Safety
Adverse reactions, rarely serious, may occur from each component of the MMR vaccine. 10% of children develop fever, malaise and a rash 5–21 days after the first vaccination; 5% develop temporary joint pain. Older women appear to be more at risk of joint pain, acute arthritis, and even (rarely) chronic arthritis. Anaphylaxis is an extremely rare but serious allergic reaction to the vaccine. One cause can be egg allergy. The vaccine product brief lists many other possible adverse reactions.
The number of reports on neurologic disorders is very small, other than evidence for an association between a form of the MMR vaccine containing the Urabe mumps strain and rare adverse events of aseptic meningitis, a transient mild form of viral meningitis. The UK National Health Service stopped using the Urabe mumps strain in the early 1990s due to cases of transient mild viral meningitis, and switched to a form using the Jeryl Lynn mumps strain instead. The Urabe strain remains in use in a number of countries; MMR with the Urabe strain is much cheaper to manufacture than with the Jeryl Lynn strain, and a strain with higher efficacy along with a somewhat higher rate of mild side effects may still have the advantage of reduced incidence of overall adverse events.
The Cochrane Library review found that, compared to placebo, MMR vaccine was associated with fewer upper respiratory tract infections, more irritability, and a similar number of other adverse effects. The review also found several problems in the quality of MMR vaccine safety studies, and recommended the adoption of standardized definitions of adverse events. The review's abstract concludes, "The design and reporting of safety outcomes in MMR vaccine studies, both pre- and post-marketing, are largely inadequate. The evidence of adverse events following immunisation with the MMR vaccine cannot be separated from its role in preventing the target diseases."
Read more about this topic: MMR Vaccine
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