In fact, the US waited to approve the vaccine for ten years after it was being used in Japan because the disease wasn't considered very dangerous. As a parent, I remember waiting impatiently for the vaccine to get approved because, while not usually dangerous, the disease was very uncomfortable for my child and we both lost a lot of sleep. I was glad when it finally got approved in time for my youngest child to get it.

But if the ten years of Japanese experience had shown serious consequences to a small number of children vaccinated, then that would have changed my calculation completely.