Why are so few people getting the latest Covid-19 vaccine?

The Covid-19 vaccines were hailed as a miracle upon their arrival. They were delivered earlier than anyone thought possible and proved exceptionally effective in preventing hospitalizations and deaths. More than 80 percent of all Americans, and more than 90 percent of adults, received at least one dose of the vaccines, remarkable penetration in a country where less than half of people get their flu shot every year.

But so far this year, just 14 percent of adults have received a dose of the new vaccine formulation that became available in September — compared...