Dr Fischer's letter highlights the difficulties in interpreting ecologic trends in studies such as this, namely, the potential association between the introduction of a new strain of measles vaccine, the change to a 2-dose MMR vaccination schedule, and the increase in the incidence of autism between 1976 and 1997 reported in our study.1
Barbaresi WJ, Katusic SK, Colligan RC, Weaver AL, Jacobsen SJ. Incidence of Autism—Reply. Arch Pediatr Adolesc Med. 2005;159(7):690–692. doi:10.1001/archpedi.159.7.691-b
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