Measles Outbreak, Measles Vaccine: Top Questions Answered
A single virus particle, or "viron", of the measles virus.
Credit: CDC/ Cynthia S. Goldsmith; William Bellini, Ph.D.

The U.S. measles outbreak now includes at least 102 infected people in 14 states. Most of the cases have been tied to Disneyland in Southern California. The outbreak has many people wondering why a disease that was eradicated from the United States in the year 2000 is now infecting so many people, and what role vaccination requirements may have had in the outbreak. We asked experts to explain how the vaccine works and why the outbreak is happening now.

Why is the outbreak happening now?

Most of the cases of measles reported so far in 2015 are part of a large, ongoing outbreak linked to Disneyland in Anaheim, California, according to the California Department of Public Health(CDPH).

The theme park has many international visitors, and measles is brought into the United States every year by unvaccinated travelers who contract the disease in other countries, especially in Western Europe, Pakistan, Vietnam and the Philippines, according to the CDPH. In addition, people who live in the U.S. and travel overseas can contract the virus and spread it to unprotected people after they return home, which can lead to an outbreak.

In 2014, there were more than 600 cases of measles in the U.S. The largest outbreak of the disease involved 383 of these cases, and occurred primarily among unvaccinated people living in Amish communities in Ohio. There were also 22 other (mostly much smaller) outbreaks, according to the CDC. Many of the U.S. cases in 2014 could be traced back to a large measles outbreak in the Philippines, the agency said.

If you've been vaccinated, can you still get measles?

Yes, people who have been vaccinated can get the measles, but there is only a small chance of this happening. About 3 percent of people who receive two doses of the measles vaccine will get measles if they come in contact with someone who has the virus, according to the CDC. 

It's not clear why some fully vaccinated people get measles, but it could be that their immune system did not respond properly to the vaccine, the CDC says. Still, if a person is fully vaccinated, and they come down with measles, they are more likely to have a mild case of the illness. [5 Dangerous Vaccine Myths]

Why do children need to receive two MMR shots?

In the original trials of the measles vaccine, the shot was 98 to 99 percent effective at protecting people against the disease, said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Nashville, Tennessee.

But in practice, the vaccine protected only 92 to 93 percent of children. When researchers took a closer look, they realized that doctors weren't always properly handling the vaccine, which needs to be kept very cold. For instance, some were storing the vaccine on the refrigerator door, which is more exposed to temperature fluctuations, he said. As a result, some children were getting a less potent form of the vaccine, Schaffner said.

To cover the remaining 7 to 8 percent of children who didn't get good protection, the CDC recommended that children get two doses of the shot.

"It's not a booster, it's a fill-in dose for those who didn't get protection the first time," Schaffner told Live Science.

Nowadays, doctors are much more aware of the proper storage guidelines, so the likelihood of getting a less potent vaccine is lower, Schaffner said.

Why can't babies get the measles shot?

If a pregnant woman has either been vaccinated against measles or had measles, she will pass antibodies against the virus on to her child through the placenta. This form of immunity is known as passive immunity.

Once they are born, babies maintain this passive immunity for about the first six to eight months of their lives, before it starts to wane. If babies were given the measles, mumps and rubella (MMR) shot during this period, the vaccine wouldn't do a good job of stimulating the immune system to fight the virus.

"So we have to wait until mom's protection wears off before we can vaccinate the baby, and then we wait a little longer just to be sure," Schaffner said.

That's why the first dose of the MMR vaccine is typically given when a baby is 12 to 15 months old, Schaffner said.

Are children protected against measles in the years between their first and second MMR shot?

Yes, a child is protected between the first and second doses of the measles vaccine, but this protection is not 100 percent, said Dr. Amesh Adalja, an infectious disease specialist and a senior associate at the University of Pittsburgh Medical Center's Center for Health Security.

"One dose confers about 95 percent protection but with a highly contagious disease such as measles, getting as close to 100 percent as possible is an important goal," Adalja said.

Two doses of the measles shot is about 97 percent effective at protecting against measles, according to the CDC.

Does the vaccine last for life?

If you got two measles vaccines when you were a child, you are considered protected for life, the CDC says.

You are also considered protected against measles if you got at least one dose of the measles vaccine when you were an adult, or if you had the measles illness.

Some adults benefit from getting two doses of the vaccine — these include people who will be at higher risk for measles, because they'll be living, working or traveling in an environment where they could more easily catch the disease, the CDC says. (High-risk individuals include college students, hospital workers and international travelers.)

Is it ever too late to get vaccinated? Does the vaccine work just as well if you receive it as an adult, compared to as a child?

It's not too late to get vaccinated as an adult. If you did not get the measles vaccine as a child, and you did not have the measles illness, you should receive the measles shot, and "it would produce effective immunity," Adalja said. People born before 1957 likely already had measles, and so do not need to be vaccinated, Adalja said.

The CDC recommends that anyone who is now age 18 or older and who was born after 1956 should get at least one dose of the MMR vaccine, unless they can show that they have either been vaccinated or had all three diseases. If you're unsure whether you've been vaccinated, check with your doctor.

Children should get two doses of the MMR vaccine: the first one at 12 to 15 months of age, the second one at 4 to 6 years of age, according to the CDC. But children can still get their second dose up to 12 years of age, as long as it is at least 28 days after the first dose.

Children ages 1 through 12 can also get a combination vaccine called MMRV, which contains both MMR and varicella (chickenpox) vaccines. The recommended ages for MMRV are the same as for MMR, but anyone who is 13 or older who has not been vaccinated should get the MMR and varicella vaccines as separate shots, the CDC says.

How is measles spread if most people are vaccinated?

Measles can spread even in a highly vaccinated population because the virus is so transmissible.

"When a virus is so easy to catch, it will find the susceptible person, and that's why even when you have a large proportion of the population that's vaccinated it can spread," Schaffner said. If someone with the measles virus sneezes in a room and then leaves, another person can walk in an hour later and become infected, Schaffner said.

Here in the United States, the susceptible people include those who are unvaccinated for personal or religious reasons, babies who are too young to be vaccinated, and those who did not develop a proper immune response to the measles vaccine. It also includes people who are immunocompromised, who cannot safely be vaccinated, such as those with leukemia or who are undergoing chemotherapy, Schaffner said.

Studies suggest that at least 95 percent of the population needs to be vaccinated in order to stop the spread of measles, Schaffner said. Clusters of susceptible people — such as a family or community with low vaccination rates — can lead to outbreaks, according to a 2011 study in the Journal of Infectious Diseases.

Can measles virus mutate, and make the vaccine less effective?

It's extremely unlikely that measles virus will become more transmissible, deadly, or that the vaccine will stop being effective in protecting people against it.

"Measles is solid as a rock. The measles virus that causes disease today is the same virus that caused disease in 1934," Schaffner said.

Although viruses such as the influenza virus and HIV are constantly mutating in significant ways, measles virus doesn't change very much. The vaccines that were developed from the measles virus circulating in the 1950s and 1960s work just as well against modern versions of the virus, according to the Journal of Infectious Diseases.

Does my state require children to be up-to-date with immunizations before entering public school?

All states require children to be up-to-date with immunizations before entering public kindergarten, but allow children to opt out of vaccinations for medical reasons. Aside from those rules, states' laws vary: In Mississippi and West Virginia, the only exemptions allowed are those with medical problems, whereas the remaining 48 states allow some exemptions based on religious beliefs. About 20 states allow parents to opt out of vaccinating their schoolchildren for philosophical reasons, including moral, personal or other beliefs, according to the National Council of State Legislators.  

Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. Originally published on Live Science.