The U.S. Food and Drug Administration is preparing to impose its most serious safety warning-a boxed, or "black box," label-on Covid-19 vaccines, according to people familiar with the agency's plans, a move that has alarmed public health experts and reopened political and scientific fault lines over the pandemic-era shots.
The proposal, first reported by CNN, has not been finalized and could still change. It remains unclear whether the warning would apply to all Covid-19 vaccines or only to mRNA shots from Pfizer and Moderna, or whether it would be limited to specific age groups. The warning is expected to be unveiled by the end of the year.
A boxed warning is the FDA's strongest alert, typically reserved for risks such as death or serious injury. Such warnings appear on opioids for addiction and overdose risks and on the smallpox vaccine ACAM2000 for heart inflammation and encephalitis.
The effort is being orchestrated by Dr. Vinay Prasad, the FDA's chief medical and scientific officer and director of the Center for Biologics Evaluation and Research, according to people familiar with the matter.
"Unless the FDA announces it, any claim about what it will do is pure speculation," said Andrew Nixon, a spokesperson for the U.S. Department of Health and Human Services.
The plan comes under President Donald Trump's second-term health leadership, with Health Secretary Robert F. Kennedy Jr., a longtime vaccine skeptic, reshaping federal vaccine policy. Kennedy has limited Covid-19 vaccine access to adults 65 and older and those with underlying conditions, despite extensive clinical trial and real-world data supporting broader use.
In a November memo to FDA staff, Prasad claimed that regulators had "found that at least 10 children have died after and because of receiving Covid-19 vaccination," though no supporting data have been publicly released.
"The FDA takes very seriously any death that is attributed to a regulated medical product," Nixon said.
Public health experts say the evidence does not support a black-box warning. A CDC presentation in June found that most myocarditis cases following vaccination resolved, with "no known deaths or heart transplants." Rates declined after the recommended interval between doses was extended.
The FDA already expanded myocarditis warnings in May, stating that "the observed risk has been highest in males 12 years through 24 years of age."
Federal data continue to show benefits. One study estimated Covid-19 vaccines prevented nearly 20 million deaths globally in their first year. A CDC report published Thursday found vaccinated children during the 2024-25 season had a "substantially lower risk" of Covid-related emergency and urgent-care visits.
Moderna said its vaccine's safety is "rigorously monitored by Moderna, the US FDA, and regulators in more than 90 countries," adding that surveillance systems "have not reported any new or undisclosed safety concerns in children or pregnant women." Pfizer issued a similar statement supporting the safety and efficacy of its vaccine.
Critics argue the FDA's approach lacks transparency. "I guess my concern is that in this case, there's not a process," said Dr. Aaron Kesselheim of Harvard University. "There isn't that same opportunity for discussion and good-faith review of the data."
A dozen former FDA commissioners echoed that concern in an open letter published in The New England Journal of Medicine, warning of "sweeping new FDA assertions about vaccine safety."
