Regardless, experts seem to think it is a cost effective measure even without accounting for the COVID-19 prevention potential. These researchers estimatd the effectiveness at £9.5 per QALY gained, which admittedly sounds too good to be true: https://www.nature.com/articles/s41430-019-0486-x
One important caveat regarding flour fortification with vitamin D3 is that if the flour is used for baking bread, you could be losing 70% or even more of the added vitamin due to thermal degradation: https://onlinelibrary.wiley.com/doi/abs/10.1111/1750-3841.14764
Regardless, experts seem to think it is a cost effective measure even without accounting for the COVID-19 prevention potential. These researchers estimatd the effectiveness at £9.5 per QALY gained, which admittedly sounds too good to be true: https://www.nature.com/articles/s41430-019-0486-x