It isn't necessarily "genetic susceptibility". It might be "nutritional susceptibility" or "epigenetic susceptibility", which can be modified by achieving high levels of Vitamin D.
There is an article on NIH by a Canadian physician (Dr. Gerry Schwalfenberg) who works in nursing homes. He has developed a protocol for reducing flu like illness among his patients. This should be tried against the corona virus in US hospitals:
"A colleague of mine and I have introduced vitamin D at doses that have achieved greater than 100 nmol/L in most of our patients for the past number of years, and we now see very few patients in our clinics with the flu or influenzalike illness. In those patients who do have influenza, we have treated them with the vitamin D hammer, as coined by my colleague. This is a 1-time 50 000 IU dose of vitamin D3 or 10 000 IU 3 times daily for 2 to 3 days. The results are dramatic, with complete resolution of symptoms in 48 to 72 hours. One-time doses of vitamin D at this level have been used safely and have never been shown to be toxic. We urgently need a study of this intervention. The cost of vitamin D is about a penny for 1000 IU, so this treatment costs less than a dollar."
Vitamin D for influenza
There is a presentation by Dr. Robert P Heaney on the healthful effects of adequate vitamin D levels:
Vitamin D Sunshine Optimal Health: Putting it all Together
There is an article by Beata M. Gruber-Bzura on NIH that describes the mechanisms by which vitamin D works with the immune system:
"There are a few arguments which support vitamin D as a likely candidate for the abovementioned “seasonal stimulus”. In summary, most important is the 1α,25(OH)2D-stimulated production of AMPs, such as defensin and cathelicidin. As mentioned, these endogenous antibiotics act directly, destroying not only microbial pathogens, but also viruses, including the influenza virus [24,48,49]. The production of cathelicidin is dose-dependent on the serum level of 1α,25(OH)2D."
Vitamin D and Influenza—Prevention or Therapy?
The Chinese virus should definitely be treated with something that works against the ordinary flu. Americans tend to be overstuffed with vitamins (among other things) and this map shows we have a higher blood level of vitamin D than a lot of countries. Among the lowest were:
Italy
Spain
China
Iran
USSR
Among the highest:
Vietnam
Thailand
(and maybe Taiwan- the map isn't clear)
INTERACTIVE MAPS ON VITAMIN D LEVEL WORLDWIDE
Note that the map was constructed as part of an investigation into osteoporosis, but there is some correlation with the current distribution of Chinese virus infections. The map suggests that Russia is headed for trouble. It also suggests an answer as to why the death rate in the US is relatively low (and how it could be lowered).
Dr. Healey came up with an optimal daily dose of Vitamin D3 of about 5,000IU. A bottle of 120 can be had for under $8.00.
Interesting. Much has been written in the lay press about Vitamin D and “immunity”. Many Americans use large doses of Vitamin D in a way analogous to the manner in which Vitamin C has been used.
Yet if the empirical observations you have listed reflect a benefit in humans regarding COVID-19 susceptibility or response to infection, prospective double blind studies must be performed. Given the massive demands on hospitals and laboratories, Vitamin D levels are not being routinely monitored on COVID-19 patients.
That is not to say that the use of Vitamin D supplements would necessarily be harmful. (although there have been warnings about pancreatic tumors and other maladies in people using large amounts). It may be useful as the regimen Hydroxychloroqune/Zithromax may be useful. Retrospective and prospective studies would be nice but people are desperate for potential remedies and hope.