The American classroom has become a battleground for parents who are threatened by the growing number of children not vaccinated against measles, one of the most highly contagious viruses in the world. The ongoing measles outbreak in the U.S. that started at Disneyland and has spread to 14 states has raised concerns over the country’s rising anti-vaccination movement, including whether the decision to vaccinate against such a dangerous disease should be left to parents, and what constitutes responsible childrearing. Should a child whose parents chose not to vaccinate be allowed to share the same pencils and playground as children whose parents did?

In general, vaccination rates against measles in the U.S. have remained high, meaning the likelihood of being exposed is still relatively low. “It’s not dangerous at all if no one has measles,” said Alison Buttenheim, assistant professor of nursing at the University of Pennsylvania School of Nursing. The problem, she said, comes down to clustering. Unvaccinated children tend to live in communities with lower vaccination rates, meaning a single case of measles would have a far greater chance of spreading among those pockets of the population.

“If an unvaccinated kid or someone else brings measles into the classroom, then the question becomes, how likely is it that that kid will bump into other people who are susceptible?” Buttenheim said. Health officials have said a child with measles would likely infect 90 percent of children around him who are susceptible, including unvaccinated children or those whose immune systems have been compromised because of other health complications.

The U.S. declared measles officially eliminated in 2000 following an aggressive vaccination campaign in the 1960s and '70s that drastically reduced the number of infections from a few million a year to fewer than 100 by the early 2000s. The campaign proved the measles vaccine was highly effective at preventing the spread of the disease, but that hasn’t meant it’s invincible. Three percent of people who receive the full dose of the vaccine could still catch the disease, the U.S. Centers for Disease Control and Prevention said. Health officials have said the number of children who need to be vaccinated to maintain low transmission rates is 92 percent to 94 percent of the population. Most states have maintained robust vaccination rates, but others, like California, have lagged. When communities dip below that threshold, it becomes more likely the virus could spread among the unvaccinated.  

As of Jan. 1, health officials had confirmed 102 cases of measles spread across 14 states linked to the Disneyland outbreak that began in December, the CDC said. Since then, cases have spread from California to neighboring Arizona and Oregon and as far away as New York. More people have become infected with measles in January 2015 alone than in nine of the past 14 years.

Overall, “it’s still relatively a small number when you consider all of the school age kids in this country,” said Sallie Permar, a pediatric infectious disease specialist at the Duke University School of Medicine in North Carolina. “Luckily, the majority of our children are vaccinated and immune to measles. The actual risk of a child aquiring measles at this stage [of the outbreak] is still pretty low.”

Measles is among the most contagious viruses in the world, far more infectious than Ebola, for instance, which exploded on the scene in West Africa last year and has led to more than 20,000 infections. “One thing Ebola had going for it was that you weren’t infectious when you were asymptotic and it was pretty hard to give it to someone,” Buttenheim said.

Measles, which causes rash, fever and, in extreme cases, deafness or brain damage, can be spread by simply being in the same room with someone who had it. Measles is an airborne disease that can survive for several hours outside the body, and patients are highly contagious. “You can be shedding the [measles] virus before you even have symptoms. It’s one of the most well-adapted and stable viruses” out there, Buttenheim said.  

Despite these risk factors, a growing number of parents, often from educated and affluent communities, have forgone vaccinations altogether. In some counties in California, often regarded as the epicenter of the anti-vaccination movement, non-vaccination rates were as high as 18 percent to 23 percent from 2010 to 2012, according to a study published last month in the journal Pediatrics.

Health experts said parents of vaccinated children don’t have to worry about their child sharing the same space as unvaccinated children. “A parent who chooses to vaccinate their child can feel good about sending their child to school,” Permar said. “There is the rare case of a vaccine not working well, but that’s rare, especially for a measles vaccine that has been so effective.”