Final answer:
Herd immunity is a phenomenon where a large portion of the population becomes immune to a disease, indirectly protecting the susceptible individuals. It is achieved through vaccination programs. To achieve herd immunity against COVID-19, a large percentage of the population needs to be vaccinated.
Step-by-step explanation:
Herd immunity is a phenomenon that occurs when a large percentage of a population becomes immune to a disease, providing indirect protection to the remaining susceptible individuals. It is achieved through vaccination programs, which reduce the number of susceptible individuals in a population. Even if some individuals are not vaccinated, as long as a certain percentage is immune, the susceptible individuals are unlikely to be exposed to the pathogen. For herd immunity against COVID-19 to be achieved in the US, experts suggest that a significant portion of the population needs to be vaccinated, ideally around 70% to 90%.