Last week, all the living former U.S. presidents, Democratic and Republican, joined together for an ad campaign touting the safety and effectiveness of the COVID-19 vaccines — except Donald Trump. At the same time, a new Yahoo News/YouGov poll found that a full 50 percent of unvaccinated 2020 Trump voters now say they will “never” get vaccinated for COVID-19, up 6 percent from last month. According to the survey of 1,629 U.S. adults, which was conducted March 4-8, no other unvaccinated group is nearly as likely to say they will “never” get inoculated: not Biden supporters (8 percent), not Black...