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In the U.S. and Canada, children are now routinely protected against 12 vaccine-preventable diseases: diphtheria, tetanus, pertussis (whooping cough), poliomyelitis, hepatitis B, invasive haemophilus influenzae disease (an invasive disease that may produce any of several clinical syndromes, including meningitis or pneumonia), invasive pneumococcal disease, measles, mumps, rubella (German measles), varicella (chicken pox) and influenza.
In general, all of these diseases are serious and may be fatal, while the vaccine adverse events, if they occur, are usually minor, such as local discomfort and/or inflammation at the site of the injection and/or mild fever or rash. To reap the benefit from these vaccines, children must be immunized and immunized on time. In Canada, the National Advisory Committee on Immunization (NACI) recommends that all children be immunized at two, four, six and eighteen months of age.
Unfortunately, immunization programs are the victims of their own success. As the diseases against which the vaccines protect become more rare, they also become less feared by the population. Vaccine-associated adverse events that are uncommon become relatively more frequent as the diseases and their manifestations become rarer. As a result, vaccines that are being used in healthy children become more feared by parents than diseases that they have never seen.
Among the most controversial allegations at present is whether childhood immunizations are associated with autism. Two hypotheses have emerged: a link between MMR (Measles-Mumps-Rubella) vaccine and autism, and exposure of young infants to excessive amounts of thimerosal, a mercury-based chemical used to stabilize vaccine preparation.
Over the last few years, a number of studies have examined the MMR-autism hypothesis. To-date, no epidemiological study has found an association between autism and MMR. Recent reviews of this hypothesis by the Institute of Medicine concluded that the evidence was in favour of its rejection. Moreover, systematic reviews of vaccine safety are regularly performed, and there had been no reports of autism as a possible adverse event following measles vaccine or MMR vaccine.
Children exposed to high doses of methyl mercury were also followed and again, no increased incidence of autism has ever been documented. (It should be noted that the thimerosal substance has never been used in the MMR vaccine, and that most vaccinations now exist in a thimerosal-free format.)
Several epidemiological designs have tested these hypotheses and found that increased prevalence of autism and related conditions (pervasive developmental disorders) was due to diagnostic switching, changes in diagnostic criteria, improved detection of autism in populations and greater awareness about the disorder in both the professional and lay audience. Since the epidemiologic data to date indicate that MMR vaccination is not associated with an increase in autism in the population, the known neurologic and other serious risks of these preventable diseases is considered to be much greater than the risk of the vaccine.
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