Investing.com -- Novavax (NASDAQ: NVAX ) shares declined 6% following a report from the Wall Street Journal that said federal regulators are requiring an additional randomized clinical trial for its Covid-19 vaccine. The Food and Drug Administration (FDA) has requested further evidence of the vaccine’s effectiveness, which could impose a significant financial burden on the Maryland-based biotech firm.

The FDA’s intervention, led by appointees under Health Secretary Robert F. Kennedy Jr., marks a divergence from the approval process for other Covid-19 vaccines. Novavax’s vaccine has already demonstrated a 90% efficacy rate in a 30,000-person trial and is currently available in the U.S. under emergency use authorization. It has also received full approval in Australia, Europe, and Japan.

The cost of a new randomized trial to evaluate the vaccine’s ability to prevent disease could run into tens of millions of dollars, according to vaccine experts. However, the FDA has left room for negotiation, suggesting the possibility of a smaller, less costly study.

Novavax has not publicly specified that the FDA requested a randomized trial, leaving investors to weigh the potential impact of the additional trial on the company’s financials and timeline for full vaccine approval.