No vaccine is 100% effective.
The vaccines were sufficiently effective to let Israel skip the alpha wave altogether with ~70% of the population covered. At the lowest point in June 2021 they were down to less than 20 new cases per day. Then the delta variant came out of the bush (from India which had much less coverage at the time, the more infections, the higher the mutation risk). It's R0 was higher than alpha's and the epidemic kicked back up.
Had there been an initial, aggressive push to vaccinate globally we might have been done with the virus by the summer of 2021.
It should also be noted that COVID has cumulative effects. The all around risk of complications is about twice as high for the second infection (I don't think there's data for more than two, so we don't know if the trend is additive, multiplicative, a combination thereof, or if it peaks there). Among other things, it culls the naive lymphocyte population weakening your immunity to bugs you haven't met yet.