Hydroa vacciniforme is a skin condition, a very rare, chronic photodermatitis

Photodermatitis

Photodermatitis, sometimes referred to as "sun poisoning" or photoallergy is a form of allergic contact dermatitis in which the allergen must be activated by light to sensitize the allergic response, and to cause a rash or other systemic effects on subsequent exposure...

with onset in childhood. It is seen in children under 10 years of age, and it disappears at puberty. It starts after being exposed to the summer sun as a vesicular eruption, then becomes umbilicated, and in the end there is a chickenpox-like scar.
It is seen on the nose, cheeks, ears, dorsum of the hand, and arms (places that are most exposed to light).