What makes a mild coronavirus infection?

We had this question in from listener Paul for virologist Chris Smith...

Chris S - It's certainly true, that if we look at the data, and we're looking at data at the moment from, largely China, because obviously they're ahead of the world in the numbers of cases they've had and their collection of the data. This will probably become clearer as other countries, certainly Italy for example, which is having a lot of coronavirus cases, and the UK too, we're going to join the party and contribute our own data.

At the moment, there are very few children there who have had this infection, or have had severe outcomes from this infection, or who have died from this infection. The vast majority of the cases have been in older people. The vast majority of the fatal cases have been in very old people, and it looks like men are disproportionately likely to die compared with women. Although it's an equal opportunities virus, it seems to infect both men and women equally. Now, why is it then that the children appear to be not getting it?

Well, actually what I think is happening, and we have to speculate because we don't know for sure yet, this will become clearer, but what I think is probably going on is that the children are not getting overt symptoms, so they're not getting tested, so they're not registering in the data as having had this infection, but they have had it. They've had it in a very mild way. And a bit similar is chicken pox. If you think about it, when we catch chickenpox as kids, and in countries like the UK, 90% of us have had chicken pox. Most of us catch it within the first few years of our lives, and we'll get two or three spots, maybe a few more than that, but we get a very mild infection. If you're unlucky enough to catch chickenpox when you're older, you tend to get a much more severe infection, and there are lots of viruses which do produce a more serious manifestation the older you get. So it may well be that in this case it's doing the same sort of thing, and that children are catching it, they're getting very trivial symptoms and we're not regarding this as, Oh, they've got this coronavirus. They've not been tested, so they're escaping from scrutiny, and it's giving us a false impression that they're either not being effected or they're immune. As far as we know, no one is immune to this virus yet because it's new. So no one's seen it before. No one has immunity, and that's why it has the potential to spread across the entire world population.