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Home » The largest safety study of COVID vaccines finds rare issues

The largest safety study of COVID vaccines finds rare issues

A new safety study that included millions of subjects has shown that the COVID-19 vaccines were linked to small increases in neurological, blood, and heart-related conditions. Despite these findings, I must restate the importance of COVID-19 vaccines since they protect against severe illness, death, and lingering long COVID symptoms from a coronavirus infection.

Because this study may be misinterpreted, I want to do what I usually do — review and critique the study. And I want to give context to it because the benefits versus risks of the vaccine cannot be understated.

Safety of COVID-19 vaccines paper

In a paper published on 12 February 2024 in the journal Vaccine, Kristýna Faksová, Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark, and colleagues used the Global Vaccine Data Network (GVDN) to search for 13 medical conditions that the group considered “adverse events of special interest” among over 99 million vaccinated individuals in eight countries, aiming to identify higher-than-expected cases after a COVID-19 vaccination. The size of the study and the use of aggregated data increased the possibility of the researchers identifying rare safety signals that might have been missed when looking only at smaller populations.

In total, 183,559,462 doses of BNT162b2 (Cominarty from Pfizer-BioNTech), 36,178,442 doses of mRNA-1273 (Spikevax from Moderna), and 23,093,399 doses of ChAdOx1 (Vaxevria from AstraZeneca) were administered across participating sites in the study period. The mRNA vaccines from Pfizer-BioNTech and Moderna are usually combined as “mRNA vaccines” in this study. The AstraZeneca vaccine will be referred to as the “viral vector vaccine.”

The researchers searched the database for the following 13 medical conditions that were considered adverse events of special interest for the COVID-19 vaccines:

medical stethoscope and mask composed with red foiled chocolate hearts
Photo by Karolina Grabowska on

Key results of the study

Here are the key safety signals that the researchers found:

  • Myocarditis was consistently identified following a first, second, and third dose of mRNA vaccines. The highest rate was after the second Moderna vaccine dose, 6.1X the expected number.
  • Researchers found that there was a 6.9X increased risk of pericarditis after the third dose of the viral vector vaccine.
  • They also found a 1.7X and 2.6X increased risk of pericarditis after the first and fourth doses of Moderna’s vaccine.
  • Researchers found a 2.5X increase in the risk of developing Guillain-Barre syndrome within 42 days with the viral vector vaccine.
  • The viral vector vaccine was linked to a 3X increase in cerebral venous sinus thrombosis, identified in 69 events (out of over 23 million doses), compared with an expected 21.
  • The researchers found a 3.8X greater risk of developing acute disseminated encephalomyelitis after the Moderna vaccine was administered, and a 2.2X increased risk after the viral vector vaccine.
  • There were possible safety signals for transverse myelitis after viral vector vaccines and acute disseminated encephalomyelitis after viral vector and mRNA vaccines. There were seven cases of acute disseminated encephalomyelitis after vaccination with the Pfizer-BioNTech vaccine (out of over 183 million doses), versus an expectation of two. 


I think several points need to be made about this study:

  1. The vast bulk of the adverse events of special interest did not show any statistically significant increased risk after receiving the vaccines.
  2. The observed adverse events are very rare. For example, acute disseminated encephalomyelitis was observed in 7 cases out of 183 million doses, which works out to 0.038 cases per 1 million doses.
  3. The risk of myocarditis from the COVID-19 vaccines is much less than the risk of it from COVID-19 itself.
  4. Finally, the size of this study is so large, that it allows the researchers to identify rare adverse events, and they could only find a handful. Furthermore, this information is taken from actual medical records, not random claims on websites.

But the most important that needs to be made is that this research shows that the COVID-19 vaccines are overwhelmingly safe. Moreover, the benefits of the vaccines, in preventing COVID-19 which itself can lead to many of these conditions, far exceed these risks.

No medical procedure is perfectly safe. For example, if you break one of the bones in your arm, you will go to a hospital to reduce the fracture because you don’t want an arm that doesn’t work. However, there are numerous and some deadly risks from reducing a fracture. Despite these risks, I bet 99.99% of people would choose to get their arm to work correctly by fixing the broken bones.

All of these adverse events that the researchers studied sound dangerous and upsetting. Again, they are extremely rare. Some are so rare that they can happen in the absence of vaccines and COVID-19, they just happen.

And there’s one last thing. When people claim that vaccine safety and effectiveness aren’t studied carefully, they couldn’t be further from the truth. This was a massive and impressive study that shows that the safety profile of COVID-19 vaccines is impressive.


Michael Simpson

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