Herd immunity, or the level of vaccination required to eliminate coronavirus, is becoming less and less likely as the pathogen continues to mutate and the pace of immunization slows. U.S. public health experts initially had hoped to reach the targeted threshold of 60%–70% of the population by this summer. Experts now calculate the herd immunity threshold to be at least 80%. If it turns out that immunized people can still spread coronavirus, another upward adjustment will be needed. Therefore, instead of eradication, scientists and public health experts expect the virus to linger for years to come. They do, however, believe the number of people who fall ill or die from it will probably decline significantly and that it will evolve into a seasonal occurrence, like influenza, and eventually become more like a common cold. “The virus is unlikely to go away,” says Rustom Antia, an evolutionary biologist at Emory University in Atlanta. “But we want to do all we can to check that it’s likely to become a mild infection.” Continued immunizations, especially for people at highest risk because of age, exposure or health status, will be crucial to limiting the severity of outbreaks, if not their frequency, experts believe.By focusing on vaccinating the most vulnerable, the United States has already brought those numbers down sharply. If the vaccination levels of that group continue to rise, the expectation is that over time the coronavirus may become seasonal, like the flu, and affect mostly the young and healthy.