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 allow their children to be 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 refused one or more vaccines.
Children in the United States who do not receive routine vaccinations still become very sick and sometimes die from 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). Hib infection, which, before the vaccine, occurred in about 20,000 children each year, is now rare in the United States. From 2009 to 2018, there were only 36 severe cases in children younger than 5 years of age, and 26 of theses cases were in children who were unvaccinated or not fully vaccinated (see Centers for Disease Control and Prevention (CDC): Epidemiology and Prevention of Vaccine-Preventable Diseases).
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 one COVID-19 vaccine dose. However, many people remain unvaccinated. Similar to other vaccine-preventable diseases, COVID-19 hospitalizations and deaths are much more common among unvaccinated people. In addition, serious side effects associated with COVID-19 vaccination are extremely rare (fewer than 10 cases per million vaccinations).
Some parents think that COVID-19 infection is not dangerous to children, but this is not the case. Although COVID-19 infection is typically milder in children than adults, it can cause serious problems and death. As of May 2023, more than 15 million children in the United States were reported to have tested positive for COVID-19 since the onset of the pandemic, resulting in 1,839 deaths. Additionally, COVID-19 can lead to multisystem inflammatory syndrome in children (MIS-C), a rare but serious condition diagnosed in nearly 10,000 children resulting in 79 deaths as of May 31, 2023. As in adults, hospitalization is more frequent in unvaccinated versus vaccinated adolescents. Also, children may develop long-lasting problems (long COVID), 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 or Post-COVID Conditions).
Measles-Mumps-Rubella (MMR) Vaccine: Concerns about Autism
In the 1990s, the public press reported concerns that the MMR vaccine may cause autism. These concerns were based on a fraudulent and brief medical report in 1998 about 12 children. 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 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 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. The percentage of children who became autistic is nearly identical in the group that was vaccinated and the group that was not vaccinated. 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 overall nor does it increase the risk in children who are at high-risk of autism because of their family history.
Other similar studies from across the world have reached similar conclusions. In addition, the research in the original widely publicized study linking autism 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 unfortunately 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
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.
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 vaccines, as well as in several vaccines intended for use in adults. Yearly flu vaccines are recommended for all children, and parents who are worried about thimerosal can ask for a flu vaccine that does not contain thimerosal. (See also CDC: Thimerosal FAQs).
The removal of thimerosal from routine childhood vaccines has had no effect on the number of children who have developed autism.
Use of Several Vaccines at the Same Time
According to the CDC's recommended vaccination schedule, children should be given several doses of vaccines against 10 or more different infections by age 6 years. To minimize the number of injections and visits, clinicians give many vaccines in combination, such as the diphtheria-tetanus-pertussis 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 are exposed to fewer vaccine antigens today 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.)
Effects of Vaccine Refusal on Public Health
Delaying or refusing vaccines 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.
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 medication (such as the medications used to treat cancer or prevent rejection of a transplant), and they cannot be given live-virus vaccines, such as the MMR or varicella vaccine.
