Ahead of Memorial Day weekend, half of U.S. adults are now fully vaccinated against COVID-19. But experts say in order to reach herd immunity, anywhere from 70 to 90 percent of the population needs their shots. Now, health leaders are pushing younger Americans to roll up their sleeves. Dr. Brian Castrucci is the president and CEO of the de Beaumont Foundation and a leading voice on public health issues. Soledad O’Brien talks with him about combatting vaccine hesitancy.