We have discussed Vitamin D3 and Covid-19 already in the past, but you should always keep in mind that we talk about our natural defense/immune system. What counts for Covid-19 will most likely be valid (at least in part) for many other virus infections and possible pandemics in the future.
This new cohort study  with 4258 individuals from the University of Chicago needs to be looked at. The researchers found that:
- Low vitamin D3 levels (<20 ng/ml) were associated with a higher risk for a positive Covid-19 PCR test result. Over 80% of patients diagnosed with Covid-19 were vitamin D3 deficient (<20 ng/ml).
- The risk of having a positive PCR-Covid-19 test for Black individuals was 2.6 times higher if these individuals had a vitamin D3 level of 30 to 39.9 ng/ml compared to a level of 40 ng/ml or above. This finding indicates that people of color should try to keep their vitamin D3 level above 40 ng/ml.
- Every increase of only 1 ng vitamin D3/ml above a 30 ng/ml level decreased the risk of showing a positive Covid-19 test by 5%.
- Vitamin D levels above 40 ng/ml showed the best protection. It is estimated that only 5-10% of the US population reaches these values. [2,3]
When placing the 4258 subjects into 4 Groups of different Vitamin D3 levels and taking their daily Vitamin D3 intake into account (Data from Table 1 in the original publication ), I get the following chart.
From Figure 1 I take that a regular average diet without any Vitamin D3 supplementation 54% of subjects show a deficient vitamin D3 level below 30 ng/ml. The other 46% are above 30 ng/ml. 24% are even above the desired 40 mg/ml level.
When subjects were supplemented with 1000 to 2000 IU/day, this did not show an increase in the relative number of subjects with more than 30 ng/ml (53%/47%). According to this data, it takes a daily supplementation of a minimum of 2000 to 4000 IU Vitamin D3 per day to increase the number of subjects with 30 ng/ml and more (61%). Supplementation of more than 4000 IU increased the number of subjects with a vitamin D3 serum level of at least 40 ng/ml (this was the level that showed the best protection) – but still, over 50% were below that desirable level.
Figure 2 shows a clear correlation between the amount of daily supplemented vitamin D3 (IU) and the relative number of subjects in each group with more than 40 ng/ml (in %). However, keep in mind that vitamin D3 is a fat-soluble compound and will accumulate in your body if you strongly overdose.
It is estimated that only 5-10% of the US population reaches 40 ng/ml values and more [2,3]. The present recommendation for vitamin D3 supplementation lies at 600-800 IU/d. Looking at the data of the Chicago study  This amount is not sufficient. The National Academy of Medicine's highest recommended values for vitamin D3 supplementation reaches 4000 IU per day and adult. However, if we look at Figure 1 and take a daily supplementation of 4000 IU into account, we could still expect 50-60% of the population with levels way below 40 ng/ml. The authors of the Chicago Study suggest considering supplementation of up to 10.000 IU even if this should increase the risk of developing hypercalcemia slightly. This would significantly boost the vitamin D3 levels of a large part of the population over 40 ng/ml and reduce the likelihood of severe virus infections like Covid-19, reducing their symptoms, death, and overall costs for the society.
Many high-quality Vitamin D3 supplements provide 5000 IU per daily serving. For me, this amount seems to be an ideal compromise for managing a well-performing immune system and a healthy normal heart- and brain function and, not to forget, solid bones and muscles.
 Meltzer D, et al. Association of Vitamin D levels, Race/Ethnicity, and Clinical Characteristics with COVID-19 Test Results. JAMA Network Open. (2021) 4(3): e214117. Doi:10.1001/jamanetworkopen.2021.4117
 Kaufman HW, Niles JK, Kroll MH, Bi C, Holick MF. SARS-CoV-2 positivity rates associated with circulating25-hydroxyvitamin D levels.PLoS One. 2020;15(9): e0239252. doi: 10.1371/journal.pone.0239252
 Looker AC, Johnson CL, Lacher DA, et al.Vitamin D Status: United States 2001–2006. NCHS Data Brief, No 59. National Center for Health Statistics. 2011.