150ng/ml (375nmol/L) is the level above which toxicity might be a concern.  Such levels might only be reached after months of supplementation with about 3 times the vitamin D3 needed for people on average to reach 60ng/ml (150nmolL).

100ng/ml (250nmol/L) is regarded as the high end of normal, but some people - such as those with MS - benefit from levels above this. 

Since 2008, leading researchers & MDs have called for all people to aim for 40 to 60ng/ml vitamin D.  July 2020: Immunologic Effects of Vitamin D on Human Health and Disease

70kg (154lb) adults, on average need 0.125mg (5000 IU) vitamin D3 a day to attain a healthy 50ng/ml vitamin D blood level.  This is 1.8 micrograms (72 IU) per kilogram body weight (33 IU/lb)

46ng/ml Average vitamin D levels of traditionaly living Maasai pastoralists and Hadzabe hunter-gatherers - and so probably of our ancestors.

40ng/ml is required to properly supply the autocrine (internal) signaling systems of hundreds of types of cell, including immune cells which kill pathogens and which limit the potentially self-destructive (cytokine storm) inflammatory actions of other immune cells.

#VitaminDStopsCOVID

COVID-19 and influenza seasonality is driven by vitamin D much more than by temperature or humidity.  45 milligrams of vitamin D3 per year for average weight adults raises average blood levels to a healthy 50ng/ml =  1/20,000,000.  Everyone should supplement vitamin D3 to aim for this.

For a given viral insult, the risk of infection would be somewhat reduced.  Symptom severity and the risk of harm and death would be greatly reduced.   Most importantly, those infected would shed far fewer viruses.  SARS-CoV-2 or flu would not spread much, at any time of year, no matter how the viruses mutate.   There would be no need for lockdowns, vaccines, social distancing or masks.

There will be many other health benefits when we end the Vitamin D Deficiency Pandemic.

Individual vitamin D levels vary a lot from these averages.  Stagi et al. sci-hub.se/10.1007/s10067-015-2970-6 report that children suffering from Kawasaki Disease average 9.2ng/ml (23nmol/L) serum vitamin D and that those with coronary artery aneurysms average just 4.9ngml (12nmol/L).   Most doctors treating children with KD and Multisystem Inflammatory Sydrome are not aware of this research.

The worst viral shedding is generally from people with lower than average vitamin D levels and so more severe symptoms due to weak and dysregulated immune responses.  Food and multivitamins contain very little vitamin D.  0.01mg (400IU) per day is a tiny amount - a gram every 274 years.  People most urgently in need of D3 supplementation: Residents and staff of hospitals, care homes and prisons; people suffering from obesity; all people with brown or black skin - especially if they cover almost all their body when outside; healthcare workers, police and emergency responders. 

Everyone more than 30° from the equator needs robust D3 supplementation to maintain healthy vitamin D levels all year round, unless they get lots of summer sunlight and use a UVB lamp in winter.  High elevation, direct sunlight on the full body (swimsuit and no sunblock) with 1/3 the exposure required to redden the skin creates ~10,000IU vitamin D3, with no benefit from more UVB exposure than this per day.  The same 297 nanometre UVB which creates vitamin D3 in the skin also damages DNA.

References for the above, and links to the latest research are at:
 
Robin Whittle   Twitter: @RobinWhittle3     I am an electronic technician, not a doctor.