Relief from the scourge of COVID-19 in the United States may be in sight. Government and private sector initiatives should be able within months to provide enough safe, effective vaccines to begin to make a dent in COVID-19 cases, hospitalizations, and deaths. But for society to approach normalcy, people will need to be able to document that they are immune to COVID-19.

Although vaccination is intended primarily to protect individuals against COVID-19 infection or reduce its severity, it can also protect the wider community when a sufficiently large fraction of the population, thought to be approximately 70% in the case of COVID-19, gains immunity to infection because of either natural infection or vaccination. At that point, the virus has difficulty finding new, susceptible hosts, and the outbreak subsides. That state is called “herd immunity.”