Whether to vaccinate or not? As a parent I chose to vaccinate our daughter. Was it scary? Some of it. But I do support a parents rights to choose the care for their own child. So what do you need to know about Disneyland and the current measles outbreak?

So far the measles have spread and new cases keep popping up. 

The California Department of Public Health said Monday most of the infections — 73— are in California. The rest are in Arizona, Utah, Washington, Colorado, Oregon, Nebraska and Mexico.

Measles has been spreading since an outbreak linked to visits to California Disney parks last month. Most of those infected were not vaccinated, and health officials have urged people to get the measles shot.

Measles is a contagious illness spread through the air by coughing and sneezing. It was largely eliminated in the U.S. in 2000, but it can enter the country from abroad.

From Katie Palmer 

The measles vaccine is actually one of the most effective vaccines in the world. According to Greg Wallace, lead of the measles, mumps, rubella and polio team at the CDC, two doses are 97 percent effective against infection. (Compare that to 88 percent for two doses of the mumps vaccine from the MMR shot.) It’s a live version of the virus, just weakened—or attenuated—so it doesn’t cause severe symptoms. The vaccine replicates just like the full-on measles virus, inciting your immune system to produce antibodies against it. Those antibodies then protect against actual measles as well.

But in some people, that response just doesn’t happen. No one knows why. Either your body doesn’t produce enough antibodies, or the ones it does produce aren’t specific enough to latch on to the virus and kill it.

That’s why the CDC recommends two doses of the vaccine: After the first dose, 5 to 7 percent of people won’t have a good enough antibody response to protect them. A second dose ensures that enough people get antibodies above that protective threshold to control the disease. “And even with two doses, you can get some failure,” says Wallace, “whether it’s because the initial response isn’t perfect, or because the response waned in some people.”

So how does that explain what happened in Disneyland? If you have a group of 1,000 people concentrated in a small space—like oh, say, hypothetically, an amusement park—about 90 percent of them will be vaccinated (hopefully). One person, maybe someone who contracted measles on a recent trip to the Philippines, moves around, spreading the virus. Measles is crazy contagious, so of the 100 people who aren’t vaccinated, about 90 will get infected. Then, of the 900 people who are vaccinated, 3 percent—27 people—get infected because they don’t have full immunity.

Now the Disneyland numbers—six vaccinated infections out of the 34 cases with known records—start to make more sense. (And considering the 16 million or so visitors the park gets every year, we might reasonably expect that number to go up.) Once vaccination levels dip below 90 or 95 percent, there aren’t enough protected people to keep the disease in check—the herd immunity that epidemiologists like to talk about so much. In the US, we’ve been doing pretty well keeping those numbers up. “But there are some fluctuations,” says Cristina Cassetti, program officer at the National Institute of Allergy and Infectious Diseases, “and if vaccination levels dip down a little, you get a situation like Disneyland.”

So, the tiered model of antibody response leaves a small percentage of vaccinated people susceptible. But note: It’s also the reason why you’re better off being vaccinated even if you end up getting infected. Your antibody levels might not be high enough to completely protect you, but they’ll still help—the CDC has seen vaccinated patients with measles who only get a rash for about an hour, says Wallace. And, importantly for octogenarians (whose immune systems are weaker) and infants, vaccinated patients are much less likely to transmit the disease to other people. Even if you’re the unlucky sucker who gets infected after vaccination, getting a shot still helps contribute to herd immunity—good job, world citizen! Taking one for the team!

Researchers are still trying to understand why people’s immune systems respond differently. When they figure it out, it might help them create a more effective version of the vaccine.

In the meantime make the best choices for your child, pray over them, and let's hope this outbreak stops. It's a terrible disease.