As part of efforts to expand access to Covid vaccines, the federal government will allow makers to release all their available doses instead of reserving booster shots, the US health secretary said Tuesday.
Alex Azar told ABC News that the manufacture of the Pfizer and Moderna vaccines was now "predictable enough that second doses are available for people from ongoing production."
This marks a break from the previous plan in which second doses were held back for those people who had received their first, to ensure that there is no delay.
The Pfizer and Moderna vaccines require boosters after three and four weeks, respectively.
President-elect Joe Biden's team had already announced that they would adopt this approach.
It comes as US rollout of the Covid vaccines has been off to a shaky start, but is showing signs of picking up.
Of the 25.4 million first doses distributed to states so far, 8.9 million had been injected into arms, or 35 percent.
"There hasn't been a focus on the last mile," Harvard health policy researcher Dr Thomas Tsai told AFP.
"There's been a focus on the vaccine and we've been able to conduct trials and produce a vaccine within a year yet that's been the entire focus of the federal response."
Azar told reporters on a press call that he would now be recommending that states begin to widen the criteria for who gets vaccinated, starting with people over 65 without health conditions and people under 65 with health conditions.
He added the rollout so far had been "over hospitalized" and needed to shift to pharmacies, local health centers, and mass vaccination sites.
According to an analysis by the Kaiser Family Foundation, all US states and the capital Washington followed recommendations by the Centers for Disease Control and Prevention (CDC) to prioritize health workers and long term care residents in "phase 1a."
Since then, they have begun to diverge from the CDC strategy and from each other, for example by expanding the age category covered by phase 1b from over-75s to over-65s, which is what Azar has now recommended anyway.
Azar also announced that Operation Warp Speed would soon change the way in which doses are allocated to states, in order to incentivize them to rapidly administer their doses and report what they are doing.
He said future allocations would be tied to the rate of administration as well as a state's population of over-65s.
But it is not clear if President-elect Joe Biden will persist with this system when he takes office next week.
After early teething troubles, cities like New York and Los Angeles are preparing mass vaccination sites.
The US is already the hardest-hit country in the world with more than 377,000 deaths and is facing its worst ever surge, with more than 3,000 people dying every day.
Epidemiologists fear it could get even worse further as newer, more contagious variants of the virus become established in the US.