Covid-19 vaccination efficacy is decreasing over time, US authorities warned Wednesday as they authorized booster shots for all Americans from September 20 starting eight months after an individual has been fully vaccinated.

The move comes as scientists and health experts grapple with how to beat back the surging Delta variant of the coronavirus, and follows extensive debate over whether a third injection of the Pfizer-BioNTech and Moderna vaccines would be appropriate.

"The available data make very clear that protection against (coronavirus) infection begins to decrease over time following the initial doses of vaccination, the nation's top health officials, including Centers for Disease Control and Prevention director Rochelle Walensky and US Surgeon General Vivek Murthy, said in a statement.

"In association with the dominance of the Delta variant we are starting to see evidence of reduced protection against mild and moderate disease," they added.

The officials warned that while the vaccines remain "remarkably effective" in reducing the risk of severe disease, hospitalization and death from the effects of Covid, protection could diminish in the months ahead without boosted immunization.

"We conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability," the officials said.

The boosters would be available beginning the week of September 20, with individuals eligible for a shot starting eight months after receiving a second dose of the Pfizer or Moderna vaccines.

The officials said they also anticipate booster shots will be needed beginning later in the year for people who received the Johnson & Johnson vaccine, which rolled out in March.

More than 620,000 people have died in the United States from Covid-19, with case numbers increasing sharply in recent months due to the spread of the Delta variant.

Last week the United States authorized an extra dose of Covid-19 vaccine for people with weakened immune systems. 

Wednesday's announcement highlights the concern over rapidly rising cases in several US states, as the country's mass vaccination program hit resistance in politically conservative regions in the South and Midwest.