'Nearly as effective' means either that more (vaccinated) people will catch it than expected and/ormore of them will become more unwell than expected (ie more hospital admissions than expected)
That may be a small %age proportion of the population, but a small proportion can mean a lot of people.
Hopefully the numbers will remain low enough that it'll all remain at the 'acceptable price' levels. I think we have all the systems in place to spot early if there are signs that will not be the case.