Officials say that 4.4 million Americans have been updated with COVID booster shots.
The Centers for Disease Control and Prevention posted the count Thursday, and the White House said 5 million people had received the omicron-targeting vaccine because of its own estimates for reporting lags.
Health experts say it is too soon to predict whether demand will meet the 171 million doses ordered for the fall.
The government said it had sent 25 million shots, mostly from Pfizer and BioNTech.
Some pharmacies were forced to cancel appointments because of a shortage of Moderna vaccine.
Some Americans said they would wait to get the shot because they had recently gotten a booster or had been infected with SARS-CoV -- 2 recently.
CDC guidance says people who recently had COVID 19 may consider delaying a primary series or booster dose by three months from symptom onset or positive test.
Increased time between vaccine and infection may result in an improved immune response to vaccination, according to studies. The agency says there has been a low risk of a re-infection in the weeks to months following the infection. Individual factors such as risk of COVID 19 severe disease, COVID 19 community level or characteristics of the predominant SARS-CoV-2 strain should be taken into account when deciding whether to delay getting a COVID 19 vaccine after infection. Some people think respiratory viruses are more easily transmitted when they are closer to the holidays and winter months.
Ashish Jha, White House COVID coordinator, said that it is likely that this will pick up in the weeks ahead. We have been thinking and talking about this as an annual vaccine like the flu vaccine. Flu vaccine season picks up in late September and early October. We are just getting started with our education campaign. We expect this to ramp up stronger, despite the fact that this was a strong start. Nearly 10 million people received the shot in the first three weeks when the U.S. approved COVID 19 boosters for older and immunocompromised people.