AP Photo/Matt Marton A basic rule of epidemiology is that the more contagious a virus is, the greater the share of the population that has to be immunized from it in order to achieve herd immunity. Which makes sense. A germ that’s very transmissible will need to have nearly every vector of transmission blocked to keep it from eventually reaching an unvaccinated person. For a super-contagious virus like the one that causes measles, upwards of 95 percent of a group needs to be vaccinated to give the other five percent a reasonable shot at avoiding infection. Initially we were told that immunizing 60-70 of the population from SARS-CoV-2 might be expected to provide herd immunity. Then Fauci admitted that he had told one of his little white lies about that, that the true threshold was probably more like 75-85 percent. But that was last year, when we were still coping with the original, slower-moving version of the virus. Now we’re waist-deep in the B.1.617 “Delta” variant, which is considerably more infectious than the B.1.1.7 “Alpha” variant, which was itself considerably more infectious than the virus that emerged in Wuhan in late 2019.