The US Food and Drug Administration has discussed with vaccine-maker Novavax the need for an additional trial of its Covid-19 vaccine as a post-approval commitment, a source familiar with the matter told CNN.

The U.S. Food and Drug Administration (FDA) is still negotiating whether to grant full approval to Novavax's COVID-19 vaccine.

Novavax Inc. shares fell after the Wall Street Journal reported that US regulators asked the company to conduct another clinical trial of its Covid vaccine.