One of the little-noticed reasons that Wyeth was attractive enough to command a $68 billion price for rival Pfizer Inc.'s planned takeover sits in a building catty-corner from the White House across Pennsylvania Avenue. That is where a special "vaccines court" hears cases brought by parents who claim their children have been harmed by routine vaccinations.

The court -- and the law that established it more than two decades ago -- buffers Wyeth and other makers of childhood-disease vaccines from much of the litigation risk that...