With the recent US measles outbreak, the issue of vaccine refusal has received growing scrutiny. Now doctors are calling attention to a similar problem: Some parents are shunning the vitamin K shot routinely given to newborns to prevent internal bleeding.
The consequences of that choice can be severe, pediatric specialists say. Infants can quickly become deficient in vitamin K, which can lead to dangerous bleeding in the intestines or the brain.
Read: Immunisation essential in preventing further measles outbreaks
"If you refuse the shot, you're rolling the dice with your child's health," said Dr. Robert Sidonio Jr., a hematologist and assistant professor of pediatrics at Emory University in Atlanta.
Vitamin K is necessary for normal blood clotting. In older children and adults, bacteria in the gut produce much of the vitamin K the body needs. But that's not the case for infants.
And breast milk does not supply enough vitamin K - no matter how careful a mother is about her diet, Sidonio said.
"All the kale in the world won't do it," Sidonio added.
Read: How much vitamin K do you need?
That's why, since 1961, US newborns have routinely received a vitamin K shot before they leave the hospital.
But some parents have started saying no to the shot. In 2013, while working at Vanderbilt University's children's hospital, in Tennessee, Sidonio saw several cases of vitamin K deficiency bleeding in succession.
It turned out that none of the infants had received their vitamin K shot. Ultimately, Sidonio and his colleagues encountered seven cases of vitamin K deficiency over an 8-month period - with five of those infants suffering gastrointestinal or brain bleeding.
An investigation by the U.S. Centers for Disease Control and Prevention found an "alarming" trend, Sidonio said. Among parents whose babies were born at private birthing centers near Vanderbilt, 28 percent had refused the vitamin K shot.
It's not clear how common such refusals are nationwide, because there are no tracking systems for them, Sidonio said.
But the trend is not confined to Tennessee. In the latest issue of the Journal of Emergency Medicine, doctors in Ohio describe their own encounter with vitamin K deficiency.
Parents brought their 10-week-old to the emergency room, saying he had become increasingly "fussy" over the previous two weeks. That morning, the mother had noticed flecks of blood in the baby's stool.
Blood tests revealed severe anemia -- a shortage of healthy red blood cells - and a CT scan showed patches of blood collecting outside the brain. Eventually, the baby was diagnosed with vitamin K deficiency, after doctors learned the family had refused both the vitamin K shot and the hepatitis B vaccine, which is normally given to newborns.
Dr. Karyn Kassis is one of the emergency physicians who treated the baby at Nationwide Children's Hospital, in Columbus.
Luckily, she said, they stopped the bleeding around the brain - with an infusion of vitamin K - before it became severe. Such brain bleeds can lead to permanent damage or even death.
So why are parents opting out of the vitamin K shot?
Sometimes parents who don't want their baby vaccinated may say "no shots," and the providers may think that includes vitamin K, Kassis said.
There are also parents who want childbirth to be completely "natural," and refuse vitamin K on those grounds.
But many parents shun the vitamin K shot because they mistakenly believe it's dangerous, Sidonio said.
"There are many stories on the Internet about vitamin K," he noted. "I think these parents are just getting bad information."
The main myth is that the shot causes leukemia, according to Sidonio. He said the myth grew out of a single, "old" study that reported a correlation between the two - a link that was refuted in follow-up studies.
The situation is almost identical to the one surrounding the MMR vaccine and autism, Sidonio noted. A study from the 1990s, later found to be fraudulent, raised the possibility that the MMR caused autism. And despite years of research showing no such connection, some parents still believe the risk is real.
Health officials say the consequences of that distrust manifested in the recent measles outbreaks.
Similarly, Sidonio said, ER doctors are now seeing the consequences of vitamin K refusal.
Kassis suggested that if parents have questions about the vitamin K shot, they talk to a doctor they trust.
Sidonio stressed three points: The vitamin K shot is safe, cheap, and effective. "There's no reason for parents to take the risk of refusing it," he said.
The US Centers for Disease Control and Prevention has more on vitamin K deficiency.