Columnist Zeynep Tufecki examines the delayed rollout of a COVID-19 vaccine for children under 5. The author writes: "As Deepta Bhattacharya, an immunologist at the University of Arizona, explained to me, a singular measure doesn't capture all the benefits of vaccination. Vaccines train the immune system to recognize and more quickly respond to a virus, giving it a head start that he notes can 'reduce severity and reduce the period of time that you are contagious.'"