“We know that children can be vectors for spreading COVID to other individuals,” Wen explains. “This is something that very much affects people in their families as well as in the surrounding community — being able to contain or to prevent children from further spreading to others. It would be very difficult for us to reach herd immunity without children also getting vaccinated.”

The other part of the equation is ethical. At some point — perhaps when the Delta wave has finally burned itself out — the vast, vaccinated majority of U.S. adults will have to accept that their unvaccinated counterparts have decided to acquire immunity the hard way (and risk suffering or even death in the process).

So vaccinated Americans — who enjoy near-perfect protection from severe illness, hospitalization and death, and who rarely spread the virus to others — won’t continue to limit their own lives indefinitely in order to protect the willfully unprotected. The U.S. doesn’t mandate masks during flu season, and even the most cautious leaders are eager to stop requiring them for people who’ve chosen, through vaccination, to reduce their personal risk from COVID to something like the flu or a cold.

But that’s unlikely to happen before 28 million younger Americans who are required to spend all day indoors with one another have had an opportunity to get vaccinated.