In a word: big.
So far the Disneyland outbreak, as of the end of January, has tallied approximately 85 cases; about 50 of those cases have occurred in California residents. Of these, the California DPH knows the vaccination status of 34 of them. Overwhelmingly they are unvaccinated: 28 of the 34 received no vaccine, and a 29th only received one of the recommended two doses. It's generated a small amount of media splash, although the number of Americans who currently have opinions about the air pressure of footballs (including prominent physicists, for instance) vastly outnumber those who are aware of what took place at the Magic Kingdom. Many more people should know about it, however: in terms of health news, it's a big story.
But how to define "big"?
Before we get there, let's first understand some very basic facts about measles. There are really only three concepts that are necessary to grasp measles in broad outline. First, it is incredibly contagious. Second, it is not particularly lethal. Third, there is a vaccine that is effective against it. (One should probably add a fourth: the vaccine is safe and does not cause autism.) From there, you can know what needs to be known about why this story is important.
How contagious is "incredibly contagious"? Simply put, it is either the most contagious virus known to man, or is among the top two. Epidemiologists use a variable known as R0 (pronounced "R nought") to describe a pathogen's transmissibility: the R0 tells you how many uninfected people a given person is likely to infect. Here is a graphic that compares some relative R0 values:
(Hang onto the bolded "Ebola" R0 value for a bit.)
It's important to understand that R0 is not a biologically fixed number: as the California measles data shows, the R0 for measles in unvaccinated people is much higher than it is for vaccinated people--especially since the total number of unvaccinated people at Disneyland was almost certainly much, much smaller than the total number of vaccinated people so the per capita infection rate of people who didn't have a Measles vaccination was much much higher. Measles, along with pertussis, are almost certainly the undisputed champs of microbes that cause human disease, with R0 values that range between 12 and 18. (Likewise, the R0 of Ebola has changed over the course of the epidemic, since people in West Africa changed their behaviors between June and October, dramatically decreasing the spread of the disease.)
So it's contagious--very contagious--yet it isn't especially deadly. About one or two children out of every 1000 who get measles will die. That's a small number, but isn't so small that it should simply be dismissed as a trivial threat. (Plus, some measles outbreaks have been significantly more deadly over history. Moreover, there are many more people living with compromised immune systems than ever before not only due to AIDS but also to the explosion of transplant medicine, and such patients are almost certainly at higher risk of complications from measles.)
But when you take a fairly small risk and combine it with a high transmission rate, you suddenly see large effects in aggregate. At the end of the last century, the World Health Organization surveyed measles mortality and didn't like what it saw: there were still nearly three quarters of a million people who died from measles in 2000 alone; it led to a massive worldwide vaccination campaign, such that the total mortality in 2013 was under 150,000 and the estimated number of lives saved over that time period is estimated at more than 15 million.
Which underscores, in very stark light, the third point, which is that the measles vaccine is safe and effective. A back-of-napkin calculation shows us what would happen in in the US if we took after places like rural Africa and Asia in the 1990s prior to the vaccination campaign (the 1990s being the time when anti-vaccination really began to gain traction in the West). If we could magically "de-vaccinate" every child in the US, and spread measles like wildfire--which, as noted, isn't that hard--here is what we would see: there are about 60 million kids under age 18, but let's be conservative and take the 40 million kids at greatest risk of measles and their complications, those under age 10. Measles has about a 90 percent infection rate, which means that it would be hard to avoid crossing its path. An absolute upper limit, then, would be 36 million infections, but again let's play this very conservatively and assume only half get infected. That's 20 million infections. If we take the more optimistic fatality rate of 0.1 rather than 0.2 percent, that would lead to a crisp two hundred thousand deaths. Based on the most recent available data, the actual number of deaths that occur in this age range is about six thousand.
So: measles is transmissible, it is deadly in populations even if not exceptionally dangerous to an individual, and people can dramatically reduce their risk of getting it by getting vaccinated (and, unfortunately, not going to Disneyland right now).
Another way to define the bigness of a story is to compare it to other stories. And there has been another virus that's been in the news of late. If you were unaware, there's an Ebola outbreak going on in West Africa. Ebola is, for the purposes of this discussion, a very convenient mirror image of measles: it is not especially transmissible, but it is very deadly (how deadly in the US, where there is better health care, is not fully clear). The R0 value of Ebola is generally estimated to be about two. When one thinks of Thomas Eric Duncan, the Liberian man who was infected there but developed disease in Dallas and ultimately died at Dallas Presbyterian Hospital, one sees a perfect display of this. Recall that Duncan was initially turned away from Presby and returned to his apartment, living in close quarters with his partner and five children, until he returned and was appropriately quarantined. During this time he not only exposed his family, he exposed people in the emergency room twice, he exposed laboratory personnel who didn't realize they were handling a lethal virus, and he exposed doctors and nurses. Then he exposed even more health care workers after he was correctly diagnosed. And the number of people he infected in the Dallas area after all of that was...two. Exactly the correct value, and a demonstration that it's hard to catch Ebola! (I would know!)
But when we compare the relative splash of the Disneyland story to Ebola, it's clear that there's a hugely misplaced anxiety in one area, and not quite enough understanding of how serious a story it is in another. Tallying Google searches is hardly precise science, but it does give some sort of general indicator of the level of buzz about a given topic. Obviously, Ebola is a really important story, so one can't just compare the total number of hits that a search for "Ebola" generates (~220 million) to that for the Disneyland story (~5 million). On the other hand, some searches show a much greater preoccupation with what are, in fact, minor threats to the public:
"Ebola doctor New York"--30.5 million hits
"Ebola Dallas"--17.5 million
"measles Disneyland"--4.8 million
"Ebola subway New York"--3.6 million*
"risk of zombie attack"--600,000
What to take away from this? The overall Ebola story is big news, but millions of Americans are aware that Craig Spencer, the MSF physician who developed disease in New York City, took the subway and went bowling before becoming ill. In terms of risk, his subway travel and bowling adventures constituted as much a threat to the population of New York City as a zombie attack, yet this became a major source of Talmudic discussion among the chattering classes who seemed not to be aware that Spencer played everything by the book (which led to nobody being infected). The Google search on this non-story yielded only a little less than the number of hits of the Disneyland measles story.
The comparison is far from perfect, but it does I think broadly indicate the relative indifference the public has toward measles. The silver lining is that such ennui may be because most people really do have faith in vaccination and simply regard themselves as immune because of it (which is mostly true). On the other hand, this outbreak serves as an important moment to underscore what this virus is capable of doing, and just how big a lifesaver the measles vaccine really is. I don't think enough people are getting the message.
(*Searching using the Boolean operators "Ebola AND subway AND New York" yields a slightly more modest 2.7 million hits. It doesn't change the Disneyland number at all.)