The United States will begin widely distributing COVID-19 booster shots in September as infections rise from the highly infectious Delta variant of coronavirus.

"The available data make very clear that protection against SARS-CoV-2 infection begins to decrease over time following the initial doses of vaccination, 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 Centres for Disease Control and Prevention said in a statement.

The White House said it is prepared to offer a third booster shot starting starting 20 September to all Americans who have received the second dose of Pfizer or Moderna’s vaccines at least eight months ago.

"At that time, the individuals who were fully vaccinated earliest in the vaccination rollout, including many health care providers, nursing home residents, and other seniors, will likely be eligible for a booster," the CDC said.

“Our top priority remains staying ahead of the virus and protecting the American people from COVID-19 with safe, effective, and long-lasting vaccines especially in the context of a constantly changing virus and epidemiologic landscape," it added.

Earlier, the US health officials had said that fully vaccinated Americans did not need booster shots at this. The decision to recommend booster shots comes amid increasing concern over the Delta variant and a rise in breakthrough cases — infections in fully vaccinated individuals.