At what point does a country achieve herd immunity? What portion of the population must acquire resistance to the coronavirus, either through infection or vaccination, in order for the disease to fade away and life to return to normal? Since the start of the pandemic, the figure that many epidemiologists have offered has been 60 to 70 percent. That range is still cited by the World Health Organization and is often repeated during discussions of the future course of the disease. Recently, a figure to whom millions of Americans look for guidance Dr. Anthony S. Fauci, an adviser to both the Trump administration and the incoming Biden administration " has begun incrementally raising his herd-immunity estimate. In the pandemic's early days, Dr. Fauci tended to cite the same 60 to 70 percent estimate that most experts did. About a month ago, he began saying "70, 75 percent" in television interviews. And last week, in an interview with CNBC News, he said "75, 80, 85 percent" and "75 to 80-plus percent." In a telephone interview the next day, Dr. Fauci acknowledged that he had slowly but deliberately been moving the goal posts. He is doing so, he said, partly based on new science, and partly on his gut feeling that the country is finally ready to hear what he really thinks. Hard as it may be to hear, he said, he believes that it may take close to 90 percent immunity to bring the virus to a halt " almost as much as is needed to stop a measles outbreak. The Centers for Disease Control and Prevention offers no herd immunity estimate, saying on its website that "experts do not know."