A lot of people have asked me today how much of a big deal this is, and the honest answer is ‘we don’t know yet’—the trial was stopped in order to figure out the answer to this question.

But let’s talk a bit about causality. https://twitter.com/helenbranswell/status/1303443741764661253

There are many ways to define what we mean when we ask the question “did the vaccine cause this adverse event?” because there are many ways to define the idea of a “cause”.

One possible way to make this question more clear is to ask “if everything else had been the same, but this person had not been vaccinated, would they still have experienced this event at that time & in this way?”

If the answer to this question is “no it would not” many will feel comfortable saying the vaccine was a cause of the event, whereas if the answer is “yes it would have happened anyway” many will feel comfortable saying it wasn’t.

But that doesn’t necessarily mean we’re correct.

Establishing that something is the cause of a one-time event is really hard! But, there are ways we can *rule out* the vaccine as a cause if it wasn’t one.

For example, if this individual was not in the experimental vaccine arm of the trial, then the experimental vaccine very likely did not cause their event.

Similarly, if the event is something that takes a long time to occur (for example, cancer) and the person only just got vaccinated, the vaccine very likely did not cause the event.

We can also use our knowledge and expertise of how biological systems work to determine when the vaccine was not a cause.

For example, if the event is death, but it occurred when the participant’s car was hit by a drunk driver, we might be confident ruling out the vaccine.

What’s the take home message? The same one it always is from #epitwitter!

Is this bad news for the vaccine? It depends on what the event is & whether there is any way to rule out the vaccine as a cause.

Be patient. This is why we do Phase 3 trials. This is the system working.