The Biden administration's plan to provide Covid-19 vaccine boosters is based on concerns that a decrease in the vaccines' ability to protect against milder infections could also mean people will have less protection against severe illness, a premise that has yet to be proven, scientists say.

Experts say the US plan requires thorough vetting by the Food and Drug Administration and a panel of outside advisers to the Centres for Disease Control and Prevention.

US officials, citing data showing waning protection against mild and moderate illness from the Pfizer-BioNTech and Moderna vaccines more than six months after inoculation, said boosters will be made widely available starting on September 20.

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