The FDA is drawing up plans to put a 'black box' warning on Covid vaccines, the agency's most serious warning over potential side effects, reports suggest.

The move was revealed to CNN by two people familiar with the matter, who spoke on condition of anonymity because they were not authorized to speak to the media.

A black box warning, which appears at the top of prescribing information, is placed on drugs or vaccines to highlight major risks such as serious side effects or restrictions.

Sources did not specify what the warning would contain, or whether it would apply to only mRNA Covid vaccines or all Covid vaccines.

One individual said the effort was being led by Dr Vinay Prasad, a hematologist-oncologist and the FDA's director for the Center for Biologics Evaluation and Research (CBER), the center that evaluates products such as vaccines for their effectiveness and safety.

Dr Prasad raised concerns over Covid vaccines late last month, saying in an email to staff that 'at least' 10 children had died after receiving the shots, although he did not reveal further details. The FDA has now launched a thorough investigation into the reports.  

Officials stressed that discussions were still in the early stages and that no changes to the warnings had been finalized.

A spokesman for the Department of Health and Human Services said: 'Unless the FDA announces it, any claim about what it will do is pure speculation. The FDA takes very seriously any death that is attributed to a regulated medical product.'

There are three Covid vaccines available in the US, the Moderna and Pfizer vaccines, which use mRNA, and the Novavax vaccine, which uses the virus's spike protein, and none currently carry a black box warning on their label.

In July, the FDA updated the warning label for the Pfizer and Moderna Covid vaccines to warn that the risk of myocarditis and pericarditis, inflammation of the membranes surrounding the heart, was highest among males aged 12 to 24 years.

The agency typically notifies the public that it is looking into a safety question around a drug or vaccine before adding a black box warning to a label. It may also convene an advisory committee, a panel of independent experts, to analyze safety data and advise whether the black box warning is warranted.

The warnings are intended to bring the attention of consumers and physicians to a serious side effect. They may be used if there is evidence of a severe, life-threatening or permanently disabling risk from a medication.

Black box warnings are on the labels for other drugs including opioids, which warn about the risks of abuse, addiction, overdose and death. The smallpox and mpox vaccine, ACAM2000, also has a warning about complications such as heart inflammation and encephalitis.

A former federal health official, who also declined to be named, expressed concern that the new label could lead to distrust in vaccines that have been shown to be safe. 

They told CNN: 'What’s happening now is death by a thousand cuts.

'What we’re watching is misinformation and lies in aggregate that will discourage the population from vaccination. It will lead us to, ultimately, to having loss of life unnecessarily, and it’s very disturbing.'

The potential decision comes after a Stanford University study published yesterday revealed how Covid mRNA shots can cause myocarditis, an extremely rare complication of the vaccine that causes inflammation of the heart muscle.

The study author, Dr Joseph Wu, a cardiologist at the university, denied that his work showed Covid mRNA vaccines should no longer be offered to young men.

He told Daily Mail: 'The incidence of myocarditis after mRNA vaccination remains very low, even among young males. Importantly, Covid infection itself poses a substantially higher risk of myocarditis than vaccination.'

He added: 'Based on current clinical evidence, existing public health recommendations continue to support vaccination, ideally guided by individualized discussions between patients and their healthcare providers.' 

Data shows about one in 125,000 children and adults under 65 who were vaccinated against the virus between 2023 and 2024 developed the condition within seven days of their inoculation. 

Among men and boys under 25 years old, however, the rate rose to one in 37,000, or 3.4 per 125,000.

Among people who are infected with Covid, a 2023 analysis suggests up to one in 950 suffer from myocarditis, equivalent to one in 950.

A CDC presentation from June also showed that most adolescents and young adults who developed myocarditis after vaccination recovered. The condition is not known to have led to any deaths or heart transplants in the US.

About 270 million people in the US, or 81 percent of the population, have received at least one dose of the mRNA Covid vaccine to date.

The US government says it has received 10,000 claims that someone suffered an injury or death after taking a Covid vaccine.

The Commonwealth fund, a foundation supporting better access in the US healthcare system, estimates that the shots saved 3million lives and prevented 18million hospitalizations in the US alone in the first two years of the rollout.

But the vaccines have been subject to increasing scrutiny in the US and other nations after officials continued to recommend them to everyone aged six months and over annually, despite the fact that children and young adults faced a low risk of hospitalization and death from the virus.

Earlier this month Dr Robert Redfield, who headed the CDC during Trump's first term, said he did not support the continued use of mRNA Covid vaccines.

In September, the CDC's vaccine panel, the Advisory Committee on Immunization Practices (ACIP), voted to remove the recommendation that everyone aged six months and older should get the Covid vaccine, and instead told Americans to get the vaccine based on 'individual decision-making'.

Health and Human Services Secretary Robert F Kennedy Junior, who selected the ACIP panel, has previously criticized the Covid vaccine rollout.