On Wednesday, the seven-day average of vaccinations fell to fewer than 182,000 per day, according to data compiled by The Washington Post. That is lower than at any time since the first days of the program.

The daily total has been in free fall for the past six weeks. On Feb. 10, the nation was averaging more than 692,000 shots a day. Booster shots have been more common than first or second doses since October, and the low rates have long caused concern among some experts.

Now, with authorities bracing for a possible increase in covid-19 cases caused by the BA.2 subvariant, 65.4 percent of Americans are fully vaccinated and just 44 percent have received a booster shot. That is substantially less than the totals in many Western European nations — which nevertheless have seen a sharp rise in cases in recent weeks and months.