The complication is a neurological disorder that can lie dormant for years and then is 100 percent fatal. Researchers don't know what causes the virus to reactivate, and there is no cure once it does. The only way to prevent the disorder is by vaccinating everyone possible against measles.

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Measles is an extremely contagious respiratory infection caused by a virus. Once common in the United States, it was eliminated nationally in 2000, but has made a comeback, mostly because of the growing number of people who refuse to vaccinate their children or delay those vaccinations, experts say.

The first MMR dose is typically administered at 12 to 15 months of age. (Babies may be vaccinated at age 6 months or older if they are at risk of exposure to measles, for instance if they are traveling to an area with an outbreak.) Unvaccinated babies can be infected with measles and later develop this complication, which is called subacute sclerosing panencephalitis, or SSPE.

Scientists once thought the risk of developing SSPE was about 1 in 100,000. Recent research in Germany among children who got measles before they turned 5 identified a rate as low as 1 in 1,700. But the new findings, by researchers at the David Geffen School of Medicine at the University of California in Los Angeles and the California public health agency, found that for babies who get measles before being vaccinated, the rate is 1 in 609.

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“This is really frightening and we need to see that everyone gets vaccinated,” James Cherry, a study author and expert on pediatrics and infectious diseases at UCLA's medical school, said at a press conference Friday. He said the findings suggest that other cases of SSPE are likely occurring and being missed.

Many parents who intentionally refuse to vaccinate their children believe "they are trying to do the right thing," said Gary Marshall, a pediatrics professor at the University of Louisville School of Medicine in Kentucky. "Unfortunately, they're mistaken. The right thing to do to protect their children is to vaccinate them to prevent them from getting measles and getting a horrible complication."

In 2014, the United States experienced a record 667 cases of measles, the largest number since the disease's elimination here, according to the Centers for Disease Control and Prevention. One major outbreak occurred primarily among unvaccinated Amish communities in Ohio.

In 2015, there were 189 cases, with 131 cases linked to an outbreak at Disneyland, that put Orange County in California at the center of the worst measles outbreak in that state in 15 years. Twenty-four cases involved children under age 2 who are now at risk of developing the always-fatal SSPE, said Jennifer Zipprich of the California Department of Public Health.

Vaccinating a very high proportion of the population — typically about 90 percent regardless of the disease — ensures herd immunity, which means those who can't be inoculated are still protected because the disease is less likely to spread. The MMR vaccine isn't recommended until after a first birthday because children retain some of their mother's antibodies until that age, making the vaccine less effective, experts say. Others who can't get vaccinated include people with immune system disorders.

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The researchers analyzed cases of California children who got measles between 1998 and 2015, finding that 1 in 1,387 were younger than 5 when they were infected. They identified 17 cases of SSPE during that period, all of whom had measles before being vaccinated. All but one of those patients have died. A 5-year-old boy, who was infected during travel to Germany, is now in hospice, unable to move or respond to commands, researchers said.

The average age of SSPE diagnosis was 12, but the range was from 3 to 35 years. Many of the patients had ongoing cognitive or movement problems before they were definitively diagnosed. SSPE typically progresses in stages. Initially, the signs are subtle, starting with behavioral problems, Cherry said. Then seizures develop until the person becomes comatose.

An additional suprising finding is that Asians are disproportionately affected by SSPE, Cherry said. He is not sure why but suspects the disorder could behave like some other diseases, such as influenza, which seem to hit Asians harder and cause higher mortality than other ethnic groups.

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Measles infection causes fever, runny nose, cough, red eyes, sore throat and rash. The virus spreads throughout the body and is usually cleared within 14 days. In rare cases, the virus spreads to the brain but then becomes dormant. Eventually, that can lead to SSPE and result in death.

Among other severe complications of measles, as many as 1 of every 20 children will develop pneumonia — actually the most common cause of death from measles in young children. And about 1 of every 1,000 children will develop encephalitis, or swelling of the brain, that can lead to convulsions and can leave the child deaf or with intellectual disability.

Because there is a 5 percent failure rate with the MMR vaccine, a second dose is given to children before they begin school. Measles is so contagious that 95 percent of people need to be vaccinated with two doses to protect those who aren't, experts say. That means all who are eligible, including adults who haven't been previously vaccinated, should receive two doses of the vaccine.

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Nearly 92 percent of U.S. children 19 to 35 months old have received the MMR vaccine, according to the CDC. Through Oct. 8, 54 people from 16 states were reported to have measles this year.

This post has been updated.

Correction: This post originally said that babies cannot be vaccinated with MMR before 12 months of age. Exceptions are made for babies 6 months or older if they are at risk of exposure to measles.