their pediatricians to delay giving one or several vaccines for a variety of reasons. The current routine immunization schedule includes 24 vaccines before age 2 years and up to six vaccines at one time. It is difficult for parents to see their infant or young child receive multiple injections in one sitting, and it causes many to wonder if exposure to so many vaccines at the same time might overwhelm a child's immune system-- particularly for an infant.

Can a baby's immune system handle multiple vaccines at once?

In a word, yes. And a brief explanation of how vaccines work will shed some light on this very common question. Vaccines contain proteins called antigens or immunogens, a piece of the bacteria or virus that the vaccine is made to protect against. The number of proteins in a vaccine is negligible compared to what an infant is exposed to from the environment starting immediately after birth. A child that receives all of the vaccines on the recommended vaccine schedule will be exposed to about 150 immunogens. In contrast, babies are exposed to trillions of bacteria each day on their skin and in their mouth and digestive tract. Each of these bacteria contain between 2,000 and 6,000 immunogens. Babies' immune systems are still developing but, from the beginning, they are well-equipped to handle the onslaught of immunological challenges.

Another common worry among parents is that multiple vaccines at one visit are stressful for a young infant. I have never enjoyed hearing my children cry when they receive vaccines. But spreading out shots and giving fewer over more visits may be even more stressful. Infants make the same amount of cortisol, a stress hormone, during each vaccination visit – whether they receive two or five shots at once.

Aren't there risks associated with vaccine additives, like aluminum?

Many parents cite concerns over vaccine additives as a reason to delay a child's immunizations. In fact, one of the most widely used "alternative" vaccine schedules is one proposed by Dr. Bob Sears, based upon the premise that it is safer to spread out aluminum-containing vaccines. Aluminum salts are widely used in vaccines as an adjuvant, or a substance that enhances the immune response to a vaccine. But, as with immunogens, vaccines contribute very little to the amount of aluminum that children are exposed to every day. Aluminum is the third-most abundant element on the earth's surface, so it is everywhere, including in the water we drink and foods we eat, and is also found in breast milk and infant formula in levels higher than in vaccines. In fact, by six months of age, while an infant could be exposed to 4 mg of aluminum from vaccines, he or she would be exposed to about 30 mg from infant formula and 120 mg from soy formula.

Some parents question whether exposure to aluminum via injection, rather than ingestion, makes a difference. In either case, aluminum that reaches the bloodstream is processed in the same way, and most of it is quickly eliminated. Just through our daily activities, all children and adults typically have a low-level of aluminum circulating in their blood. Injected vaccines do not raise that level. Plus, aluminum levels in people with symptoms of toxicity are 100 to 1,000 times the typical circulating amount, so it would take a lot of aluminum to cause problems. In reality, aluminum toxicity is only seen in people who have poorly or non-functioning kidneys and long-term exposure to high doses of aluminum, such as from medications like antacids.

Is the growing number of requests to delay vaccination a consequence of the success of vaccines?

How can we increase confidence in vaccines?

Could an alternative schedule still be any safer?

Spreading out vaccination may, in fact, be less safe, and it is certainly without clear benefit. The rise in outbreaks of vaccine-preventable diseases such as measles and pertussis in communities with higher rates of unvaccinated children indicates that delaying vaccination matters. Vaccines may not be as effective when given on different schedules, and most importantly, the child will be at risk of getting a vaccine-preventable disease during the period of delay.

All of the evidence reviewed by the Institute of Medicine supports the safety of the current schedule, yet vaccine safety concerns persist and some parents will inevitably doubt the conclusions. As a pediatrician, this lack of trust is disheartening. We depend on science for many things that make our lives safer and healthier; vaccines represent one of the most remarkable achievements in science. Unfortunately, vaccine-preventable diseases can easily re-emerge and cause significant illness and death.

So, for me, recommending vaccination is a responsibility built upon the principle of "do no harm." It is one of the most important things I can do to support the health of my patients and ensure the health of the communities in which they live. Even more importantly, I am a parent. Vaccines and the immunization schedule are developed by my colleagues – scientists, epidemiologists and health care providers who are also parents. Our goal is to make the best and safest decisions when it comes to the health of our children – and yours.

Kristen A. Feemster, M.D.,M.P.H., is an assistant professor of pediatrics at Children's Hospital of Philadelphia, where she sees patients and conducts research on infectious disease epidemiology and vaccine policy.

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