The theory was that some large percentage of population is either immune or at least significantly more resistant to covid. Do once the initial 20% get it the other 50-% are resistant so you get your 70% number that way
Barring concrete evidence that is the case, it's a very dangerous assertion to make. It would also mean that the transmissibility of SARS-CoV-2 is much higher than we originally thought, amongst those without "natural immunity".
Even if it were the case that half the population was naturally immune, we would want to understand why. The leading explanation at the moment is T-cells and previous exposure to other coronaviruses. Problem is, there's a good chance that previous exposures would be less likely in certain populations, like children, which could be especially problematic as we're debating sending kids back to school.
At the very least, we need more data on T-cell prevalence/reactance to SARS-CoV-2 before we can jump to the conclusion that people are already immune.
But right now, it's far more likely that we've seen drops because of the drastic measures that have been taken and the changes in daily behavior across the population.
While kids are less likely to get sick from it (less likely doesn't mean zero cases or even deaths btw), they can sure as hell spread it. There was some research suggesting that they're spreading the virus as much as (or not detectably less than) adults [1]. Yet at the same time, there are also some indications that kids might be less likely to become infected [2]. How that will affect school reopenings is anyone's guess.
It's still a stretch to say that the virus is "harmless" to children.
But that's beside the point. The real point about children is that they can spread the virus just as well, even when they don't get sick - especially since it's also really hard for at least smaller children to keep distance or wear masks.
