Despite the strong vaccine safety systems in place in the United States, some 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 likely to develop in children whose parents have refused one or more vaccines.
Specifically, unvaccinated children are
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.
The public press has 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 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
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 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.
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.
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 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.
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.