According to today's interview with a NY Times reporter on The Daily podcast, the immunity is explained by the close relationship to the common cold. Children tend to get lots of common colds. Around 1/3rd of common colds are a form of coronavirus, because of this, their immune systems are more prepared for this new coronavirus. Over time, this immunity fades, but it takes many years.

https://www.nytimes.com/2020/02/27/podcasts/the-daily/corona...

This is the aspect of the disease that is so interesting to me. With many diseases, like the flu, the most at risk are the elderly and the very young. But barely 1% of infections in China are children younger than 10, and 0 deaths.

Can it be that smoking plays big part in this? As this also explains gender statistics.

An early preprint from the team of China’s leading expert Zhong Nanshan tagged smokers and non-smokers among the 1099 cases they analyzed. 85.4% supposedly never smoked, with 14.5% of those cases severe. So it’s not like non-smokers are very much immune.

https://www.medrxiv.org/content/10.1101/2020.02.06.20020974v...

https://www.medrxiv.org/content/10.1101/2020.02.06.20020974v...

I would say that it really dismisses smoking as a big factor then.

I wouldn't: https://news.ycombinator.com/item?id=22435408

The percentage of severe cases among smokers is definitely higher, although the sample size is small.

Looking at the numbers from Wei-jie Guan et al (https://www.medrxiv.org/content/10.1101/2020.02.06.20020974v...):

nonsmokers = 927 cases, 793 non-severe, 134 severe

exsmokers = 21 cases, 12 non-severe, 9 severe

smokers = 137 cases, 108 non-severe, 29 severe

That gives:

nonsmokers: 14% severe cases of presented

ex-smokers: 43% severe cases of presented

smokers: 21% severe cases of presented

The low number of ex-smokers as well as other factors (did they quit because of advanced smoking-related diseases, etc.?) makes that number hard to interpret.

But smoking carries a normalised average 40% increased risk of severe cases.

    100 * (21-14)/((21 + 14)/2) = 40

Isn't smoking much more common among the Chinese than this sample suggests? I would expect a random sample to include more smokers than not. It may imply that the act of smoking or the associated health states reduce the likelihood of infection, even if they increase the likelihood of a bad infection if it does land.

Or length of time exposed to air pollution, where smoking habit increases the likelihood of mortality?