The Trump administration's effort to impose new requirements on Novavax's COVID-19 vaccine -- the nation's only traditional protein-based option for the coronavirus -- is sowing uncertainty about updates to other vaccines too.

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.

Former government health officials fear the Trump administration is moving to slow-walk vaccine approvals, including by imposing new regulatory hurdles on drugmakers, such as changing the requirements for approval or seeking additional clinical trial data.