![]() But this percentage isn’t a “magic threshold” that we need to cross-and it’s not just dependent on the level of population immunity. The more contagious an infection is, the higher the proportion of the population that needs immunity before infection rates start to decline. The hope is that the population can develop a high enough level of immunity to keep spread low. The same idea works for any infectious agent, including coronavirus. As a result, even though we still see localized outbreaks of measles in the U.S., those outbreaks generally die down without starting a nationwide epidemic. ![]() If a person with measles were to come to the U.S., for example, nine out every 10 people that person could infect would be immune, making it very hard for measles to spread in the population. This level of vaccination provides protection to the population as a whole-even to those who aren’t vaccinated-by decreasing viral circulation and the chance someone who is unvaccinated will encounter the virus. are vaccinated against measles, mumps, and rubella by their second birthday. When most of a population is immune to an infectious disease, this provides indirect protection-also called population immunity, herd immunity, or herd protection-to those who are not immune to the disease.įor example, currently over 90% of all children in the U.S. What do epidemiologists mean when they talk about “herd immunity”? In this Q&A, epidemiologists Gypsyamber D’Souza and David Dowdy explain that herd immunity is still possible for COVID-19, but that we might need to think a bit differently about what that means in this phase of the pandemic. With this changing perspective, how should we be thinking about herd immunity? As vaccination has rolled out, variants have emerged, and as cases surge once again, we are learning more about the nuances of SARS-CoV-2 infection and what short- and long-term immunity to this virus may look like. In the months following the emergence of SARS-CoV-2, “herd immunity” was frequently cited as the long-term destination of the COVID-19 pandemic. ![]()
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