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Correlations and Clots

Apr 21, 2021

Shop Talk

April 21, 2021

The word on the street is that US health officials will end the nationwide pause on the Johnson & Johnson coronavirus vaccine on Friday. While we wait, I want to share a secret.

·         Remember when you took statistics?

·         Remember learning about correlation?

·         Remember how adamant your teacher was that correlation is not causation

I do. And now that I teach correlation, I use a bunch of absurd examples to drive home the point. Here in Wisconsin, there is a correlation between snowblower sales and hot cocoa consumption. Buying a snowblower does not make us drink more cocoa. The two things are associated because they both make winter a bit more pleasant. But here’s the secret. Scientists often look to correlation for clues about causality. And that’s part of what’s going on with the Johnson & Johnson vaccine.

Correlation #1 

Six cases of rare blood clots have been reported among recipients of the Johnson & Johnson vaccine.

Correlation #2

As of March, European health officials identified 86 blood clots among 25 million recipients of the AstraZeneca coronavirus vaccine. 

Correlation #3

The clots observed in the Johnson & Johnson recipients are the same type as those seen in the AstraZeneca recipients.

Correlation #4

Johnson & Johnson and AstraZeneca are both viral vector vaccines.

These correlations do NOT mean that the vaccine causes blood clots. It could be a coincidence just like it could be a coincidence that Col. Mustard was in the kitchen with the candlestick when Miss Scarlett fell and hit her head. The police would interview Col. Mustard, and scientists want to take a closer look at the blood clot data to see if there is a causal link to vaccination. 

The good news is that the scientific process is working. Since the start of the pandemic, several vaccines have been developed and tested in clinical trials. The vaccines were up to 95% effective, and no serious side effects were reported. The FDA issued emergency use authorizations, and pharmacies all over the country began administering vaccine “in the wild.” Adverse events (causal or not) are being reported and investigated. After only 6 blood clots, US health officials paused the use of the Johnson & Johnson vaccine to understand its relation to the clots. New guidance is expected by Friday.

If everything is working, why do we feel uneasy? I think it’s uncertainty that’s making us all uncomfortable. While it’s possible that the 6 blood clots reported to date are a coincidence, it’s also possible that they are a rare side effect caused by the Johnson & Johnson vaccine. A preliminary report in the New England Journal of Medicine suggests that the AstraZeneca vaccine can, in rare instances, result in a blood clot. Conclusions were drawn from a case series and mechanism-based reasoning. If we really wanted to get to the bottom of this, we could keep administering the Johnson & Johnson and AstraZeneca vaccines and compare the number of clots reported in those recipients to the number of clots in individuals who got the Pfizer or Moderna. That’s a good old fashioned retrospective cohort study.

But will that happen? Should it happen? I’m not sure.

Health regulators may advise against the Johnson & Johnson vaccine for younger women before a large enough sample is obtained for a conclusive cohort study. Johnson & Johnson halted the rollout of its vaccine in Europe. These may be prudent decisions given that other vaccines are available. But you can say bye-bye to the data set you need to figure this out. What about the developing world and hard to reach regions of the US? That’s where the single-dose, regular frozen (not super frozen) vaccines could be a big help. Maybe someday we’ll have a data set from that population. But if the vaccine isn’t good enough for our aunts, sisters, and mothers, should we administer it to women in developing nations? I cannot answer that question, nor do I envy those who must. I think it depends, in part, on the relative risk of COVID-19 vs. blood clots. Regardless, I think we need to get used to that uneasy feeling we get when we are forced to make decisions amid uncertainty because this is one mystery that we may not solve for a while.

As always you can learn more from the materials below. If you are so inclined, share this piece with a friend and encourage them to subscribe. All they need to do is e-mail us at [email protected] and say, “Add me to the list.” You can also find all our posts and support Shop Talk at

Thanks for reading, and until next time, stay well.

Learn more: 

1.       My source for the word on the street:

2.       The CDC and FDA joint statement on vaccination and blood clots:

3.       The European report on vaccines and blood clots:

4.       New England Journal of Medicine article on AstraZeneca vaccine

5.       More information about retrospective cohort studies:

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