Disneyland in Anaheim, Calif. (H. Lorren Au Jr./Orange County Register via AP)

In 2000, the United States declared that measles had been eliminated. But the country experienced a record number of measles cases last year, while an outbreak this month that began at Disneyland has been linked to dozens of cases in California and other states.

This has raised obvious concerns, both because of the overall anti-vaccination movement in this country and because it’s not considered a good thing for a virus to suddenly surge back into circulation. So, how did we get here? How did we travel from elimination to a record number of cases and a new, dangerous outbreak in just 15 years? Let us take a walk through the recent history of measles and the United States.

ELIMINATION

First, it is important to know that “elimination” does not really mean “eradication.” When the United States said that measles had been eliminated in the country, this did not mean there would be no other cases within the nation’s borders; rather, it meant that it was no longer endemic. In other words, any measles cases that occurred would come from people traveling to the country. (The Centers for Disease Control and Prevention defines measles elimination as 12 months or more without “continuous disease transmission” in a geographic area.)

BEFORE AND AFTER ELIMINATION

Before the virus was eliminated and before vaccinations became routine in 1963 (well, for the most part, but we’ll get back to that in a moment), it infected millions of people each year and was responsible for hundreds of deaths. The CDC says that until that point, every year 3 to 4 million people were infected, 48,000 of them went to the hospital and 400 to 500 people died.

After the elimination declaration in 2000, between 37 and 220 people reported having measles each year in the United States — until last year.

WHAT HAPPENED LAST YEAR?

The number of measles cases in the country skyrocketed, nearly tripling the highest number seen in the preceding 13 years. As noted above, there were no more than 220 cases in a given year between 2001 and 2013. In 2014, there were 644 cases stemming from nearly two dozen outbreaks.

The jump was dramatic, with more cases just last year than there were in the five preceding years combined.

(CDC)
(CDC)

VACCINATIONS AND THE ANTI-VACCINE MOVEMENT

Obviously, the recent surge in cases is highlighting the still-extant anti-vaccination movement in the United States. Celebrities like Jenny McCarthy, among others, have been given large platforms from which to express their views about the dangers of vaccinations, even though the gas that seemingly fueled the anti-vaccination movement for so long — a medical study published in 1998 — was retracted in 2012. (My colleague Terry McCoy has much more here about the doctor who wrote the retracted paper.)

There are other reasons a person may not get vaccinated, of course, ranging from religious or philosophical reasons to someone moving to the country and not being aware of or able to obtain the vaccination. Yet there is a different danger posed by people who choose not to get vaccinations for their children: A recent study found that these people are clustered together, which means that measles, a highly-contagious disease, can more easily spread to additional children.

Ultimately, the vast majority of children in the United States are vaccinated for measles. The majority of people who get measles have not been vaccinated. In California, for example, half of the people with measles in the current outbreak have not been vaccinated (and vaccination status is still not known for many of the others).

Still, even if people aren’t getting vaccinated, they aren’t spontaneously developing measles. The disease has to come from somewhere. A huge number of measles cases originate outside the United States, as measles remains common in other countries. (The CDC estimates that each year, about 20 million people around the world get measles and that 122,000 people die from it.)

“The only way it can come is from other countries, usually from U.S. travelers who travel abroad and who are not protected through vaccinations, who get infected overseas and bring it back,” Jane Seward, deputy director of the CDC’s viral disease division, told me during one of the outbreaks last year.

A study published in the Journal of the American Medical Association — Pediatrics last year found that 88 percent of measles cases in the country “were internationally imported or epidemiologically or virologically linked to importation.” There were a few cases not linked to any kind of importation over a period from 2001 to 2011, but they did not seem to be endemic, researchers said. The study ultimately found that while endemic measles was still eliminated, international importation remained a serious risk.

As long as people are traveling to and from other countries, and as long as some of these people may be unvaccinated and they may come into contact with other unvaccinated people, the risk remains of measles cases in the United States.

WHAT IS HAPPENING IN CALIFORNIA?

There are 59 confirmed cases of measles in California residents this month, and 42 of those cases were tied to exposure at Disneyland or Disney’s California Adventure Park, the state’s Department of Public Health said Wednesday. (The CDC says there are also cases in five other states.)

Those 59 people ranged from seven months to 70 years old. The vaccination status of 34 of these people is known, the health department said, and all but six of those people had not been vaccinated. One other person had received one of the two recommended doses.

The situation in the state points to a specific danger presented by an infectious disease that can be spread by people who have traveled overseas and those who have not been vaccinated. Travelers from overseas who do not live in the United States or visit it frequently will often go to tourist attractions such as theme parks, which draw very large crowds. If those crowds contain people who have not been vaccinated, they can potentially contract a disease and bring it back to a community full of other people who may be susceptible.

For public health officials in the state, they are monitoring possible measles cases and expecting additional cases to emerge in the near future. They also have a clear message about what they say can help stave off this sort of thing. “The best way to prevent measles and its spread is to get vaccinated,” Ron Chapman, director of the health department, said Wednesday.