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Childhood Vaccinations

By Michael J. Smith, MD, MSCE, University of Louisville School of Medicine

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Children should be vaccinated to protect them against infectious diseases. Vaccines contain either noninfectious fragments of bacteria or viruses or whole forms of these organisms that have been weakened so that they do not cause disease. Giving a vaccine (usually by injection) stimulates the body's immune system to defend against that disease. Vaccination produces a state of immunity to disease and is thus sometimes termed immunization.

Vaccine Effectiveness

Vaccines have eliminated smallpox and have nearly eliminated other infections, such as polio and measles, that were once common childhood scourges in the United States. Despite this success, it is important for health care practitioners to continue to vaccinate children. Many of the diseases prevented by vaccination are still present in the United States and remain common in other parts of the world. These diseases can spread rapidly among unvaccinated children, who, because of the ease of modern travel, can be exposed even if they live in areas where a disease is not common.

How Many Cases of a Disease Can Be Prevented by Vaccines?


Before the Vaccine Was Developed*

In 2010 or 2008 (in parenthesis)




Haemophilus influenzae type b infection

20,000 (estimated)


Hepatitis A



Hepatitis B (acute)









Pertussis (whooping cough)



Pneumococcal infections that are severe—in all age groups



Pneumococcal infections that are severe—in children under 5 years



Polio that causes paralysis



Rotavirus infections that required hospitalization—in children under 5 years















*The average number of cases each year during the 20th century.

Adapted from Appendix G: Data and statistics: Impact of vaccines in the 20th and 21st centuries. In Epidemiology and Prevention of Vaccine-Preventable Diseases: The Pink Book, ed. 12. Centers for Disease Control and Prevention, 2012. Available at the Centers for Disease Control and Prevention web site.

Vaccine Safety

No vaccine is 100% effective and 100% safe. A few vaccinated children do not become immune, and a few develop side effects. Most often, the side effects are minor, such as pain at the injection site, a rash, or a mild fever. Very rarely, there are more serious problems.

Vaccines are continuously undergoing improvements to ensure safety and effectiveness. Recent improvements include the use of

  • Acellular pertussis vaccine (typically combined with diphtheria and tetanus vaccine—DTaP), which has a much lower chance of having side effects than the previously used whole-cell pertussis vaccine (also combined with diphtheria, and tetanus—DPT)

  • An inactivated, injectable polio vaccine instead of the previously used oral polio vaccine

The oral polio vaccine, which is made of a live, weakened virus, can cause polio if the weakened virus mutates, which happens once in every 2.4 million children. Although this risk is extremely low, it has led doctors in the United States to completely switch to the injectable polio vaccine.

Febrile seizures (seizures that are triggered by a fever) occur in about 3 in 10,000 children after receiving the measles-mumps-rubella (MMR) vaccine.

Before a vaccine can be licensed, it (like any medical product) is tested in controlled clinical trials (see The Science of Medicine : How Doctors Try to Learn What Works). Such trials compare the new vaccine to a placebo or to a previously existing vaccine for the same disease. Such trials show whether the vaccine is effective and identify common side effects. However, some side effects are too rare to be detected in any reasonably sized clinical trial and do not become apparent until after a vaccine is used routinely in many people. Thus, a surveillance system called the Vaccine Adverse Event Reporting System (see VAERS) was created to monitor the safety of vaccines that are used in the general public. VAERS collects reports from people who believe that they had a side effect after a recent vaccination and from health care practitioners who identify certain side effects after a vaccine was given, even if they are unsure the effects are related to the vaccine.

To help people evaluate the risks of vaccination, the federal government requires doctors to give parents a Vaccine Information Statement each time a child is vaccinated. Also, a federal Vaccine Injury Compensation Program has been established to compensate people with proven vaccine-related injuries. This program was established because doctors and health authorities want as many children as possible to be protected from life-threatening diseases. When considering the risks of vaccination, parents must remember that the benefits of vaccination far outweigh the risks.

