Here is the link to the study.
Griffin, George, et al. “Perspective: Vitamin D supplementation prevents rickets and acute respiratory infections when given as daily maintenance but not as intermittent bolus: implications for COVID-19.” Clinical Medicine 21.2 (2021).
The study showed that daily vitamin D supplements reduced risk of rickets, acute respiratory infection, and tuberculosis — but not weekly, monthly, or less frequent doses.
However, the study also found that a large dose, such as a single dose of 150,000 IU of Vitamin D, raised blood vitamin D levels much faster than a daily dose. The implication is that, for the benefits that vitamin D might offer against Covid-19, the best approach would be an initial large dose, to correct a deficiency as soon as possible, followed by daily dosing.
Covid.us.org is now recommending that any adult who has not been taking vitamin D daily at a level of at least 10,000 IU per day, should take 50,000 IU per day for 5 days (total of 250,000 IU) — in order to quickly raise blood vitamin D levels — and then begin taking 10,000 IU per day continuously for benefits against Covid-19.
If you become sick with Covid-19, whether you are taking vitamin D3 at the above stated level or not, consider taking 100,000 IU of Vitamin D3 per day for the first 5 days (total of 500,000 IU), then resume taking 10,000 IU per day.
See the Vitamin D and Covid-19 studies on which this recommendation is based here.