Just 40% of fully vaccinated Americans have received a booster dose, according to the Centers for Disease Control and Prevention. And the average number of booster shots dispensed per day in the U.S. has plummeted from a peak of 1 million in early December to about 490,000 as of last week.

Also, a new poll from The Associated Press-NORC Center for Public Affairs Research found that Americans are more likely to see the initial vaccinations — rather than a booster — as essential.

“It’s clear that the booster effort is falling short,” said Jason Schwartz, a vaccine policy expert at Yale University.