tootyfruitypickle
The vaccines work by telling our body to recognise specific parts of the virus. If the mutations come about in the right places, it is like the virus is in disguise!
The mutations you refer to come from two closely related but slight different mechanisms.
1)
When a virus infects someone it has to make copies of itself to spread through their body. As it does this, it makes errors in copying it’s code. It does this all the time and usually it doesn’t come to anything, but sometimes, randomly, lead to advantageous mutations which mean that “new version” will then propagate more.
When a virus “rips through” a population it is like putting the copying process on fast forward and you are more likely to see mutations.
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With regard to the vaccine (and indeed natural immunity from getting the virus) it is called evolutionary pressure. This means that the mutations which are advantageous for the virus will be the ones which prevent the vaccine from working so well and allow the virus to ”escape”.
You would see this in people from natural immunity from catching the virus but what we are hoping to achieve with vaccination, is generating immunity in a lot more people a lot more quickly. So it is like putting that evolutionary pressure on fast forward.