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.

At FDA, full vaccine approval is the gold standard. The agency was on track to sign off on Novavax’s license by its April 1 target date, according to two people with direct knowledge of the situation who spoke on condition of anonymity to discuss confidential agency matters.