How can vaccinated people contract measles?

Although most of those infected with measles this year aren't fully vaccinated, the disease is so contagious that it could wind up infecting many people who have had all of their recommended shots, health officials say.

California health officials have said that 82% of measles patients in that state were not fully vaccinated, suggesting that 18% have had a measles shot.

Does that mean that 18% of vaccinated people will come down with measles?

Not at all, says Gregory Wallace of the Centers for Disease Control and Prevention.

To understand why, Wallace says people should remember that measles is one of the most contagious viruses known.

The virus can linger in the air "like a ghost," infecting people even two hours after the sick person has left the room, says Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. Infected people also can spread measles for four days before they develop the tell-tale red rash, as well as four days after the rash goes away.

One dose of measles vaccine protects 93% of patients, while two doses protect 97%, Wallace says.

But even with a vaccine failure rate of only 3%, a large measles outbreak can end up infecting a lot of fully vaccinated people, Wallace says.

To illustrate why, Wallace suggested considering a scenario in which 1,000 people are exposed to measles, with 900 fully vaccinated and 100 unvaccinated.

Doctors would expect 90% of the unvaccinated people -- or 90 people -- to get sick. They would also expect 3% of the vaccinated people -- or 27 people -- to get sick, for a total of 117 people.

Clearly, the vaccinated people here are much better protected. But the percentage of patients were vaccinated in this scenario is 23% -- about the same proportion that health officials tend to see in real outbreaks, Wallace says.

"People look at these numbers and they say the vaccine doesn't work," Wallace says. "It does work... Nothing in the numbers we're seeing from the current outbreak suggest the vaccine isn't working as well as it always has."

Wallace notes that vaccination can have advantages, even in the rare cases in which vaccinated people get measles. These people are typically less contagious than people with measles who were never vaccinated, so they spread the virus to fewer people.

He notes that people without measles symptoms cannot act as "carriers" to silently spread the virus. If people don't have measles symptoms, such as a cough, fever or rash, they can't spread the virus to others, Wallace says.