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.

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.

Novavax says its closely watched COVID-19 vaccine is on track for full U.S. approval after additional discussions with the Food and Drug Administration.