Why do they need to study this? There really shouldn't be any reason why vitamins should stop infections:

Infection rates in older people living at home are not helped by taking vitamin or mineral tablets, researchers have found.

The Aberdeen University study looked at the effects of daily multivitamins compared with dummy placebo tablets.

It found that taking supplements seemed to make no difference in infection rates between the groups.

("Vitamins 'do not stop infections'", BBC, Aug.4)