Transmissibility is not fixed: It can vary based on a population’s behavior, demographics and health. We’ve seen this in action with Covid-19, which has spread far more quickly in some populations, as a result of differences in disease-mitigation efforts, housing density, age, occupations and health conditions. New coronavirus variants such as B.1.1.7, which is now the predominant strain in the United States, are more transmissible. And that increases the herd immunity threshold. Still, countries with high rates of vaccination like the United States may be able to move past many of the restrictions of pandemic life before reaching that goal. The seasonal flu, a virus that has a lower fatality rate but is also highly transmissible, offers a good example of how this works: Most years the flu does not become a disastrous epidemic, even though vaccination rates aren’t sufficient to get to the threshold and even though the efficacy of flu vaccines varies from year to year. With Covid-19, some communities will likely reach the herd immunity threshold, even if the entire United States does not.