On Tuesday morning, a Fox News contributor claimed on Twitter that the Centers for Disease Control and Prevention was set to mandate that schoolchildren get coronavirus vaccines. By Tuesday evening, the claim was being repeated by the nation's most popular cable news show, and had been amplified to millions more on social media online. "The CDC is about to add the Covid vaccine to the childhood immunization schedule, which would make the vax mandatory for kids to attend school," host Tucker Carlson tweeted, sharing a segment from his show that has been viewed more than 1.5 million times online. But...