When looking at studies that show Vit D is not helpful, it pays to understand whether the dose is significant or not.
In this case, 2,000UI is not a significant amount.
Someone in the UK can generate 10,000UI - 20,000UI from spending 5 mins in the midday summer sun.
This amount various depending on whether they are in the countryside, where about 25% of UVB radiation is absorbed by plants or sunbathing on a concrete/sand surface can reflect 100% of the UVB upwards. If sunbathing in a polluted area, the low level ozone can also reduce the UVB radiation reaching the ground depending on the pollution levels.
The Vit D Receptor has a zinc finger which is seems to only be structural, so it would seem that without enough zinc in the diet, even the 2776 VDR's in the human body according to the Wellcome Trust, Cambridge Aug 2010, most of which are concentrated around the immune system genes wont function optimally.
A point worth noting about Vit D3 supplements derived from Lanolin, aka Sheep Fat/Oil, as most sheep are dipped in organophosphates (Nerve Agents), there is a chance that you could be exposing yourself to harmful residual levels of organophosphates. I've yet to see any legislation which monitors this in Vit D supplements in any region of the world (if you know of any please reply so I can update), so megadosing Vit D3 supplements could be increasing the tiny residual amounts of organophosphates to significant and possibly harmful levels. In a study on different Vit D3 specific supplements, the amounts quoted also varied significantly.
Its also worth noting that due to the way patents work, manufacturers of Vit D serum levels measuring equipment use different methods and markers to calculate your serum 25(O)HD levels, 10 years on, the medical equipment is still not standardised as noted here. https://ods.od.nih.gov/Research/vdsp.aspx
In my own limited tests using NHS equipment, the amounts measured varied by 45%.
Like all fat soluble vitamins, D3 (25(O)HD) also has a half life of about 28days so you could do what I have done in the past and purchase some UVB reptile flo tubes and bask under them during the winter months for a few hours a day to monitor serum levels. https://www.reptilecentre.com/blog/2016/09/difference-t8-t5-...
If you want to know how UVB levels vary around the world, this is a good resource http://www.uvguide.co.uk/ reptiles like some other mammals also need UVB radiation for good health.
I could go on, I dont agree with some professors in this field, sometimes science seems to be a scientific Ash conformity experiment which can prevent you from becoming a qualified expert and harm your health but as always with health, there is no panacea and a variety of factors influence our health. For example if your kidneys and/or liver are not healthy you wont be able to make as much Vit D. Diet also plays a part, but if you should find yourself getting into hypercalcaemia territory, increase your Vit K intake, eg eat Brussel Sprouts, and increase your Leucine and Lysine intake as these keto amino acids also stimulate all type collagen synthesis and can reduce the calcium in the blood stream.
As the airways are the main entry point, it would seem logical to increase Vit A intake because this is required by all epithilial cells as is choline for cell membrane integrity, and choline also helps to reduce the pressure in alveoli to facilitate gas exchange which also increases physical performance but could make it easier to breath when on a ventilator and Choline also helps clean the blood through methylation. Epithelial cells secrete things, so those two supplements will enable the lungs and airway secretions to work properly producing enough functional mucous in order to trap pathogens before it even gets into the lungs where the real damage can take place. Like I said it would seem logical to do this, but I'm not an expert, just a self taught IT geek like many of you guys. :-)