Despite the rigorous vaccine safety systems in place in the US, many parents remain concerned about the safety of the childhood vaccines and immunization schedule. These concerns have led some parents to not allow their children to receive some or all of the recommended vaccines. In the US, rates of vaccine exemptions increased from 1% in 2006 to 2% in 2011; some states reported that 6% of children received exemptions. The rate of vaccine-preventable diseases is higher in children whose parents have refused ≥ 1 vaccines for nonmedical reasons. Specifically, they are 123 times more likely to contract pertussis, 28.6 times more likely to contract varicella, and 36.5 times more likely to contract pneumococcal disease. Children in the US still die from vaccine-preventable diseases. 4In 2008, there were 5 cases (one fatal) of invasive Haemophilus influenza type B infection in Minnesota, the most since 1992. Three of the infected children, including the child who died, had received no vaccines because their parents had deferred or refused the vaccine.
The decision to defer or refuse vaccines also affects public health. When the proportion of the overall population that is immune to a disease (herd immunity) decreases, disease prevalence increases, increasing the possibility of disease in people at risk. People may be at risk because
Parents hesitate to vaccinate their children for many reasons. Two of the more prominent parental concerns over the past decade have been that
1Glanz JM, et al
: Parental refusal of pertussis vaccination is associated with an increased risk of pertussis infection in children. Pediatrics
2Glanz JM, et al:
Parental refusal of varicella vaccination and the associated risk of varicella infection in children. Arch Pediatr Adolesc Med
3Glanz JM, et al:
Parental decline of pneumococcal vaccination and risk of pneumococcal related disease in children. Vaccine
MMR vaccine and autism:
In 1998, Andrew Wakefield and colleagues published a brief report in The Lancet. This report concerned 12 children with developmental disorders and GI problems; 9 of them also had autism. According to the report, parents claimed that 8 of the 12 children had received the combined measles-mumps-rubella (MMR) vaccine within 1 mo before the development of symptoms. Wakefield postulated that the measles virus in the MMR vaccine traveled to the intestine where it caused inflammation, enabling proteins from the GI tract to enter the bloodstream, travel to the brain, and cause autism. This study received significant media attention worldwide, and many parents began to doubt the safety of the MMR vaccine. In another study, Wakefield claimed to find the measles virus in intestinal biopsy specimens of 75 of 90 children with autism and in only 5 of 70 control patients, leading to speculation that the live measles virus in the MMR vaccine was somehow implicated in autism.
Because Wakefield's methodology could show only a temporal association rather than a cause-and-effect relationship, numerous other researchers studied the possible connection between the MMR vaccine and autism. Gerber and Offit1 reviewed at least 13 large epidemiologic studies, all of which failed to support an association between MMR vaccine and autism. Many of these studies showed that national trends of MMR vaccination were not directly associated with national trends in the diagnosis of autism. For example, in the UK between 1988 and 1999, the rate of MMR vaccination did not change, but the rate of autism increased.
Other studies compared the risk of autism in individual children who did or did not receive the MMR vaccine. In the largest and most compelling of these studies, Madsen et al2 assessed 537,303 Danish children born between 1991 and 1998, 82% of whom had received MMR vaccine. After controlling for possible confounders, they found no difference in relative risk of autism or other autism-spectrum disorders in vaccinated and unvaccinated children. Overall incidence of autism or an autistic-spectrum disorder was 608 of 440,655 (0.138%) in the vaccinated group and 130 of 96,648 (0.135%) in the unvaccinated group. Other population-based studies from across the world have reached similar conclusions.
In response to Wakefield's increased detection of measles virus in intestinal biopsy specimens from autistic children, Hornig et al3 searched for the measles virus in biopsy samples taken from 38 children who had GI symptoms and were having a colonoscopy; 25 children had autism, and 13 did not. The measles virus was not detected more often in the children with autism than in those without.
1Gerber JS, Offit PA
: Vaccines and autism: A tale of shifting hypotheses, Clin Infect Dis
2Madsen KM, et al
: A population-based study of measles, mumps, and rubella vaccination and autism. N Engl J Med
3Hornig M, et al
: Lack of association between measles virus vaccine and autism with enteropathy: A case-control study. PLoS ONE
, 3(9):e3140, 2008.
