The most rigorous and comprehensive analysis of scientific studies conducted on the efficacy of masks for reducing the spread of respiratory illnesses — including Covid-19 — was published late last month. Its conclusions, said Tom Jefferson, the Oxford epidemiologist who is its lead author, were unambiguous. “There is just no evidence that they” — masks — “make any difference,” he told the journalist Maryanne Demasi. “Full stop.” But, wait, hold on. What about N-95 masks, as opposed to lower-quality surgical or cloth masks? “Makes no difference — none of it,” said Jefferson. What about the studies that initially persuaded policymakers...