Monday, October 04, 2021

Why Preprints Are Only Preprints

Credit ACIP/CDC  


For one brief, shining moment last month, the social media anti-vaccine contingent thought they'd found the `Holy Grail' in the form of a preprint published by researchers at the University of Ottawa Heart Institute which (Aha!) proved that mRNA COVID vaccines were responsible for a much higher rate of myocarditis in young adults than previously reported.     

It was a big deal, because instead of making up or distorting the facts - which they have been known to do - they could quote an authoritative source.       

But preprints, no matter how useful they have become in getting important information out quickly during this pandemic - are not peer reviewed - which means they have not (yet) been scrutinized by other scientists, which is when errors and omissions are often discovered and corrected. 

Having been a computer programmer for over two decades, I know how easy it is for a simple math error to find its way into one's code, skewing the final result (GIGO). And no matter how many times you look at your own source code, it can hide stubbornly in plain sight. 

In this case, there was an error (and a sizeable one), which was picked up quickly by readers of the preprint - and called to the attention of the authors - after the paper was published.  While the authors quickly retracted the study, by that time the damage had been done, as the news of the initial findings had spread like wildfire.  

I genuinely feel badly for these researchers, who undoubtedly believed their data to be sound, and trusted that the disclaimer of `Not yet Peer-reviewed' would mean something outside of academic circles.    

But of course, no such rules apply on the internet. 

Because this (withdrawn) paper continues to be touted by some die-hard anti-vaccine advocates online, I've posted the author's notice below.  

WITHDRAWN Comments (34)

Tahir Kafil, Mariana M Lamacie, Sophie Chenier, Heather Taggart, Nina Ghosh, Alexander Dick, Gary Small, Peter Liu, Rob S Beanlands, Lisa Mielniczuk, David Birnie, Andrew M Crean

This article is a preprint and has not been peer-reviewed [what does this mean?]. It reports new medical research that has yet to be evaluated and so should not be used to guide clinical practice.

Preview PDF


During the process of open peer review on MedRxiv we quickly received a number of messages from reviewers concerned that there was a problem with our reported incidence of myocarditis post mRNA vaccination. Our reported incidence appeared vastly inflated by an incorrectly small denominator (ie number of doses administered over the time period of the study). We reviewed the data available at Open Ottawa and found that there had indeed been a major underestimation, with the actual number of administered doses being more than 800,000 (much higher than quoted in the paper).

In order to avoid misleading either colleagues or the general public and press, we the authors unanimously wish to withdraw this paper on the grounds of incorrect incidence data. We thank the many peer reviewers who went out of their way to contact us and point out our error. We apologize to anyone who may have been upset or disturbed by our report.

In summary, the authors have withdrawn this manuscript because of a major error pertaining to the quoted incidence data. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

(Continue . . . )


There is a slight risk of myocarditis following COVID vaccination, particularly in adolescents and young adults, which must be better understood (see CDC Myocarditis and Pericarditis web page), making this a legitimate area of scientific investigation. 

Cases, however, have generally been mild, and have usually resolved quickly, leading most researchers continue to conclude the benefits of the vaccine outweigh its risks. 

While a lot of people love to cherry pick studies that purport to agree with, or support, their personal point of view - the simple truth is that all studies, even peer reviewed ones - have flaws, or limitations.  And when a study announces something new, or unusual, we really need to see their findings replicated by others. 

A reminder that gaining scientific knowledge is a process . . . one that evolves over time and often involves detours, setbacks, and constant reevaluation. Assuming scientific certainty about anything is often the first step towards a humbling. 

For some past examinations of why all studies (even peer-reviewed ones) should be taken with a grain of salt, you may wish to revisit.

When Studies Collide (COVID-19 Edition)

When Flu Vaccine Studies Collide

When Studies Collide (Revisited)

Despite their increased potential for error, and their occasional misuse by the tabloid press and on social media, preprints have provided us with an abundance of good, and timely, information during a fast-moving pandemic. 

Mistakes, and sometimes even intentional fraud (see When Scientists Behave Badly), often make it into peer-reviewed studies.  An watchdog site ( dedicated to tracking, and publicizing, these sorts of problems makes for some eye-opening reading. 

As always, regardless of the source: Caveat Lector.