State of play: Boosters would be given in two main scenarios: A new variant arises that is vaccine-resistant, or data eventually shows that vaccinated populations' level of protection wanes over time. But protection against the virus doesn't just disappear all at once. And it's possible that the mRNA vaccines could be 70% effective against a new variant, for example, which is still relatively good but less than their current efficacy of more than 90%. Less protection could mean that more people could get sick, but with only mild infections that infrequently lead to hospitalization or death. Giving Americans a third shot isn't cost-free, financially or politically. “If you’re going to give a third dose, that is a massive logistic exercise — administratively, logistically, and in terms of the cost. So it’s not a trivial decision," said Cornell virologist John Moore.