CFR is just confirmed deaths divided by confirmed cases. It's absolutely certain, but that's also why it's not a great basis for a serious analysis of most things. We can comfortably say the numerator is roughly correct, but the denominator is likely too low because we probably missed a lot of cases, given limited testing capacity and just how asymptomatic COVID often is.
IFR is actual deaths divided by actual infections. It's the most important number, but also much harder to figure out, because the delta between confirmed cases and actual infections is, by definition, unknown.
So, in context, if we look at CFRs, it looks like Israelis are 3x less likely to die from COVID than Americans. Maybe that's vitamin D, or maybe instead Israel tested and identified every single person with COVID, whereas America missed two for very case they identified. If we could instead see the IFRs in that case, they'd actually be roughly the same, and it'd turn out (all other things being equal) that vitamin D was irrelevant.
IFR is actual deaths divided by actual infections. It's the most important number, but also much harder to figure out, because the delta between confirmed cases and actual infections is, by definition, unknown.
So, in context, if we look at CFRs, it looks like Israelis are 3x less likely to die from COVID than Americans. Maybe that's vitamin D, or maybe instead Israel tested and identified every single person with COVID, whereas America missed two for very case they identified. If we could instead see the IFRs in that case, they'd actually be roughly the same, and it'd turn out (all other things being equal) that vitamin D was irrelevant.