The boosters are rolling out. In places likes the U.S., Britain, and Israel, authorities are providing additional Covid-19 vaccines with the goal of bolstering immune systems and shoring up their economies. Though vaccines such as Moderna and Pfizer-BioNTech have proven highly effective against the virus, efficacy wanes after six months, rendering older adults and those with weakened immune systems more vulnerable. Yet a debate about fairness, when much of the planet is unvaccinated, is growing.  Learn more about your ad choices. Visit megaphone.fm/adchoices