The US government said on Wednesday it plans to make Covid-19 vaccine booster shots widely available to all Americans starting on 20 September 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 White House pandemic response coordinator, Jeff Zients, said the coronavirus vaccine booster shots will be free for all Americans. “It will be just as easy and convenient to get a booster shot as it is to get a first shot today,” Zients said.

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 authorized in the United States begins to diminish in the months after the shots are given.

The officials included Joe Biden’s chief medical adviser, Anthony Fauci, as well as the heads of the Centers for Disease Control and Prevention, the Food and Drug Administration and the National Institutes of Health.

“The available data make very clear that protection against SARS-CoV-2 infection begins to decrease over time … and in association with the dominance of the Delta variant, we are starting to see evidence of reduced protection against mild and moderate disease,” the officials said.

“We conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability,” they added.

The officials said that they expect that people who received Johnson & Johnson’s single-dose Covid-19 vaccine will also need boosters.

US health officials previously authorized a third dose of vaccines from Pfizer Inc and Moderna Inc for people with weak immune systems. The broader booster program follows mounting evidence that protection from the vaccines wanes after six or more months, particularly in older people with underlying health conditions.

Vaccinations have been widely available in the United States, unlike many other countries, and yet the Delta variant has caused what health experts describe as a pandemic of the unvaccinated as a significant number of people choose not to get inoculated.

Even with the announcement on boosters, Zients said the Biden administration remained committed to convincing more Americans to get their first vaccine dose.

“This remains a pandemic of the unvaccinated,” Zients said in a briefing on Wednesday morning.

The new Covid-19 cases also include a number of people who have been vaccinated, though they are far less likely to experience severe disease or death than the unvaccinated.

In recent weeks, several other countries have also decided to offer booster shots to older adults as well as people with weak immune systems, including Israel, France and Germany.

The decision announced by the US officials represented a shift from the optimism of health authorities in May in curbing the pandemic when Biden set a goal to vaccinate 70% of American adults with at least one dose by 4 July. That goal was achieved about a month late.

Daily cases in the United States soared from fewer than 10,000 in early July to more than 150,000 in August as the far more infectious Delta variant took hold.

More than 1 million Americans had independently sought an extra vaccine dose before the official decision on boosters was announced, according to federal data.