While this is a theoretical article, the theory is very plausible. If you have many vaccinated people in an environment in which the virus is circulating strongly because there are also many unvaccinated people, then soon or later the virus will discover how to get through the vaccine barrier among the vaccinated.
UK scientists believe it is 'almost certain' a coronavirus variant will emerge that beats current vaccines
The Delta variant is as contagious as chickenpox,
The delta variant of the coronavirus appears to cause more severe illness than earlier variants and spreads as easily as chickenpox, according to an internal federal health document that argues officials must “acknowledge the war has changed.” The document is an internal Centers for Disease Control and Prevention slide presentation, shared within the CDC and obtained by The Washington Post.