Hviid A, et al. - In this nationwide cohort study involving 657,461 children born in Denmark, researchers ascertained if the risk for autism is increased after receiving the measles, mumps, rubella (MMR) vaccine in children, subgroups of children, or time periods after vaccination. There were 6,517 children diagnosed with autism during 5,025,754 person-years of follow-up. The investigation strongly supports that MMR vaccination does not increase autism risk, does not trigger autism in susceptible children, and is not associated with post-vaccination clustering of autism cases. It adds significant additional statistical power to previous studies and addresses susceptible subgroup hypotheses and case clustering. A fully adjusted autism hazard ratio of 0.93 was seen when comparing MMR-vaccinated with MMR-unvaccinated children.