Prevalence of vitamin A deficiency. Vitamin A deficiency (VAD) is a lack of vitamin A in humans. It is common in poorer countries but rarely seen in more developed countries. Nyctalopia (night blindness) is one of the first signs of VAD.Xerophthalmia, keratomalacia, and complete blindness can also occur since Vitamin A has a major role in phototransduction.yctalopia (night blindness) is one of the first signs of VAD. Xerophthalmia, keratomalacia, and complete blindness can also occur since Vitamin A has a major role in phototransduction.