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.

Federal regulators are asking Novavax to complete an additional randomized clinical trial on its Covid-19 vaccine after previously delaying approval, people familiar with the matter said, a request that could be so prohibitively expensive the company might not be able to fulfill it.