Journal of Child Psychology and Psychiatry Volume 46, Issue 6
No effect of MMR withdrawal on the incidence of autism: a total population study
Methods: This study examined cumulative incidence of ASD up to age seven for children born from 1988 to 1996 in Kohoku Ward (population approximately 300,000), Yokohama, Japan. ASD cases included all cases of pervasive developmental disorders according to ICD
Results: The MMR vaccination rate in the city of Yokohama declined significantly in the birth cohorts of years 1988 through 1992, and not a single vaccination was administered in 1993 or thereafter. In contrast, cumulative incidence of ASD up to age seven increased significantly in the birth cohorts of years 1988 through 1996 and most notably rose dramatically beginning with the birth cohort of 1993.
Conclusions: The significance of this finding is that MMR vaccination is most unlikely to be a main cause of ASD, that it cannot explain the rise over time in the incidence of ASD, and that withdrawal of MMR in countries where it is still being used cannot be expected to lead to a reduction in the incidence of ASD.
Before rejecting the causal hypothesis, it is essential to consider possible objections to our study. There are five that need attention. First, Wakefield and his colleagues have postulated that the autism associated with MMR almost always involves developmental regression (Furlano et al., 2001; Torrente et al., 2002; Wakefield et al., 2000). Accordingly, it could be suggested that we needed to focus on autism with regression. We found no change in the incidence of ASD with regression between the periods before and after withdrawal of MMR. Three British studies (Fombonne & Chakrabarti, 2001; Taylor et al., 2002; Fombonne et al., 2004) have also shown no change in the rate of regression across time periods beginning before introduction of MMR and continuing during a time of high take‐up of the vaccine. In any case, the Spitzer, Aitken, Dell’Aniello, and Davis (2001) finding on a large sample of cases of ASD supposed to be attributable to the MMR vaccine found only 39% with regression, a proportion broadly in line with that reported during the pre‐MMR era.
Second, it could be claimed that the proportion of cases of autism due to the vaccine is too low to be detectable in a total population study of time trends in incidence. However, this runs counter to the argument that the effect was big enough to result in an overall rise in the frequency of autism (Wakefield, 1999). If it was sufficient to cause an overall rise, the cessation of MMR usage should be sufficient to result in a measurable fall. Also, the Spitzer et al. study in the UK concerned 325 cases of ASD thought to be due to MMR; this is a sizeable number.
Third, it has been argued that previous studies have been misleading because their follow‐up has not extended over at least three years (Spitzer et al., 2001). That objection cannot be applied to our study because we have deliberately focused on incidence in birth cohorts followed to age seven, meaning an age roughly six years after MMR (if used).
Fourth, it might be suggested that our surveillance system missed many cases of autism. That is extremely unlikely because our system is unusually thorough and because our overall incidence figure of 88.5 per 10,000 for ASD is in line with some other recent estimates (Kadesjö, Gillberg, & Hagberg, 1999; Wing, 1996) and higher than the 60 per 10,000 that has been put forward as the best estimate (Medical Research Council, 2001) based on high quality total population epidemiological data (Baird et al., 2000; Chakrabarti & Fombonne, 2001).