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 available data make very clear that protection against (coronavirus) 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 nation's top health officials said in a statement.

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