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.

Under emergency use, vaccines can be removed even after a health emergency is considered to be over. Full approval from the FDA means a product can stay on the market even after a health emergency ends. (The World Health Organization declared an end to the COVID emergency in May 2023.)