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The P.1 (Brazil) variant includes mutations that cause a small structural change to the S protein. That’s the protein that enables cell entry and which is primarily targeted by the human immune system (i.e. the antigen). You can think of the S protein like a key to the locks (ACE2 receptors) on the membrane protecting your cells. The antibodies your body produces to fight an infection (same ones produced by a vaccination) are like a lock that binds to the keys on SARS-CoV-2 virions. Once that lock is stuck on there, the key no longer works.

The problem is that if that key changes a little, the locks (antibodies) may not be able to bind as effectively. If you throw enough locks at it, one may get lucky and “click” into place, but suddenly you have to throw a lot more at it for that to happen. Otherwise, those keys are still working. Mild infections of the April 2020 variant of SARS-CoV-2 produce a relatively low number of antibodies in most people. Severe infections and vaccinations produce much higher numbers. Most infections (about 80%) with COVID-19 are mild or asymptomatic, so those people all have a low number of those locks (antibodies) in their systems. If they get exposed to the April 2020 variant, that’s still enough to prevent reinfection in 9,999 out of 10,000 times. But with P.1, those odds are much, much worse because of that small change in the key (antigen) structure.

Hopefully that makes sense?

Here’s a short video (under 1 minute) that shows what the normal antibody binding process looks like: https://www.youtube.com/watch?v=lrYlZJiuf18