A huge number of people will need to receive a vaccine in order to stop the spread of the coronavirus and establish herd immunity — the point at which most of the population is protected against an infectious disease. To achieve herd immunity against Covid-19, experts estimate that around 70% or more of the population may need to be immune. And, since no vaccine is 100% effective, the more people who are vaccinated, the better. Getting a vaccine to all the people who need one will take a massive effort, both in the United States and around the world. There has never been a global immunization campaign on the scale that will be needed to distribute a Covid-19 vaccine.