Moderna Takes on Unusual FDA Request: Do Its Vaccines Cause Long Covid?

Bloomberg Prognosis Newsletter - December 15, 2025 - By Gerry Smith.  

Moderna is moving forward with an unusual study of its Covid vaccine at the FDA’s request.

The US Food and Drug Administration has asked Moderna Inc. to conduct a study to determine if its Covid-19 vaccine could cause long Covid, according to people familiar with the matter.

The request is unusual because it would require finding study subjects who never had Covid and never took the vaccine - a group that's increasingly hard to find years after the pandemic. The FDA wants Moderna to compare rates of long Covid symptoms in people who received the vaccine versus those who didn't, said the people, who asked not to be identified because the discussions are private.

Moderna is planning to propose a study design to the FDA in the coming weeks, one of the people said. A spokesperson for Moderna declined to comment. An FDA spokesperson didn't respond to a request for comment.

The request comes as the Trump administration has raised questions about the safety of Covid vaccines, with Health and Human Services Secretary Robert F. Kennedy Jr. vowing to investigate potential links to chronic illnesses. The FDA's request reflects ongoing scrutiny of the vaccines' long-term effects, even as public health officials maintain they are safe and effective.

Long COVID, characterized by symptoms like fatigue, brain fog and shortness of breath that persist for months or years, has affected millions of people. While most cases are linked to Covid infection, some researchers have explored whether vaccines could play a role in rare instances.

Moderna's vaccine, known as Spikevax, has been administered to hundreds of millions of people worldwide. The company has said extensive data show no evidence linking the shot to long COVID.

The proposed study could take years to complete and may face challenges recruiting participants who meet the criteria of being unvaccinated and never infected. Blood tests can detect prior infection, but self-reported data has limitations.

The FDA has previously required vaccine makers to conduct post-marketing studies on safety issues like myocarditis, a rare heart inflammation linked to mRNA vaccines. But a study specifically on long COVID would be novel.

Source: Bloomberg Prognosis Newsletter (December 15, 2025) – Full article (behind paywall) available here.

This development highlights growing official recognition of potential vaccine-related chronic symptoms - something React19 members have experienced and advocated for since the beginning.