Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule Childhood Vaccination Schedule Most doctors follow the vaccination schedule recommended by the Centers for Disease Control and Prevention (CDC—see the schedule for infants and children and the schedule for older children... read more 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 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.
COVID-19 vaccines
The COVID-19 pandemic COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more has brought vaccine hesitancy back to the forefront. The first COVID-19 vaccine COVID-19 Vaccine Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines... read more 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), including excessive blood clotting (thrombosis), low platelet levels (thrombocytopenia Overview of Thrombocytopenia Thrombocytopenia is a low number of platelets (thrombocytes) in the blood, which increases the risk of bleeding. Thrombocytopenia occurs when the bone marrow makes too few platelets or when... read more 
), and inflammation of the heart muscles (myocarditis Myocarditis Myocarditis is inflammation of the muscle tissue of the heart (myocardium) that causes tissue death. Myocarditis may be caused by many disorders, including infection, toxins and drugs that affect... read more and pericarditis Acute Pericarditis Acute pericarditis is inflammation of the pericardium (the flexible two-layered sac that envelops the heart) that begins suddenly, is often painful, and causes fluid and blood components such... read more 
).
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. Millions of children in the United States have been infected during the pandemic, leading to more than 6,400 cases of the rare but serious multisystem inflammatory syndrome in children Symptoms COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more (MIS-C) and more than 50 deaths. 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.
Measles-mumps-rubella (MMR) vaccine and concerns about autism
In the 1990s, the public press has reported concerns that the MMR vaccine Measles, Mumps, and Rubella Vaccine The measles, mumps, and rubella (MMR) vaccine is a combination vaccine that helps protect against these three serious viral infections. The vaccine contains live but weakened measles, mumps... read more may cause autism Autism Spectrum Disorders Autism spectrum disorders are conditions in which people have difficulty developing normal social relationships, use language abnormally or not at all, and show restricted or repetitive behaviors... read more . 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.
Thimerosal and 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.
Because of these 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. (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, provides additional support 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, which do not contain thimerosal.
Use of several vaccines at the same time
According to the recommended schedule, children should be given several doses of vaccines against 10 or more 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 Diphtheria-Tetanus-Pertussis Vaccine The diphtheria, tetanus, and pertussis vaccine is a combination vaccine that protects against these three diseases: Diphtheria usually causes inflammation of the throat and mucous membranes... read more and others, have been developed so that children can receive more vaccines with fewer injections and visits.
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. 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.
Effects of vaccine refusal on public health
Delaying or excluding 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 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.
