Millions of free covid-19 rapid tests arriving in Americans’ mailboxes are long-awaited vindication for Dr. Michael Mina, who, as a Harvard assistant professor, had been advocating for two years that the best way to limit covid is to identify it quickly, cheaply, and widely with rapid antigen tests so infected people know to isolate themselves. “Rapid Tests Are the Answer to Living With Covid-19” was the headline on an October New York Times op-ed he co-authored. The Atlantic called him “America’s biggest antigen-test advocate.” In much of the world, rapid tests are “free for people and sold t…