For some of us, booster shots have finally arrived. But they’ve charted quite a meandering course to get here. First, last month, President Joe Biden announced that most Americans would be able to nab third doses of mRNA vaccines eight months after their second shots. Then, last week the FDA narrowed the eligible population, before a CDC advisory committee suggested tightening the boundaries even further. Hours after that panel shared its recommendation, the agency’s director, Rochelle Walensky, reversed course and ballooned the guidance back out to more closely align with...