The United States purchased 200 million more doses of the Pfizer/BioNTech vaccine for flexibility if booster shots are later needed and the shots are approved for younger children, Reuters quotes the White House statement as saying.

The vaccine makers announced the government's purchase earlier on Friday.

The White House is setting its sights on immunising children under the age of 12, who are not yet eligible, and potentially deploying booster doses if new data shows they are necessary, according to officials.