Comment by m4burns

Good points. Here is some data on vitamin D deficiency in the UK (where the original study was conducted) that also addresses seasonality: https://www.bjfm.co.uk/prevention-of-vitamin-d-deficiency

"Low serum levels of vitamin D are found in significant numbers of all population groups in the UK: in winter 30-40% of all age groups in the general population are classed as vitamin D deficient. Even towards the end of summer 8% of adults and 13% of adolescents remain deficient."

where deficient is defined as < 25 nmol/L of 25(OH)D