The one nutrient that I've seen mentioned more than any other over the last few months is vitamin D. We already know that vitamin D is a very important nutrient for bone health as well as for immune function against organ cancers. However the latest research shows a significant reduction in respiratory infections in children when supplemented with vitamin D.
This was conducted in Mongolia, where sunlight is limited so natural vitamin D production would be limited, revealing the importance of children getting adequate sunlight exposure. The UK isn't exactly sun central so children with repeated respiratory infections may benefit from supplementation.
Vitamin D can be stored in the liver so you make lots during your hot summer holidays to use the rest of the year, but I don't tan easily so don't spend alot of time in the sun and take vitamin D in the winter months to keep myself topped up. You can also get vitamin D through you diet by eating eggs and fish, so if you're vegan supplementation is much more important.