healthHealth and Medicine

Study Of COVID-19 mRNA Vaccines Found No Immediate Severe Side Effects Across 6.2 Million People


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockOct 8 2021, 17:37 UTC

Image Credit: Mongkolchon Akesin/

The COVID-19 mRNA vaccines have been a true medical revolution, and have now immunized billions of people globally. Despite many mRNA vaccines being in the works for years before COVID, the urgency of the pandemic led to an influx of public and private support, that led to the successful fast-tracking of such interventions.

But no medical intervention is without risks and despite its benefits, it is necessary to look at what those may be. Since the clinical trials, there have been discussions on side effects, most of them common with vaccination. From localized pain to fatigue and even nausea. But what about more serious and life-threatening side effects? Some important answers to that question come from a new paper published in JAMA. And it is a positive one.


Both the Moderna and Pfizer/BioNTech mRNA COVID-19 vaccines do not appear to lead to an increase in serious health outcomes. The research was carried out by federal and private insurance scientists and they looked at 23 specific serious and often deadly conditions. Among them, there were heart attack, stroke, appendicitis, blood clot formation, Bell’s palsy, and Guillain-Barré syndrome.

Data from 6.2 million individuals were collected across the first six weeks post-vaccination. And the incidence of severe health outcomes was compared between the first three weeks and the latter three weeks. Severe conditions are constantly happening in the population but there was no indication that they were caused by the vaccine – incidence of serious outcomes was not significantly higher in the first 21 days post-vaccination than in days 22 to 42 days post-vaccination. The team also estimated that the confirmed number of anaphylaxis events was around 5 per million doses of vaccine injected.

While the study is encouraging about the side effects, it has severe limitations. The team is upfront that the power of the statistical analysis on the earlier data is not as good or as strong as would be preferable. Further data will solve that. There might be specific health outcomes not evaluated in this study that might be important, or some might have been missed if it took longer to manifest than the six weeks of the study.

It is also important to remember that this is not a study of long-term effects and that the researchers can only estimate effects that have been brought to the attention of health care physicians in the institutions involved, which brings a level of societal bias.  


The research was funded by the Center for Disease Control as part of the Vaccine Safety Datalink (VSD). The initiative, started in 1990, conducts research on important vaccine safety questions in large populations

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