The US Centers for Disease Control and Prevention has said phases of vaccinating priority groups against COVID-19 may overlap, clarifying guidance that many states are already applying as a way to get shots to as many Americans as possible.

The CDC has said healthcare workers and nursing home residents should have priority for vaccines, to be followed by people over 75 and essential workers. On Friday, the CDC changed its website to underscore that these phases may overlap, essentially greenlighting what many states are already doing.

Every state has its own plan for vaccine distribution, with little oversight and involvement from the federal government. This week, US Health and Human Services Secretary Alex Azar said the CDC's priority recommendations were just that, and should not get in the way of getting shots to Americans.

Although the federal government has distributed more than 21 million doses, only about 6 million have been administered.

A CDC spokesperson said the information was previously available on a section of its site that outlined priority recommendations from its outside advisory panel.