No, the current boosters are the same vaccine as the original vaccinations. The booster shots are given as the antibody count in your blood drops over time and the booster shot keeps them at the maximum for another half year or so.

We don't have precise data about the required level of antibodies to stay healthy yet and in quite a few countries the infection rates are currently high enough to pose a real risk for vaccinated patients in the vulnerable group. The safe play is to give booster shots to keep the immune response at its best.

Excuse me, the correct number seems to be 88%[1] effective against symptomatic COVID from the delta variant after 2 doses vs 95% for the alpha variant.

[1] https://www.nejm.org/doi/full/10.1056/NEJMoa2108891

[edit] I'm sure with more data that number might change up or down a little, but it seems in line with other findings.