The Food and Drug Administration is reportedly preparing to add a “black box” warning to Covid-19 vaccines, a move that would label the shots with the agency’s most serious safety alert and cast a shadow over one of the signal achievements of President Trump’s first term. 

The plan, confirmed to CNN by two sources familiar with the matter, marks a significant escalation in the Trump administration’s approach to vaccine regulation. 

The initiative is being overseen by Dr. Vinay Prasad, the FDA’s chief medical and scientific officer and director of the Center for Biologics Evaluation and Research. While the plan has not been finalized and could still change, it is expected to be unveiled by the end of the year, the sources said.

The “black box” warning — named for the prominent black border surrounding the text — is reserved for medications that carry risks of death or life-threatening reactions. It is currently found on products such as opioids, which carry risks of overdose and addiction, and Accutane, which poses risks for birth defects. 

It remains unclear whether the new warning would apply to all Covid-19 vaccines or specifically to mRNA shots produced by Pfizer and Moderna. The warnings also could be targeted at specific age groups.

The mRNA vaccines were rapidly developed during the Covid pandemic in 2020 at the urging of Mr. Trump, who hailed them at the time as a “monumental national achievement” and predicted they would “save millions and millions of lives all over the world.” However the president has had little to say about the doubts being cast on the vaccines by his hand-picked Health and Human Services secretary, Robert F. Kennedy, Jr.

HHS has not officially confirmed the new policy. “Unless the FDA announces it, any claim about what it will do is pure speculation,” said an agency spokesman, Andrew Nixon. He added, “The FDA takes very seriously any death that is attributed to a regulated medical product.”

Both Pfizer and Moderna have defended the safety profiles of their vaccines. In a statement issued in September, Moderna emphasized that its vaccine, Spikevax, is “rigorously monitored by Moderna, the FDA, and regulators in more than 90 countries.” The company noted that after the distribution of more than 1 billion doses, monitoring systems “have not reported any new or undisclosed safety concerns in children or pregnant women.”

The proposed warning comes as Mr. Kennedy continues to challenge established vaccine protocols. The secretary’s nonprofit, Children’s Health Defense, filed a citizen petition on Tuesday demanding the FDA revoke the licenses for Covid-19 vaccines, arguing they are misbranded.

Mr. Kennedy’s influence has extended to other areas of public health as well. Recently, his federal vaccine advisory committee voted to end the recommendation that newborns receive Hepatitis B injections — a standard procedure credited with preventing thousands of infant deaths.