https://en.wikipedia.org/wiki/Marek%27s_disease

My understanding is that COVID vaccines do limit spread, but as can be seen from forums like Reddit's "COVID19 positive" (https://www.reddit.com/r/COVID19positive/) many people who are fully vaccinated do still contract COVID and health officials have stated that it's possible to still spread COVID in these cases.

This seems to be a growing issue as more recent variants seem to have adapted to circumvent existing vaccines and we're seeing growing numbers of people now become infected despite being fully vaccinated. Of course, the argument is that if you're fully vaccinated then you're likely to have milder symptoms, but as with the vaccine for Marek's disease we can assume this would then enviably reduce selection pressure against highly virulent strains.

I'm really hoping someone can explain to me why this isn't a concern. Admittedly the person who made me aware of this is fairly conspiratorial, but so far I have no good argument against what they're suggesting could happen.