Story at a glance

  • More side effects were reported among patients who received the Moderna vaccine rather than the Pfizer shot.
  • The side effects tend to be benign, with fatigue, injection site pain and headaches being among the most common outcomes.
  • Public health experts still advocate that the benefits of being vaccinated greatly outweighing any risks.

Individuals who receive the Moderna vaccine are more likely to experience post-vaccine reactions and side effects, new data from the U.S. Centers for Disease Control and Prevention (CDC) shows.

Published in the online journal JAMA, the study gathered patient data from the CDC’s V-Safe surveillance system that documents adverse reactions to the COVID-19 vaccines. The data collected reflected episodes of injection site pain, fatigue, itching and headaches, among other effects.

Anaphylactic episodes and other allergic reactions were not included.


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Looking at this data gathered from Dec. 14, 2020 to Feb. 28, 2021, Moderna’s vaccine appears to have a higher rate of side effects, with about 74 percent of V-safe respondents reported incidences following a single dose as opposed to the 65.4 percent who reported episodes following a single shot of the Pfizer vaccine.

This pattern strengthened with a second dose, where 82 percent of Moderna recipients reported an adverse reaction. Pfizer recipients also saw a slight uptick in reactions, with 68.6 percent reporting a reaction after a second shot.

The side effects of the vaccines, however, tend to be mild; the vast majority of reactions include pain at the injection site.

More systemic reactions following the vaccine were similarly benign, with fatigue, headache and myalgia being the most common reported outcomes. 

All of the local and systemic adverse effects became more frequent following the second dosage. Symptoms like fever, joint pain and body chills were reported more frequently. 

More severe outcomes, such as nausea, abdominal pain, vomiting and diarrhea remain relatively uncommon among the various adverse reactions.

Notably, CDC researchers say the V-safe responses measured in this study represent less than 10 percent of all vaccinated individuals. The authors recommend clinicians consult with patients and discuss the likelihood of adverse reactions following a second dosage of either vaccine. 

Following the publication of the article, the National Institutes of Health (NIH) announced a parallel study is underway to examine the instances of allergic and anaphylactic reactions following a dose of the Moderna or Pfizer vaccine.

The forthcoming research will specifically focus on mast cell disorders, and if individuals with this condition are more likely to have an allergic reaction to either vaccine.

“The public understandably has been concerned about reports of rare, severe allergic reactions to the Moderna and Pfizer-BioNTech COVID-19 vaccines,” said Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, part of the NIH. “The information gathered during this trial will help doctors advise people who are highly allergic or have a mast cell disorder about the risks and benefits of receiving these two vaccines. However, for most people, the benefits of COVID-19 vaccination far outweigh the risks.”


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Published on Apr 09, 2021