Thread by @GidMK

The important thing to remember is that (outside of a trial) REPORTED side-effects are common. Side-effects LINKED TO THE VACCINE are much less so

https://twitter.com/DoctorChristian/status/1334221833952956418

Worth clarifying here that what I mean is that there will undoubtedly be many reports of problems that people have following vaccination - this is NORMAL

It's just the law of large numbers at work. If enough people are given a vaccine, sooner or later one of them will be, say, hit by a car shortly after

The question is whether this is related to the vaccine or not!

Now, we know that common/minor things, like headaches, probably are

We've excluded common/severe things, like heart attacks, in the trials

But what about rare/severe?

Well, we've tested enough people that even rare/severe things are extremely unlikely. But there will almost certainly be very rare side-effects that can't be captured even with 10,000s of people

If something only happens 1 in a million times, it's hard to pick it up in even the biggest trial

What this means is that there will be some issues that we only find out about once the vaccine has been rolled out

This is what Phase 4 (post-license) studies are all about. To sort out whether the things that happen to people after getting vaccinated are related to the vaccine or not