The official reasoning for this, is likely that 70% is the threshold for herd immunity, though in the US that was supposed to be natural infection + vaccination. The other aspect is that the threshold isn’t actually established yet - so when strains break through or people keep getting infected, they’ll raise the percentage and say that it’s due to “new science”.

Secondly, while there are some cases of children transmitting the virus, there’s a decent bit of evidence that it doesn’t spread as well in children, especially younger children. Something to do with T cells and the thalamus.

So there’s an argument to be made that vaccinating children isn’t going to have a big impact on reducing spread. The other element is that children are the lowest risk group for infection, and so distribution should focus on inoculating adults as they are more likely to have negative outcomes.

The third factor is that if our demographics are like America, we’ll see voluntary vaccination heavily taper off around the 65% mark. That’s about the percentage that want it. So if you tie full rights to 70%, you have to get a good chunk of the people who don’t want it to take the shot.

This is part of the reason they’re saying “be social, tell people you’ve gotten it, if you know somebody who is hesitant get them to talk to a friend who had it”. The aim is to put social pressure on those who don’t want it to get it, that the vaccine is our “ticket to freedom” (expect to hear this phrase more). Of course, once more of the “hesitant” people get it, their will be greater social pressure among those people to give in and get it too.