Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule of vaccines in children. These concerns can lead some parents to vaccine hesitancy. Vaccine hesitancy is when parents delay or do not consent to their children being given some or all of the recommended vaccines despite the availability of vaccine services. Diseases that can be prevented by vaccines are much more likely to develop in children whose parents have not consented to one or more vaccines.
Children in the United States who do not receive routine vaccinations may become very sick and sometimes die of vaccine-preventable diseases. For example, before a vaccine was developed, Haemophilus influenzae type b (Hib) was the leading cause of bacterial meningitis in children (which can cause brain damage or deafness) (see Centers for Disease Control and Prevention (CDC): Epidemiology and Prevention of Vaccine-Preventable Diseases: Chapter 8: Haemophilus influenzae).
COVID-19 Vaccines
The COVID-19 pandemic brought vaccine hesitancy back to the forefront. The first COVID-19 vaccine received Emergency Use Authorization (EUA) from the U.S. Food and Drug Administration (FDA) in December 2020. Since that time, hundreds of millions of Americans have received at least 1 COVID-19 vaccine dose. However, many people remain unvaccinated. Similar to other vaccine-preventable diseases, hospitalizations and emergency department visits resulting from COVID-19 infection are more common among unvaccinated people. In addition, serious side effects associated with COVID-19 vaccination are extremely rare.
COVID-19 infection can affect children and adolescents and cause serious, body-wide health problems. As in adults, hospitalization is more frequent in unvaccinated versus vaccinated children and adolescents. Also, children and adolescents may develop long-lasting problems such as long COVID (a chronic condition that occurs after SARS-CoV-2 infection and is present for at least 3 months), even if the COVID-19 infection was mild or did not cause symptoms. Research suggests that people who get a COVID-19 infection after vaccination are less likely to report long COVID compared to people who are unvaccinated (see CDC: Long COVID Basics).
Measles-Mumps-Rubella (MMR) Vaccine: Concerns About Autism Spectrum Disorder
In the 1990s, the media reported concerns that the MMR vaccine may cause autism spectrum disorder. These concerns were based on a brief medical report in 1998 that was later found to be fraudulent and was withdrawn by the medical journal that published it. Since this report, doctors have done many studies to look for a connection between the vaccine and autism spectrum disorder. No such connection was found in any of the many 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 spectrum disorder than those who were not vaccinated.
Autism spectrum 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. The percentage of children with autism spectrum disorder is nearly identical between the 2 groups. A follow-up study of all children born in Denmark between 1999 and 2010, a total of 657,461 children, concluded that MMR does not cause autism spectrum disorder nor does it increase the risk in children who are at high-risk of autism spectrum disorder because of their family history or other risk factors.
Other similar studies from across the world have reached similar conclusions. In addition, the research in the original widely publicized study linking autism spectrum disorder and the MMR vaccine has been found to have serious scientific flaws and has been discredited by the medical and scientific communities.
Despite the overwhelming evidence to support the safety of the MMR vaccines, many parents may remain unconvinced. As a result, in 2019 the United States experienced its largest measles outbreak since 1992. According to the Centers for Disease Control and Prevention (CDC), most of the infected people were not vaccinated (see CDC: Measles Cases and Outbreaks).
Thimerosal: Concerns About Autism Spectrum Disorder
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 spectrum disorder, in children.
The World Health Organization (WHO) has not recommended thimerosal be removed from any vaccines because there is no evidence that routine use causes any harm. However, because of theoretical concerns and even though 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. In these countries, small amounts of thimerosal continue to be used in certain influenza (flu) vaccines, as well as in several vaccines intended for use in adults. Yearly flu vaccines are recommended for all children starting at 6 months of age, and parents who are worried about thimerosal can ask for a flu vaccine that does not contain thimerosal. (See also CDC: Thimerosal and Vaccines).
The removal of thimerosal from routine childhood vaccines has had no effect on the number of children who have developed autism spectrum disorder.
Receiving Several Vaccines at the Same Time
According to the CDC's recommended vaccination schedule, children should be given by age 6 years several doses of vaccines that prevent 10 or more different infections. To minimize the number of injections and visits, clinicians give many vaccines in combination, such as the diphtheria-tetanus-pertussis (DTaP) vaccine and others.
However, some parents worry that the 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, the recommended schedule is designed to give the various vaccines at the ages when children begin to need protection against the diseases. Thus, not following the schedule puts children at increased risk of infection. Furthermore, because current vaccines contain fewer antigens overall (because key antigens have been better identified and purified), children now are exposed to fewer vaccine antigens than they were for most of the 20th century.
Also, vaccines, even combination vaccines, contain very few antigens compared to what people encounter in daily life. 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. A child's immune system is not stressed or overwhelmed by combination vaccinations. (See also CDC: Multiple Vaccines at Once.)
