When Dr. Cassidy vaccinated 36,000 children, their records were added to Louisiana’s immunization registry. When Dr. Mores knew an African patient was immune to Ebola, the patient was employed to relieve exhausted health-care workers. Increasing the number of immune people and knowing who is immune fundamentally changes things in a health crisis.

To restart the economy, the government needs to set up coronavirus-immunity registries. At the same time, widespread testing is necessary to document immunity in those who haven’t fallen sick. A recent report from China found that 100% of patients tested two weeks after symptoms cleared had antibodies for the coronavirus. Recovering from a known coronavirus infection or having a positive antibody test is likely to indicate immunity lasting for at least some time. Those who so demonstrate that they are immune can be allowed to return to work. The whole community is freer when herd immunity is established.

Healthy young people appear to be at lower risk of contracting Covid-19. This, combined with herd immunity, would allow children and college students to return to school. Any adult who is immune could safely teach. As herd immunity establishes itself, the 30% of Americans who are under 25 could safely return to shops and restaurants.