People vaccinated against Covid-19 in high risk parts of the United States should resume wearing masks indoors, the top health authority said, a major reversal in guidance that underscored the country's struggle to suppress the Delta variant.

US President Joe Biden said the announcement showed that America needs to “do better” on vaccinations, adding that a vaccine mandate for the country's more than two million federal workers was now “under consideration”.

Centers for Disease Control and Prevention director Rochelle Walensky cited new data that shows rare breakthrough cases involving Delta have an increased risk of onward transmission.

“In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings,” she said.