Actually the real reason the vaccines effectiveness reduces over time is because of the "2 weeks after injection = vaccinated" definition which creates a overestemation of the effectiveness by moving the denominator 2 weeks in front of the numerator. When you then have a surge in infections, then the number goes up, which means you are comparing an infected number 2 weeks ago with a vaccinated population from today. This skews the data in favor of the vaccinated during the uptick of a surge in infections, however likewise the opposite is true once the surge in infections goes down after a few weeks, and then it reverses so that it favors the unvaccinated during the downturn. This is why the danish government pulled the data last year around this time and why they opened up Denmark 1 year ago DURING the surge. Because once the number of infected goes down, then it starts favor the unvaccinated because they are 2 weeks ahead of the vaccinated. And 2 weeks ahead is lower numbers when it goes down week for week.

The UK professor Fenton explained this very well and they did a research paper on it.