US officials optimistic Covid booster rollout will start on 20 September

But they insist shots won't be rolled out without health agencies' authorization, leaving open possibility of delays

US officials have expressed optimism that Covid-19 booster shot delivery can start for all adults on 20 September, the goal set by President Joe Biden, as cases continue to rage across the country fueled by the highly transmissible Delta variant.

The officials insist, however, that boosters will not be rolled out without US health agencies' authorization, leaving open the possibility of delays.