I could find no research on measles severity and mortality in relation to circulating 25-hydroxyvitamin D levels. However, like every other disease, measles would be less severe and so transmit less (fewer virus particles shed) the better the immune system works. It can only function well with about 50 ng/mL (125 nmol/L = 1 part in 20,000,000 by mass) or more circulating 25-hydroxyvitamin D, while many people (except in summer and/or for those who supplement vitamin D3 properly) have half or less of this level.

UK measles seasonality is high in winter-spring and low in summer autumn: Fig 1 in arxiv.org/abs/2405.09664. This would be caused, in large part, by variations in circulating 25-hydroxyvitamin D (made in the liver from ingested or UV-B -> skin produced vitamin D3, and which is measured in "vitamin D" blood tests), due to UV-B flux in winter being so low compared to that in summer.

Please see the research cited and discussed at my long web page on the vitamin D compounds, primarily 25-hydroxyvitamin D, and the immune system: vitamindstopscovid.info…. There, you will find peer-reviewed research articles which clearly show that the immune system can only work properly, at least against the bacterial pathogens which cause post-operative infections, if the level of circulating 25-hydroxyvitamin D is 50 ng/mL. The same is likely true of immune responses to cancer and both viral and fungal infections.

Without proper vitamin D3 supplementation - in quantities which while small, are 5 to 10 times those recommended by governments and many doctors - and without recent high level ultraviolet B exposure of ideally white skin, most people have 25 ng/mL (75 nmol/L) or less circulating 25-hydroxyvitamin D. Some have only one tenth - especially the elderly and/or those with dark or black skin who are living far from the equator.

This page begins with recommendations from New Jersey based Professor of Medicine, Sunil Wimalawansa on the average daily supplemental intake quantities of vitamin D3 which will attain least 50 ng/mL circulating 25-hydroxyvitamin D, over several months, without the need for blood tests or medical monitoring:

70 to 90 IU / kg body weight for those not suffering from obesity (BMI < 30).

100 to 130 IU / kg body weight for obesity I & II (BMI 30 to 39).

140 to 180 IU / kg body weight for obesity III (BMI > 39).

For 70 kg (154 lb) body weight without obesity, this is about 0.125 milligrams (125 micrograms 5000 IU) a day. This takes several months to attain the desired > 50 ng/mL circulating 25-hydroxyvitamin D. This is 8 or more times what most governments recommend. "5000 IU" a day sounds like a lot, but it is a gram every 22 years - and pharma-grade vitamin D costs about USD$2.50 a gram ex-factory.