The U.S. Centers for Disease Control and Prevention say it has administered more than a million doses of COVID-19 vaccines as of Wednesday.
The CDC also says they have distributed more than 9.46 million doses across the country.
In the early phase of the nationwide vaccination program, priority is currently being given to health workers and residents of retirement homes. A private research institute says the number of people in these groups exceeds 17 million.
The CDC plans to expand the scope of the vaccination program in stages. Next in line for the vaccine will be police officers, firefighters, people 75 and older, teachers and groceries.
Meanwhile, the CDC says it has received a report that six people who received the injection developed serious allergic reactions.
Officials from the CDC and the United States Food and Drug Administration are investigating the cases. Officials have established guidelines for safe vaccinations. They called for a system where recipients are observed after vaccination and treated immediately in case of side effects.
US Secretary of Health and Human Services Alex Azar has said he expects a total of 100 million doses of the Pfizer and Moderna vaccines to be distributed by the end of February.
He added that more doses should be available if Johnson & Johnson and the AstraZeneca vaccines are approved.