The US Food and Drug Administration (FDA) is planning to introduce a so-called black box warning on COVID-19 vaccines, according to information from two sources with insight into the agency’s work. Such a warning is the most serious type of safety labeling and is used for drugs where there is a risk of death or life-threatening side effects that must be weighed against the benefits.

A black box warning is placed at the top of the drug’s product information and is used for opioids, where risks of addiction and death are warned, as well as for certain vaccines where serious side effects have been identified. For example, the smallpox vaccine ACAM2000 has such a warning due to the risk of brain inflammation and heart problems, as reported by CNN.

The plans are said to be driven by FDA’s chief medical officer and chief scientific officer, Vinay Prasad, who also leads the agency’s Center for Biologics Evaluation and Research. However, the information has not been officially confirmed yet, and the decision is not final. It is unclear if the warning would apply to all COVID vaccines or only mRNA vaccines, as well as which age groups would be affected. Currently, FDA has approved three COVID vaccines in the US, of which two – from Pfizer and Moderna – are based on mRNA technology.

The US Department of Health and Human Services (HHS) emphasizes that no decisions have been made.