> "I cite the UK's response to COVID19 as evidence."

Here's some things I would love to see an alternate-history version of:

1) Vitamin D has some involvement in the immune system. The US Department of Health[1] says "Your immune system needs vitamin D to fight off invading bacteria and viruses.". Harvard School of Publich Health says[2] "laboratory studies show that vitamin D can reduce cancer cell growth, help control infections and reduce inflammation", "a large meta-analysis of individual participant data indicated that daily or weekly vitamin D supplementation lowers risk of acute respiratory infections"

2) The UK NHS page on Vitamin D does not mention immune function at all[3] but does strongly imply that everyone in the UK is deficient during winter when it recommends "since it's difficult for people to get enough vitamin D from food alone, everyone (including pregnant and breastfeeding women) should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter."

3) The Harvard page linked earlier says a randomized controlled trial with 340 Japanese school children given either Vitamin D or a placebo, the Vitamin D group had 40% fewer flu infections in winter.

What do we know about COVID? It's an infection, it's expected to be more prominent in winter, some of the knock-on effects are to do with inflammation of tissues all around the body - lung, heart, brain, nerves.

So, would anything have played out differently if during the early days of no vaccines and no effective treatment, the NHS had leaned hard into Vitamin D testing and supplementation? Anyone presenting to a doctor or hospital or care home of any kind for any medical problem gets a routine blood test for VitD levels as well, any blood tests happening for anything also test for VitD levels, high risk people picked out specifically and called for testing, generic supplments freely available from GPs and pharmacies even without prescription using the NHS's large scale buying and negotiating power, anyone found deficient given a strong dose or large injection to start with, public relations push for the public to supplement or get checked, kept up all through the year leading into the first winter. Would it have made a difference to the ease of it spreading, to the amount of dead people, to the amount of hospitalized people, to the amount of long-term complications, would it have flattened the curve, helped the NHS, reduced or eliminated the lockdowns?

I am indoors most of the time, but I eat a lot of the recommended vitamin D foods - dairy, eggs, red meat, sardine, mackerel - and still had 'severely deficient' levels the first time I paid for my own test out of my own curiosity[4], and then 'insufficient' the next time.

That seems like the kind of thing a "national health service" would be well placed and incentivised to do, whereas a for-profit expensive-pills-and-surgery-and-insurance-profit "service" isn't.

[1] https://ods.od.nih.gov/factsheets/VitaminD-Consumer/

[2] https://www.hsph.harvard.edu/nutritionsource/vitamin-d/ (click to expand the 'immune function' section)

[3] https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-...

[4] (by a UK NHS lab, one which doesn't sell supplements so it's not incentivised to report misleadingly low figures)