Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

A deadly measles complication that kills kids years after they seemingly recover may not be as rare as doctors thought, researchers said Friday.

The fatal and incurable complication has killed at least 16 California adults and children, the researchers in Los Angeles and San Francisco said.

Jacqueline Fletcher of the Knox County Health Department prepares a dose of the measles, mumps and rubella (MMR) vaccine Noah Addis / for NBC News, file

They say they’re afraid the condition is far more common than anyone thought, and say it strongly reinforces the need for vaccinating every single child who can be.

“This is really frightening,” said Dr. James Cherry of the University of California, Los Angeles medical school.

The victims all had a condition called subacute sclerosing panencephalitis or SSPE. It’s caused when the measles virus stays in the brain, usually for years, after a young child is infected and has recovered.

For reasons that no one understands, it can reawaken and causes an immune response that leads to seizures, coma and death. There’s no treatment and no one has been known to survive it.

“It takes having had measles many years before."

Once thought very rare, a recent study in Germany and now the California study suggest SSPE is far more common than previously believed. Original estimates suggested it affected 1 in 100,000 kids, but in the California cases 1 in 600 people who got measles as infants developed SSPE, the researchers said.

They said 1 in 1,400 kids under 5 who got measles developed SSPE.

“Not only is SSPE still here and still diagnosed but it actually may be more common than we thought it was,” said Dr. Gary Marshall of the University of Louisville School of Medicine, who was not involved in the study.

Related: Disneyland Measles Outbreak Came From Overseas

Cherry says the cases involve people infected with measles since a major outbreak in the U.S. that started in 1988. Between 1989 and 1991, 55,000 measles cases were reported and 123 children died. Since then rates have been just a fraction of that and measles was declared eliminated in the U.S.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Still, up to several hundred are reported every year when travelers bring measles back from other countries and occasionally spark small outbreaks.

The team started to look at possible SSPE cases after the German cases were reported. They found 16 people in California alone killed by SSPE – all diagnosed by autopsy after they died. A 17th case is in hospice care now, they told a meeting of infectious disease specialists in New Orleans.

Measles is one of the most infectious viruses known. It can be transmitted in the air and infects most people exposed to it unless they are vaccinated. It’s easily prevented with a vaccine.

Symptoms include high fever, a distinctive rash and irritated eyes. It causes severe complications including blindness, encephalitis, severe diarrhea and pneumonia, and is sometimes fatal.

“When you get measles, the virus goes all over the body. A surprising amount goes to the central nervous system,” Cherry told a briefing at the Infectious Disease Week meeting.

Most people clear the virus, meaning the immune system eliminates it from the body. But not always.

“In the case of SSPE it’s gone to the brain and it is all over the brain,” Cherry said.

Related: Measles Eliminated From the Americas

The team found 17 cases of SSPE reported in California between 1998 and 2015. The average age was 12 but three were adults, aged 20, 30 and 35. Every one had measles before getting vaccinated and the researchers say there is no evidence the measles vaccine can cause SSPE.

“It takes having had measles many years before,” Cherry said.

Many more cases across the country may have gone unreported, Cherry says.

The researchers say the findings reinforce the need to make sure children get two doses of the measles, mumps and rubella (MMR) vaccine as recommended by the Centers for Disease Control and Prevention.

A 2015 report showed nearly 9 million U.S. children are not fully vaccinated against measles and risk getting infected.

“Parents of infants who have not yet been vaccinated should avoid putting their children at risk."

Because babies younger than 1 cannot be vaccinated, populations around them need to be fully vaccinated to protect them.

The U.S. has a small but heated controversy over vaccinating children. Health experts say pockets of well-meaning and well-to-do vaccine doubters can help fuel outbreaks when they delay vaccination or refuse to vaccinate their kids at all.

The latest outbreak affected Disney theme parks in 2014 and 2015 and spread to a half-dozen U.S. states, Mexico and Canada. The outbreak sickened 147 people in the U.S., including 131 in California. No one died.

Travelers from other countries can carry measles to the U.S. Measles is far more common even in western European countries than it is in the U.S.

Globally, there are around 20 million cases of measles and more than 145,000 children die of measles every year.

“Parents of infants who have not yet been vaccinated should avoid putting their children at risk,” Cherry said. “For example, they should postpone trips overseas – including to Europe – where measles is endemic and epidemic until after their baby has been vaccinated with two doses. It’s just not worth the risk.”