This is a sensationalist headline, in my opinion. However, it is speaking the truth, only just published by Trinity College Dublin this month! These clever researchers bring home the point that many of us are deficient in Vitamin D and that it's essential in our body's fight against illness, e.g. Covid-19.
Vitamin D is a vitamin made in our skin upon sun exposure. It is absolutely vital for proper immune function. These days, we are chronically deficient in it.
If you are working indoors, over 50 and/or cocooning (or know anybody who is), listen up: think about Vitamin D supplementation now. Take your recommended RDA (expressed in IU's). I personally recommend taking it year round this year.
All of us have been asked to stay indoors for the most part of the spring already, so will likely miss many hours of sunshine our skin needs to make Vitamin D.
I recommend you take between 1,000-3,000 IUs per day in supplementary form to keep bug fighting fit. Food sources of Vitamin D include: eggs, oily fish, almonds, cheese, full fat milk.
A good quality low cost Vitamin D supplement I can recommend: https://www.grassrootsnutrition.ie/product-page/d3-lemon-melts