Thimerosal and autism:
Thimerosal is a mercury compound previously used as a preservative in many multidose vaccine vials; preservatives are not needed in single-dose vials and cannot be used in live-virus vaccines. Thimerosal is metabolized to ethylmercury, which is eliminated quickly from the body. Because environmental 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 US, Europe, and several other countries by 2001. However, in these countries, thimerosal continues to be used in certain influenza vaccines and in several other vaccines intended for use in adults (see Thimerosal Content in Some US Licensed Vaccines). It is also used in many vaccines produced in developing countries; the WHO has not recommended its removal because there is no clinical evidence of toxicity due to routine use.
Despite the removal of thimerosal, rates of autism have continued to increase, strongly suggesting that thimerosal in vaccines does not cause autism. Also, 2 separate Vaccine Safety Datalink (VSD) studies have concluded that there is no association between thimerosal and autism. In a cohort study of 124,170 children in 3 managed care organizations (MCOs); Verstraeten et al1 found no association between thimerosal and autism or other developmental conditions, although inconsistent associations (ie, seen in one MCO but not another) were seen between thimerosal and certain language disorders. In a case-control study of 1000 children (256 with an autism-spectrum disorder and 752 matched controls without autism), Price et al2, using regression analysis, found no association between exposure to thimerosal and autism.
Practitioners who work with parents who are still concerned about thimerosal in the influenza vaccine may use single-dose vials or give live-attenuated influenza vaccine; neither of them contains thimerosal.
1Verstraeten T, et al
: Safety of thimerosal-containing vaccines: A two-phased study of computerized health maintenance organization databases. Pediatrics
2Price CS, et al
: Prenatal and infant exposure to thimerosal from vaccines and immunoglobulins and risk of autism. Pediatrics
Use of multiple, simultaneous vaccines:
A nationally representative survey done in the late 1990s revealed that nearly one fourth of all parents felt that their children receive more immunizations than they should. Since then, additional vaccines have been added to the immunization schedule so that by age 6, children are now recommended to receive multiple doses of vaccines for 15 different infections (Table 2: Recommended Immunization Schedule for Ages 0–6 yr). To minimize the number of injections and visits, practitioners give many vaccines as combination products (eg, diphtheria-tetanus-pertussis, measles-mumps-rubella). However, some parents have become concerned that children's (particularly infants') immune system cannot handle multiple simultaneously presented antigens. This concern has caused some parents to request alternative immunization schedules that delay and sometimes completely exclude certain vaccines. A recent nationally representative survey found that 13% of parents use such a schedule.
The use of alternative schedules is risky and scientifically unfounded. The official schedule is designed to protect children against diseases when they are most susceptible. Delaying vaccination increases the amount of time children are at risk of acquiring these diseases. In addition, although parents may plan to only delay vaccination, the increased number of visits needed for alternative schedules increases the difficulty of adherence and thus the risk that children will not receive a full series of vaccines. Regarding the immunologic challenges, parents should be informed that the amount and number of antigens contained in vaccines is miniscule compared with that encountered in everyday life. Even at birth, an infant's immune system is prepared to respond to the hundreds of antigens the infant is exposed to while passing through the birth canal and being handled by the (unsterile) mother. Children typically encounter and respond immunologically to dozens and perhaps hundreds of antigens during an ordinary day without difficulty. A typical infection with a single organism stimulates an immune response to multiple antigens of that organism (perhaps 4 to 10 in a typical URI). Furthermore, because current vaccines contain fewer antigens overall (ie, 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.
In summary, alternative vaccine schedules are not evidence-based and put children at increased risk of infectious diseases. More importantly, they offer no advantage. Using data from the VSD, Smith and Woods1 compared neurodevelopmental outcomes in a group of children who received all vaccines on time with those who did not. The children in the delayed group did not do better on any of the 42 outcomes tested. These results should reassure parents who are concerned that children receive too many vaccines too soon.
1Smith MJ, Woods CR
: On-time vaccine receipt in the first year does not adversely affect neuropsychological outcomes. Pediatrics
125(6)1134 -1141, 2010.
Last full review/revision September 2013 by Michael J. Smith
Content last modified September 2013