The UK response - Part II - read OP

I know. Nobody says that variants can't evolve and spread when there are low infections and zero vaccinations; just that you optimise the conditions for the development, spread and dominance of vaccine-resistant variants by removing infection control restrictions at a time when half the population is vaccinated and half is not.

The argument, as I understand it, is:

(a) Having a large pool of unvaccinated people maximises the number of people who get infected. Since genetic variation occurs randomly, the more infected people there are the more variants will emerge. Many of these will be non-viable, or will be less well-adapted than the already prevalent variants; they won't survive or propagate. But it's the new variants that are more easily transmitted, or cause more severe disease, or both, that should worry us. And, the more variants that emerge, the greater the likelihood that a variant will emerge that possesses these characteristics, and that is resistant to the current vaccines.

(b) Having a large pool of vaccinated people means that vaccine-resistant variants will enjoy a darwinian advantage - the pool of people who can (a) catch, and (b) transmit vaccine-resistant covid variants will (if the population is 50% vaccinated) be twice as large as the pool of people who can catch nonresistant variants . So resistant variants will tend to spread more quickly and more widely than the nonresistant variants.

So, having 50% vaccinated and 50% unvaccinated is a lousy combination, and not the time at which to start dismantling infection control measures.

If you get to a point where it's, say, 90% vaccinated and 10% not (and if you get to this point before vaccine-resistant variants are widespread) that's a much safer place to be. In this situation only vaccine-resistant variants are ever likely to spread because, if you have a nonresistant variant, 90% of the people you come in contact with will be immune to it. But vaccine-resistant variants are less likely to emerge (or will emerge less frequently) because, with 90% of the population vaccinated and vaccine-resistant variants not yet prevalent, the total number of infected people will be small, and as a result the rate at which new variants emerge will be greatly reduced. This doesn't mean that a vaccine-resistant variant won't arise by mutation, but fewer of them will arise, and at a slower pace, and it will be easier to focus attention and resources to develop treatments and new vaccines that are effective against them.

Basically, on this view, you don't have enduring herd immunity until the great bulk of the population - well over 50% - are vaccinated with vaccines that are substantially effective against all endemic variants of the virus.

The reason you vaccinate anybody against any infectious disease is only partly on account of the risk to themselves; it's also on account of the risk they pose to others, should they become infected. So if children are susceptible to infection and can become infectious, there is a benefit to society, and not just to the children concerned, in vaccinating them.

It doesn't follow that you should vaccinate children. You have to weigh the benefit against actual or possible detriments - is the vaccine safe for children? Is it effective in children? Do children respond differently to the vaccine than adults do? (This is common.) You can't just map the data you have about adults onto children to answer these questions; they have to be investigated in their own right. I don't know the answers to these questions, and therefore I don't have a view as to whether we should vaccinate children or not, beyond saying "certainly not until the answers to these questions are known".

I'd also add that the 90% figure I mentioned in my post is plucked out of the air for illustration. I don't know what level of vaccination is needed to arrive at enduring herd immunity. The answer to that question depends on things like, how effective are the vaccines we have against different variants? How transmissible are the different variants? These aren't simple binaries. Maybe we've got good herd immunity at 65% vaccination; maybe it takes 90%. Which throws up another question that has to be considered when it comes to vaccinating children; do we need to vaccinate children in order to acheive enduring herd immunity? If we don't, it becomes a lot easier to decide not to vaccinate them. If we do, a whole other ball game. Maybe in that scenario the choice becomes "do we vaccinate children, or put up with some level of social distancing/restriction for ever, or accept an endemic disease that will have signficant ongoing mortality and other detriments for ever?"

I don't know. The terminology is beyond me.