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 said on Thursday.

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, on Wednesday said boosters will be made widely available starting on Sept 20...