'Kay. So helmets are effective at reducing the severity of non-concussion head injury, IF YOU HIT YOUR HEAD while cycling.
Unfortunately for helmetfags, the risk of head injury is already low enough regardless for it to not matter, and risk was raised significantly by the effects of mandatory helmet law due to safety-in-numbers effects. So in general bicycle helmets remain unnecessary and mandatory helmet law remains counterproductive. Useful if you're racing or mountain biking or whatever though.