Vaccination Concerns

Despite the strong vaccine safety systems in place in the United States, many parents remain concerned about the safety of childhood vaccines and the immunization schedule. These concerns have led some parents to not allow their children to be given some or all of the recommended vaccines. Diseases that can be prevented by vaccines are much more common among children whose parents have refused one or more vaccines. Specifically, unvaccinated children are

  • 23 times more likely to contract pertussis (whooping cough)

  • 8.6 times more likely to contract varicella (chickenpox)

  • 6.5 times more likely to contract pneumococcal disease

Children in the United States still die from vaccine-preventable diseases. In 2008, five children developed life-threatening Haemophilus influenza type B infection in Minnesota, the most since 1992. Three of the infected children, including one child who died, had been given no vaccine because their parents had delayed or refused the vaccine.

MMR vaccine and autism

The public press has reported concerns that the MMR vaccine may cause autism. These concerns were based on a brief medical report in 1998 about 12 children with developmental disorders such as autism. Their parents reported that eight of the children had received the MMR vaccine within a month before they developed symptoms. Because this chain of events could also have occurred by chance, doctors have since done many studies to look for a connection between the vaccine and autism. No such connection was found in any of the studies. The largest of these studies looked at 537,303 Danish children who were born between 1991 and 1998. Most (82%) of these children had been given the MMR vaccine. Doctors found that the children who had been vaccinated were no more likely to develop autism than those who were not vaccinated. Autism or a related disorder occurred in 608 of 440,655 (0.138%) children who were vaccinated and in 130 of 96,648 (0.135%) of those who were not vaccinated.

Other similar studies from across the world have reached similar conclusions.

Thimerosal and autism

People have also been concerned about possible side effects of thimerosal. Thimerosal was previously used as a preservative in vials that contain more than one dose of a vaccine (multidose vials). Preservatives are not needed in vials that contain only one dose (single-dose vials), and they cannot be used in live-virus vaccines (such as rubella and varicella). Thimerosal, which contains mercury, is broken down by the body into ethylmercury, which is eliminated quickly from the body. Because methylmercury (which is a different compound that is not eliminated from the body quickly) is toxic to humans, there was concern that the very small amounts of thimerosal used in vaccines might cause neurologic problems, particularly autism, in children.

Because of these theoretical concerns, although no studies had shown evidence of harm, thimerosal was removed from routine childhood vaccines in the United States, Europe, and several other countries by 2001. (Thimerosal is still used in vials that contain more than one dose of influenza vaccine and in several other vaccines intended for use in adults.) The World Health Organization (WHO) has not recommended its removal from any vaccines because there is no evidence that routine use causes any harm. The removal of thimerosal from childhood vaccines has had no effect on the number of children who have developed autism. Several studies have shown that rates of autism have continued to increase despite removal of thimerosal from routine childhood immunizations. This increase, which occurred despite the removal of thimerosal, strongly suggests that thimerosal in vaccines does not cause autism.

However, people who are still concerned about thimerosal in the influenza vaccine may ask their doctor to use single-dose vials or live-attenuated influenza vaccine, neither of which contains thimerosal.

Use of several vaccines at the same time

According to the recommended schedule, children should be given several doses of vaccines against 15 different infections by age 6 years. Because some parents have been concerned about the number of injections and doctor visits, combination vaccines, such as the diphtheria-tetanus-pertussis vaccine, have been developed so that children can receive more vaccines with fewer injections and visits.

However, some parents now worry that children's immune system cannot handle so many antigens given at once. Antigens are the substances in vaccines that are derived from the virus or bacteria and that cause the body’s immune system to produce antibodies to fight disease. Sometimes parents who are worried ask for a different vaccine schedule or ask to delay or exclude certain vaccines. However, because the recommended schedule is designed to give the vaccine at the age when children are first at risk of a disease, not following the schedule puts children at increased risk of infection. Also, vaccines, even combination vaccines, contain very few antigens. Starting at birth, children encounter dozens and possibly hundreds of antigens during a single ordinary day. Their immune system handles these antigens without difficulty. Even a mild cold exposes children to 4 to 10 virus antigens. The immune system of children is not stressed or overwhelmed by combination vaccinations.

Delaying or excluding vaccines also affects public health. When fewer people are vaccinated, the percentage of a population that is immune to disease decreases. Then, the disease becomes more common, particularly among people who are at increased risk of getting a disease. These people may be at increased risk because

  • They were vaccinated but did not become immune.

  • They were vaccinated, but their immunity has decreased over time, as may occur as people age.

  • Their immune system is impaired by a disorder or a drug (such as the drugs used to prevent cancer or rejection of a transplant), and they cannot be given live-virus vaccines, such as the MMR or varicella vaccine.

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