When there was the story about how the TB vaccine could maybe help in this pandemic, and I was looking into the broader background of such claims, there was actually some interesting stuff on that note. I forgot the detail, but something along the lines of that vaccine seeming to make very little apparent difference in some places, and theories about why that is. One of them involves the idea that if people had already naturally picked up certain bacterial infections, those infections had done a similar 'immune system priming' job to that the vaccine can offer.
Polio is another one. The theory is that increased public health/hygiene/clean environment in some countries meant polio epidemics could occur, often affecting middle class families at the time, in a way that simply didnt used to happen in the past. Because in the old days the mother passed some immunity to their child, and the child was likely to be exposed to polio in the environment early, while this immunity was still offering them some protection. But when the environment was cleaner, the young were catching it much later on, when the temporary immunity they had gotten from their mother had long since passed, and the risk of complications was greater.
www.walesonline.co.uk
