This is incorrect. A vaccine is not an antibiotic; the presence of one does not cause virulent strains. Vaccinations trigger the body's immune response to the virus; nothing else. They provide for a more effective response faster, but do not otherwise provide any forcing function.

Mutations and variants occur all the time as the virus reproduces. Variants are not a response to any vaccine; they have appeared primarily (entirely? I haven't been following all of them) in regions where the vaccine is not yet widely available because that is where the virus is able to multiply (and thus mutate) the fastest.

The general theory is that a virus avoids evolving in a way that is too dangerous to its host because the sooner the host dies the less potential the virus has to spread. As the vaccinated have a weaker response to the virus there is no longer as strong evolutionary pressure against the virus becoming more lethal.

A real life/modern example of this can be found in Marek's disease in chickens. https://en.wikipedia.org/wiki/Marek%27s_disease#Prevention

The counter claim seems to be that because vaccination reduces the abundance of the virus there is a lower chance of mutations. It's hard to say which effect is stronger and it probably varies greatly by virus and by vaccine!

Disclaimer: I'm not an expert, just an armchair enthusiast.

I think it's too soon to say yet if the vaccines encourage mutation. But my understanding is quite a few of the significant variants come from places where there was significant vaccine testing: - Brazil (Gamma) - South Africa (Beta) - India (Delta) - Alpha (UK)

The causation may be backwards here. You need a decent enough medical system both to run clinical trials, and to detect variants.

Interesting theory though, I don't think it's true right now, but potentially there may be evidence I'm not aware of.

What is a leaky vaccine?

I’m not qualified to really discuss, but here’s a study I recently heard brought up. It would be interesting to hear an unbiased opinion of someone with a medical background

“Vaccines that let the hosts survive but do not prevent the spread of the pathogen relax this selection, allowing the evolution of hotter pathogens to occur. This type of vaccine is often called a leaky vaccine.”

https://journals.plos.org/plosbiology/article?id=10.1371/jou...