Thread by @sargsyanz

When an adverse health event happens shortly after a covid vaccine, it's hard not to try to make a connection.

"I got the shot Monday and had a heart attack Wednesday... is it really a coincidence?"

Let's do some quick math:

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Let's round the US population to about 300 million.

Around 3 million people per day are getting a vaccine dose.

Now let's look at heart attack rates in the US (we'll abbreviate them MI for myocardial infarction):

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There are 1.5 million MIs in US per year

That's 1/200 ppl per year (1.5 M / 300 M)

That's 1/73,000 ppl per day (1/200 / 365 d)

That means that every single day, out of every 3 million people, ~40 have an MI.

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So if you are vaccinating 3 million people per day, 40 of them will have an MI the next day. Another 40 the day after, another 40 the day after that..

280 within the week! It feels creepy..

But all of those were "statistically supposed to" happen with or without the shot.

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Yet most folks--both patients and doctors--would wonder: things were fine and then two days after the shot, you get a heart attack... was it really coincidence?

And the answer in most cases (like in those 280 heart attacks in the week after each day's shots), is yes.

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This doesn't mean that vaccines can't cause side effects. Obviously sore arms and tiredness and headaches are pretty common.

And many interventions can have more serious side effects too.

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But that's why we do randomized trials, flipping a coin of who gets the real thing and who doesn't, and then comparing rates of all of these things to see if there's really a difference.

And the trials showed that these vaccines are remarkably effective and safe.

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Now: some very rare side effects, like J&J's vaccine-induced thrombotic thrombocytopenia (VITT) that might have a rate of 1/250,000 or 1/1,000,000, may be missed even in a large trial with 20,000 people.

Luckily, this would mean it's rare. But it could still be important.

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Which is why post-trial reporting and tools like @CDCgov 's V-Safe after Vaccination Health Checker texts are key.

So when health events happen, we have data to compare to their "usual" rates, and to see if it's occurring at a higher rate after vaccine.

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But it has to be looked at in aggregate. An individual event or a clinician's observations are going to be extremely limited. Tons of people get sick all the time. And pretty much everyone (hopefully) is getting vaccinated. So there's gonna be a lot of "funny" timing.

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The cerebral venous thrombosis and thrombocytopenia from VITT is not only serious, but also just doesn't happen out of the blue, which is why it was studied closely and found to be a real causation.

Again, thankfully, it's very rare.

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nejm.org/doi/full/10.10…

Anyway, wanted to write this for any non-medical folks who might happen across it... but ALSO for fellow clinicians, as I know that many including myself have a tendency to see patterns as causation, rather than the likely coincidence that they are.

Feedback welcome.

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