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COVID Vaccine May Lower your Risk by Only 1%

Let me start by saying I'm generally in favor of vaccines: I got a tetanus shot when I fell off my bike onto the pavement; I got a flu shot last year because I didn't think I could fight off both the flu and COVID if I was unlucky enough to get both. But one reason I haven't gotten a COVID shot is because there's no way of knowing the long-term effects, if any, of the new technology it uses. 

Now I have another reason. For people like me (not old, diabetic, overweight or suffering from heart disease), the clinical trials of the vaccines showed a reduced absolute risk of getting COVID of...wait for it...around 1%. 

Regular readers probably know about absolute risk vs. relative risk. Reducing your relative risk of something by 70% or 95%, as the vaccines do for symptomatic COVID infections, sounds like they are extremely effective. But when your risk of something is low enough to start with, reducing it further may not mean much. This graphic of a hypothetical vaccine trial illustrates relative vs. absolute risk. Absolute risk reduction (ARR) is 1%, while relative risk reduction (RRR) is 50%. One less person out of a hundred got sick with the vaccine--but that's half as many as got sick with the placebo. That's how you can say "50% risk reduction!" with a straight face. 

Hypotethical example of a vaccine clinical trial (1)

The author of the article where this graphic appears notes that vaccine trials are supposed to report absolute risk, since reporting only relative risk can lead people to make suboptimal decisions. But "the manufacturers [Moderna and Pfizer] did not report a corresponding absolute risk reduction..." 

If relative vs. absolute risk is still confusing, the article helpfully states the number needed to vaccinate to avoid a symptomatic case of COVID: 142 people for the Pfizer vaccine; 88 for the Moderna vaccine. 

Sebastian Rushworth likewise reports the Astra Zenica vaccine offers an absolute risk reduction of 1.2% and notes their trial had some problems: two arms of the trial didn't use a real placebo but meningococcal vaccine instead; there were relatively few elderly people in the trial (the people most at risk from COVID); and people with a variety of health conditions were also excluded. So between that, the short length of the trial, and the size of the trial, there's a good deal of uncertainty around the safety of the vaccine. He says he would not take the vaccine (even though he's a junior doctor who presumably sees patients) because he's young and healthy and has a low risk of getting COVID. (2)


"Outcome Reporting Bias in COVID-19 mRNA Vaccine Clinical Trials" by Ronald B. Brown. Europe PMC, February 26, 2021.

COVID: Why Most of What you Know is Wrong by Sebastian Rushworth. Karneval Publishing, Stockholm. 2021, pp. 112-131.


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