Background: We examined whether, in the UK, there is an increased risk of autism (AD) following exposures, in early life, to: (1) wild measles; (2) live attenuated measles, alone or in combination as MMR; and (3) the alteration of the mumps strain within MMR.
Method: We conducted time trend analyses of 2407 AD subjects born between 1959-93; and for comparison, 4640 Down's syndrome (DS) subjects born between 1966-93. Between 1968-86, we correlated variations in AD and DS births with wild measles incidence. Between 1959-93, we tested for abrupt changes in the long-term AD birth trend for the effects of introducing: (1) monovalent measles vaccines in 1968; (2) MMR immunization in 1988; and (3) the 'overnight switch' from mixed use of Urabe MMR to exclusive use of Jeryl-Lynn MMR in 1992. Incidence rate ratios (IRRs) were used as measures of association.
Results: We found no significant association between AD births and exposure (prenatal and postnatal up to 18 months age) to population rates of measles infections, and no 'step-up' increase in AD births associated with the introduction of monovalent measles and MMR vaccines, and changing mumps strain. An unexpected reduction in AD births of 21% (95% CI 6.9-33.3%; P=0.005) among the post-1987 birth cohorts was detected.
Conclusion: No increased risk of AD following exposures to wild measles and vaccinations with monovalent measles, and Urabe or Jeryl-Lynn variants of MMR was detected. The precise meaning of the detected AD births reduction is unclear. Our study cannot exclude rare complications of MMR, given its correlational design.