Pfizer Inc plans to ask US regulators to authorise a booster dose of its Covid-19 vaccine within the next month, the drugmaker's top scientist said, based on evidence of greater risk of reinfection six months after inoculation and the spread of the highly contagious Delta variant.

The US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) said, however, in a joint statement that Americans who have been fully vaccinated do not need a booster Covid-19 shot at this time.

Some scientists have also questioned the need for booster shots, Reuters reports.

Pfizer's chief scientific officer, Mikael Dolsten, said the recently reported dip in the vaccine's effectiveness in Israel was mostly due to infections in people who had been vaccinated in January or February.