Literally billions of people around the world have been vaccinated against Coronavirus now. How high can the chance of serious short-term side effects be?
If you look into the Australia, Canada and Norway data you see that the numbers are higher than initially expected. Serious side effects like TTS were thought to happen around 1 in 100k, but they now think it would be something like 1 in 20 000. Perhaps less, if you account for people that don't even realise they're getting it.
The vaccine are something like 5x reduction in mortality, but they are not perfect.
mistakes are normal, to assume none is truly unfathomably dense....
In the case of Coronavirus vaccines, the sample size is 1/3 the population of Earth.
If hundreds of millions of vaccinated people were developing heart problems it would be noticeable.