Scientists have discovered a deadly complication with the measles virus that could potentially kill children years after suffering from an outbreak, a new study reveals.

The condition called subacute sclerosing panencephalitis or SSPE is when the measles virus stays dormant in the brain after contraction and seeming recovery from it. The frightening condition has seen at least 16 adults and children die and scientists are unaware of how or why it happens. Not only that, but it is thought to be far more common than previously thought.

Once the virus has awakened, it can cause a trigger to the immune system which can lead to subtle changes in behavior, slowly developing seizures, possible coma and death. So far, no one has survived a case of SSPE.

Dr. James Cherry of the University of California, Los Angeles medical school, who was involved in the study, says the victims of SSPE caught the virus during the measles outbreak in California which started in 1988 and ended in 1991. That outbreak saw 55,000 people contract the virus with 123 children dying.

SSPE highlights the importance of getting as many people as possible vaccinated from the highly contagious virus. Many vulnerable people such as babies under a year old who are too young to get the vaccination, will benefit from more people being immune and ultimately keeping the virus from spreading, according to the Washington Post.

The MMR vaccination has been controversial for some, with many parents refusing to get their children vaccinated. It’s this that has caused some people to blame a comeback of the measles virus. A 2015 report showed nearly 9 million children in the U.S. are not vaccinated and are at risk of getting infected.

Details of the study were announced at IDWeek 2016.

For more information on measles, visit the Centers for Disease Control and Prevention website.