Vitamin D status has a linear association with seasonal infections and lung function in British adults.
> Each 10 nmol/l increase in 25(OH)D was associated with a 7 % lower risk of infection
Most UK adults have a 25(OH)D vitamin d level around 30nmol/l and if they could raise that to above 130nmol/l that would virtually eliminate their risk of seasonal colds/flu.
Initially each 1000iu extra vitamin d3 daily raises 25(OH)D approx 25nmol/l however this drops to about 18nmol/l when you get to 70nmol/l and it drops further the nearer you get to 125nmol/l when each extra 1000iu/daily produces only a 12nmol/l increase.
So as you get nearer the point of natural vitamin d homeostasis the less new vitamin d your body absorbs (and if it were summer the less extra vitamin d would be created on your skin)
It's never too late to improve your vitamin d status the sooner you start the sooner your immune function will improve and the better able you will cope with infection. If you could find some 5000iu oil based gel cap vitamin D3 capsules then for the next 8~12 weeks you could safely take 10,000iu daily (2 capsules every day with food) and then drop down to 1 x 5000iu/daily through the summer.
Grassrootshealth.org is a vitamin d information charity that will send postal 25(OH)D tests so you can check your vitamin d level after the intital 8~12 weeks to be sure that is sufficient to get your level to around the 150nmol/l = 60ng/ml mark.
I find it cheapest to buy vitamin D3 direct from the discount providers in the USA. Amazon and Ebay have people supplying them but are generally more expensive. Aim for £6.00 for 200 x 5000iu plus £2.50 postage. VAT is now charged on orders above £15 so you can safely order 2 pots approx £12 + £2.50 postage and you'll save money compared to buying from the UK. (unless someone here knows a cheaper sources)
For ideas that may help shorten the current cold you may like to look at this study.
Efficacy and safety of a multiherbal formula with vitamin C and zinc (Immumax) in the management of the common cold. and then see if you've any garlic, Echinacea, ginseng vitamin c or zinc in the medicine cabinet. Garlic Zinc and Vitamin C are fairly standard suggestions for improving immune function. and echinacea & ginseng have a lot of fans as well.