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Perspective
NEJM

07 November 2002

Vaccines represent one of the most effective interventions in medicine. They can protect whole populations from potentially dangerous diseases. Because vaccines are usually given to healthy people, especially children, any concern about the safety of vaccines has to be taken very seriously. Even with close scrutiny, the overall safety record of vaccines is extraordinarily good.

Despite this safety record, for a surprising number of vaccines, there have been widely publicized but unsubstantiated claims of adverse effects (see Table). When suspicions about a vaccine begin to spread, there may be dangerous consequences for the public health. Such is currently the case with the live attenuated measles virus in the vaccine against measles, mumps, and rubella (MMR), which some people erroneously believe is a cause of autism.

Concern about autism and the measles component of the MMR vaccine began with the awareness that encephalitis is a rare but devastating complication of measles. Some survivors of measles have immune-mediated postinfectious encephalomyelitis with seizures and mental retardation. The hypothesis about the measles vaccine and autism also rests in part on some widely publicized research involving a small number of young children with chronic gastrointestinal symptoms and behavioral abnormalities. The researchers speculated on a causal relation among MMR vaccination, abnormalities in lymphoid tissue, and developmental disorders such as autism. Together, these findings have led some to conclude that the measles virus in the MMR vaccine is a cause of autism in children.

When a child has autism, the parents begin to notice problems during the second and third years of life. Sometimes, there is severe regression in language and social skills in young children who initially seemed to be developing quite normally. Since the MMR vaccine is given at the beginning of the second year of life, there is a temporal association between vaccination and the recognition of autism. The situation is further complicated because the frequency of autism appears to have increased by at least a factor of five during the past 20 years. This increase is probably the result of better reporting and wider use of this diagnosis to describe children with unexplained cognitive and behavioral disorders. However, fears only grow when there appears to be an increase in the prevalence of a severe and unexplained disease in children.

A report in this issue of the Journal (pages 1477�1482) provides an objective analysis of the rates of autism in relation to MMR-vaccination status in a well-defined population of more than half a million children in Denmark, about one fifth of whom had not received the MMR vaccine. This careful and convincing study shows that there is no association between autism and MMR vaccination. Other studies have also found no such association.

Unfortunately, objective data are not likely to put an end to the controversy. Strongly held beliefs are difficult to change. We live in an era in which the public does not have a high degree of trust in the vaccine manufacturers, the government, or the medical establishment. Consumers have become highly sensitive about safety, and their confidence has not been bolstered by recent recalls of approved drugs or by controversies such as that over mercury-containing preservatives in vaccines (thimerosal, which has now been removed). The large number of approved vaccines and the laws that require vaccination for school entry only increase the tension. Some anti-vaccine groups have received wide publicity, and they try to convince worried parents that avoiding vaccination is "playing it safe." Internet sites are filled with accusations about damage from the MMR vaccine and other vaccines. Some parents tell moving stories about their children who showed the first signs of autism in the months after vaccination. Such experiences have already led to congressional hearings. But however painful these stories may be, anecdotes are not proof. The association of autism with MMR vaccination appears to be only a predictable coincidence, since nearly 90 percent of children in this country receive the MMR vaccine at about 15 months of age.

Unsubstantiated accusations that a vaccine causes harm can have serious consequences. Some people begin to avoid vaccination, increasing the number at risk, particularly in communities where anti-vaccine activists are most successful. Once a vaccine has been tied repeatedly to even an unsupported claim of an adverse effect, costly legal action usually follows, even if the vaccine is eventually proved to be safe. The companies that produce vaccines come to associate vaccines with adverse economic effects for themselves, with little profit and huge liability. Effective vaccines may be withdrawn from the market. It becomes harder to run vaccination programs, including those in developing countries where the needs are greatest.

Those arguing against vaccination forget how bad the illnesses and their complications can be. They forget that in this country before the development of the measles vaccine, thousands of children had severe illness from measles, sometimes resulting in permanent disability. In 1960, 400 children died from measles. But the biggest tragedy is that outside of the developed countries, more than a million children still die from this disease each year. Those deaths are preventable by a measles vaccine that is cost effective and safe. Children everywhere deserve the protection that carefully developed, carefully monitored vaccines can provide against so much disease.

Edward W. Campion, M.D.