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.

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.

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