US Food and Drug Administration scientists said on Wednesday that booster doses of Pfizer's Covid-19 vaccine may not be needed, even though the third shot generates a higher immune response in recipients.

The FDA staff members said in a document prepared for outside advisors that it is still unproven that the efficacy of Comirnaty - the Covid-19 vaccine Pfizer developed with Germany's BioNTech SE - is declining.

"Some observational studies have suggested...