The seven-day average of daily first doses was about 272,000 by last Friday, according to the CDC, making it the slowest week of first-dose immunizations since mid-July. On Tuesday, fewer than 21,000 individuals were injected with their first shot, tentative figures from the CDC show, potentially making it the slowest day since Christmas.

This slowdown is partly because millions of Americans have been either fully or partially vaccinated. About 55 percent of Americans are fully vaccinated, while about 64 percent have received at least one dose of a coronavirus vaccine. But it also comes amid an unwelcome comeback of infections and deaths across the country, and as the United States falls behind in overall vaccination rates globally, despite having had a months-long head start in immunizing its population.

Vaccination rates mostly rose from December 2020 through April in America as more people became eligible for vaccination, the CDC data show. At the peak, April 8, nearly 2.6 million people received their first shot. Rates fell again until July, when the highly transmissible delta variant increased coronavirus case numbers, pushing more people to get vaccinated. In August, vaccination rates started dropping again.