Actual recommended daily d3 intake for adults is 8-10k IU, maintenance level. Doing studies on 10% of that dose is meaningless

The short version: 8,000–10,000 IU daily is well above what most trials test, and the few that tested higher doses found no benefit for respiratory infections and some bone-density harm.

The evidence you have is mostly about 400–2,000 IU daily, not 8,000–10,000 IU. A 2009 trial of 2,000 IU daily in adults with decent baseline levels (mean 64 nmol/L) found zero reduction in colds or flu [24]. A Finnish trial of 400 IU daily in soldiers found nothing [17]. The Japanese trial that found a 42% reduction in influenza A used 1,200 IU daily [17]. None of these tested the 8,000–10,000 IU range you are asking about.

The one trial that did test a high dose, 10,000 IU daily for three years, found something worrying: significantly lower bone mineral density in the radius and tibia [15]. That is the opposite of what vitamin D is supposed to do. The same commentary that reported this argued that recommending 10,000 IU daily to reduce COVID-19 risk was inappropriate given insufficient evidence and potential harm [15].

The dose-response data in the evidence is modest. For each 100 IU increase in intake, serum 25(OH)D rises by about 0.68 ng/mL [1]. That means 8,000 IU would raise serum levels by roughly 54 ng/mL from baseline. If your baseline is 20 ng/mL, that puts you at 74 ng/mL, well above the 38 ng/mL threshold where one cohort study found lower respiratory infection risk [16]. But that cohort study was observational, not a trial, and the trials that tried to replicate the benefit at lower doses mostly failed [24].

The mechanistic picture is also not simple. A mouse study found that calcitriol (active vitamin D) reduced early inflammation from H9N2 influenza but worsened inflammation later in the infection [19]. That is not a clean protective signal.

The claim that 8,000–10,000 IU is the "actual recommended daily intake" is not supported by the evidence here. The evidence shows that 2,000 IU did nothing for respiratory infections, and 10,000 IU caused bone loss. The studies that found benefit used 1,200 IU daily in deficient populations. The dose-response curve for respiratory protection may not be linear, and higher doses may be worse, not better.

My call: 8,000–10,000 IU daily is not supported by the evidence here for respiratory infection prevention, and the one trial at that dose showed harm. Confidence: moderate. The evidence is thin at high doses, and what exists is negative.