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Home » Bivalent COVID vaccines are not linked to stroke — new research

Bivalent COVID vaccines are not linked to stroke — new research


Newly published research provides strong evidence that the bivalent mRNA COVID vaccines do not increase the risk of stroke, debunking a claim that is widely made about these vaccines. This provides more evidence that the bivalent mRNA COVID vaccines are demonstrably safe.

To be fair, there were some safety signals that the bivalent vaccines were linked to stroke, but it had not been confirmed in large epidemiological or clinical studies. This new research is a large study that dismisses these signals.

As I usually do, I will discuss this new research and critique it, if necessary.

What are strokes?

According to the National Institutes of Health (NIH), a stroke occurs when reduced blood flow within the brain causes many brain cells to die. There are two main types of stroke:

  • Ischemic stroke:  It is usually caused by a blood clot that blocks or plugs a blood vessel in the brain. This keeps blood from flowing to the brain. Within minutes, brain cells begin to die. Another cause is a stenosis or narrowing of the artery. This can happen because of atherosclerosis, a disease in which plaque builds up inside your arteries.
  • Hemorrhagic stroke: This is the less common type of stroke. It happens when a blood vessel breaks and bleeds into the brain. Within minutes, brain cells begin to die. Causes include a bleeding aneurysm, an arteriovenous malformation (AVM), or an artery wall that breaks open.

Both types of stroke need to be treated quickly or they can lead to permanent damage to the brain or death.

As I mentioned above, some research provided contradictory evidence that COVID vaccines may or may not have been linked to an increased risk of stroke. This new research appears to have cleared up the contradictions.

an elderly woman having a headache covid vaccines stroke
Photo by Kindel Media on Pexels.com

COVID vaccines and stroke paper

In a paper published on 19 March 2024 in the respected journal JAMA, Yun Lu, Ph.D., a statistician of the FDA in Silver Spring, MD, and colleagues examined the medical records of nearly 5.4 million Medicare beneficiaries who received either brand (Pfizer-BioNTech or Moderna) of the COVID-19 bivalent vaccine.

Here are the key results:

  • The researchers found no evidence of a significantly elevated risk for stroke at 1-21 days or 22-42 days after receiving either mRNA COVID vaccine distributed during the 2022-23 respiratory seasons. This was compared to a control 43-90 day window.
  • There was a significant risk of non-hemorrhagic stroke for people who received the Pfizer-BioNTech vaccine plus a high-dose or adjuvanted influenza vaccine during the 22-42-day vaccination window.
  • There was also a significant increase in the risk of transient ischemic attack, often called a mini-stroke, for those who received the Moderna vaccine plus a high-dose or adjuvanted influenza vaccine during the 1-21-day risk window.
  • Finally, the researchers found that people who received the high dose or adjuvanted influenza without a COVID vaccine were at elevated risk of non-hemorrhagic stroke or transient ischemic attack in both the 1-21-day and 22-42-day window.

The researchers concluded:

Among Medicare beneficiaries aged 65 years or older who experienced stroke after receiving either brand of the COVID-19 bivalent vaccine, there was no evidence of a significantly elevated risk for stroke during the days immediately after vaccination.

Summary

This research has some complicated results that may not give satisfactory answers to most people. The research found no increased risk of stroke after receiving either brand of the COVID bivalent mRNA vaccines. That is the key point.

However, the research seems to show that receiving the flu vaccine itself or with the COVID vaccines increases the risk of certain types of stroke. On the other hand, there is strong evidence that flu vaccines are not linked to stroke.

I am not sure I know what to make of the results of this research. It does provide strong evidence that COVID vaccines are safe concerning strokes. However, it seems to provide some concerns about the flu vaccine, although most research has not shown any link previously. In addition, there could be some demographic factors or confounding data that would might make people who get the flu vaccine more susceptible to stroke.

If I were to redo this research I would have four cohorts to compare to one another — COVID vaccine only, flu vaccine only, COVID+flu vaccine, and unvaccinated with either (though that would be a very small portion of Medicare patients). I believe that would give us some strong information that would clear up some of the confusion.

At this point, there just isn’t much evidence that the COVID vaccine is linked to risks of stroke.

Citations

Michael Simpson

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