Netherlands has 40% completely unvaccinated to feed a Covid wave.
Partly vaccinated at least won't suffer worst effects and spread less mostly.
No surprise, no vaccination failure, by what can be expected.

Older people were prioritized for vaccination and essential workers making up most of the numbers.
Unvaccinated proportion is much higher in young, mobile, clubbing, social group.
In the places where I have seen reports of of cases, its pretty much these unvaccinated driving things, younger than previous waves.

Understandably they are the most impatient, at the time of their lives when people are out socializing, travelling, finding partners etc.
So once the older were protected, and restrictions relaxed a bit, they moved rapidly to 'its all over' mode.

Sydney reports seem to be also saying they are seeing younger people, and reporting a lack of co-operation typical of young people.
They have also been consuming messages that play down the risks for younger people for a year or more.

Answer is the same.
We need to get a large proportion of 'all' groups fully vaccinated, plus a few weeks for the second shot to be effective.
Ok protect older groups first as they are vulnerable to the worst effects and filling hospitals, essential workers and such.

Job is not done until mid age, and younger groups also have 'high proportion' of full vaccination.