The only thing that's changed with the vaccines is that we have a new, more efficient variant with Delta. Their effectiveness was expected to decline over time. The max length of protection for these vaccines that I recall was about a year. Delta is probably shortening that immune period because of its efficiency.
If we don't drive down cases and spread with vaccines and other mitigation methods, an even more contagious variant will emerge. And now I hear there's a "Delta Plus" variant that is even more contagious. Those who argue that evolution should select for more highly contagious and less virulent strains (quacks like Geert Vanden Bossche, Peter Navarro) fail to realize that virulence has no limiting effect on spread and evolution of greater virulence if the disease progresses and kills its victims slowly. Indeed, the Delta variant is both a more efficient virus that spreads easily and it appears to be more virulent in terms of the percentage of victims requiring hospital care now.
The analysis of more than 40,000 coronavirus infections in England adds to evidence suggesting that Delta may cause more severe illness than other variants do.
www.nytimes.com
Breakthroughs are Delta cases defeating vaccines taken months ago for other strains. New vaccines need to be developed that better neutralize newer dominant strains. Boosters are said to boost antibody levels exceeding those after the second shot of the first round of vaccinations. What has changed, as Turk reported, is the strength of naturally acquired immunity (NI) over vaccine immunity (VI) is emerging in the data now whereas that wasn't clear in the first few months after innoculation. There were reports that VI persons had higher titers of antibodies than NI people and immunity was supposed to be higher. That VI immunity seems to have eroded faster than NI over time.