During the Disney measles outbreak, there's been a lot of focus on unvaccinated kids. But what about adults who may have only been partially vaccinated? What should they do?
I've consulted with the experts, and here's what I've learned:
What is the vaccine recommendation for kids?
Children should get two doses of the measles vaccine: one dose when they're about one year old, and a second dose when they enter kindergarten. One dose is considered 95 percent effective; with two doses, the vaccine is more than 99 percent effective.
What's recommended for adults?
With adults, it's a little complicated: If you were born before 1957, health officials say you likely got the measles - or were at least exposed to it – so you're considered to have natural immunity.
But a lot of adults born after that point probably only got one dose of the vaccine.
Here's why: The measles vaccine was first introduced in 1963. (At that point, there were two types of measles shots on the market - an inactivated, "killed," measles vaccine, and the more effective, "live," virus-containing vaccine. The inactivated vaccine was withdrawn in 1967, because it wasn't effective.)
It wasn't until 1989 that health officials started recommending two doses of the live vaccine. There was a catch-up program in 1989, so some people who were in grade school at that time might have gotten the second shot, according to state health officials. California started requiring two doses for kids entering kindergarten in 1997.
There are other reasons some adults born before 1989 may have gotten their second shot: It's recommended for healthcare workers, kids going off to college, and people traveling to countries where measles still circulates.
Should adults who only received one dose of the MMR vaccine get a second dose?
It depends on whom you ask.
"Certainly, if you've only received one vaccine, and you're an adult, then it is the safe thing to do to go ahead and get that second booster shot for measles," says Dr. Shruti Gohil, with the Division of Infectious Diseases at University of California Irvine Health.
Dr. Daniel Vigil, a family medicine doctor at UCLA, agrees, saying, "oh yes, yes I would, that's an easy yes-no question, and my answer is an emphatic yes."
But during a media call earlier this week, Dr. Kathleen Harriman, head of the California Department of Public Health's vaccine-preventable disease section, was less... emphatic.
"Any adult who thinks they've had only one dose of MMR [the measles, mumps, rubella vaccine], or isn't sure, can either get a dose, or get blood serology done to ensure if they are immune," she said.
Can get or should get? The California Department of Public Health clarified its answer today, saying that it "recommends anyone who hasn't already received two doses of the MMR vaccine to do so."
There's another caveat: People who only received the inactivated vaccine between 1963 and 1967 are not considered protected, according to the state health department.
State epidemiologist Gil Chavez adds: "Unfortunately, most persons born in the 1960s do not have immunization records. If such people do not know if they ever received a dose of live vaccine, they are recommended to receive it."
What does the federal Centers for Disease Control and Prevention say about all of this?
Greg Wallace, who heads the CDC’s domestic measles, mumps, rubella and polio program, says the CDC is not recommending American adults get a second dose of the measles vaccine.
Now, understand that he's coming at this from a big picture, public health perspective: Most measles transmission, he says, is not occurring between adults with one dose of vaccine. Instead, outbreaks like the one we're having now occur when the disease spreads throughout a community with a high proportion of unvaccinated people.
So, if I'm one of those adults who only got the one dose when I was a kid, what should I do?
Remember: Even with the one dose, there's a 95 percent chance that you are immune.
If you're not sure if you got one or two shots, your doctor can do a simple blood test to check if you have immunity. But the CDC's Wallace points out that the blood test can cost more than getting the second shot of the vaccine. So, he says, if you're really worried, there's certainly no harm in getting a second dose.
Again, Wallace says, the CDC's greatest concern is getting unvaccinated people vaccinated, because that's the primary way a disease like measles is going to spread.
Do you have other questions about measles? Read our FAQ here.
This post was updated on Jan. 24, 2015.