Health officials are reporting four Tennessee cases of a rare infant illness that have been linked to parents refusing a routine shot for newborns.

The four Nashville-area infants suffered brain or stomach bleeding earlier this year. Their parents had declined vitamin K shots, which have routinely been given to newborns since 1961. The shots improve blood clotting and prevent internal bleeding. All four children were treated with vitamin K and survived.

On Thursday, the Centers for Disease Control and Prevention said the illnesses were the first tied to parents opting out of the shots; reports of vitamin K deficiency are rare. The CDC said the parents cited a belief that the shots weren't necessary or cause leukemia. Officials say neither is true.