“The essential thing was to save the greatest possible number of persons from dying and being doomed to unending separation. And to do this there was only one resource: to fight the plague. There was nothing admirable about this attitude; it was merely logical.”—Albert Camus, The Plague
Altruism is the rational response to an irrational world.
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