The United States government has said it plans to make Covid-19 vaccine booster shots widely available to all Americans starting on Sept 20 as infections rise from the Delta variant of the coronavirus.

The White House is prepared to offer a third booster shot starting on that date to all Americans who completed their initial inoculation at least eight months ago, the US Department of Health and Human Services said in a statement.

The booster shots initially will be given primarily to healthcare workers, nursing home residents and older people, all of whom were among the first groups to be vaccinated in late 2020 and early 2021, the department said.

Top US health officials said in a joint statement that they based their decision to offer boosters on data showing that the protectiveness of Covid-19 shots currently authorised in the United States begins to diminish in the months after the shots are given, according to a Reuters report.