Pfizer Inc chief executive officer Albert Bourla said on Monday he was unsure about the need for a fourth dose of Covid-19 vaccine and that a shot targeting the highly contagious Omicron variant would be ready in March.

The comments contrasted with those made by Moderna Inc CEO Stephen Bancel, who said last week people could need another shot in the fall of 2022 as the efficacy of boosters was likely to decline over the next few months.