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What would it take for measles to not be eliminated anymore in the United States?

There used to be a time in the history of the United States when measles was a big deal. The measles vaccine changed that. Starting in 1964, cases of measles in the United States dropped precipitously when the measles vaccine was licensed. A second dose was recommended in the late 1980s, and the total number of measles cases each year really dropped to almost none. The level was so low that measles was declared “eliminated” in North America (after Mexico and Canada also reached near-universal vaccine coverage).

Note that these are cases of measles, not deaths.

Note that these are cases of measles, not deaths.

But then we had another change. Beginning with a fraudulent paper in the late 1990s and continuing with all sorts of misinformation and exaggeration of the risks of the measles vaccine (a part of the Meases-Mumps-Rubella [MMR] vaccine), levels of vaccination coverage only flirted with the level needed to prevent outbreaks. There were still enough who were not vaccinated and traveled to parts of the world where measles is still endemic (always present, though at low levels) or places where measles is epidemic (present at levels higher than normal or expected). Those people came back and brought back measles. Combine this with the fact that the MMR vaccine is not 100% effective, and you have a good proportion of the US population that is not immune, kicking off measles outbreaks.

So far in 2014, the Centers for Disease Control and Prevention (CDC), has reported on hundreds of cases from dozens of outbreaks in the US since the disease was declared “eliminated”. All of these cases and outbreaks can be attributed to the importation of measles by non-immune people. They were non-immune because they were never vaccinated, vaccinated but the vaccine did not take (it’s not 100% effective), or never otherwise developed immunity to it (e.g. didn’t have measles as children). Again, the good news is that we don’t have continuous transmission of measles… Yet.

A paper published in the American Journal of Epidemiology looks at these outbreaks and analyzes what it would take for measles to come back and be endemic in the United States again, or, at the very least, cause sustained epidemics. The paper is titled “Identifying Postelimination Trends for the Introduction and Transmissibility of Measles in the United States” by Blumberg et al. The authors looked at the sizes of transmission chains in outbreaks of measles and developed a mathematical model for determining the infectivity of measles and what several milestones would be for sustained transmission. In the paper, the authors concluded that the average size of a transmission chain is about 2 cases and that about half of cases are imported to the United States. In other words, a case goes out and brings measles and then infects one more person here, on average. Some chains are considerably larger. Other chains are not chains at all but single cases who return to highly immune communities. This assumes that all cases are properly reported to public health, which is not always the case from my experience.

On its face, this is good news, right? Someone is foolish enough to not be vaccinated, goes to an endemic place, gets measles and comes back. But, because the rest of us are sensible to the needs of our collective public health, we “cocoon” the infected person and keep them from infecting many others. According to the authors of the paper I cited above, there is only a 15% chance that this person kicks off an outbreak. But the problem is a little more complicated than that. The authors note that CDC has reported that up to 12% of cases don’t have a known exposure. That changes the math a little bit. Instead of having 2.1 people in the average chain, the chain is now 2.2 people. A small-but-significant change if you consider how many of these cases we’re seeing.

And then it gets even more complicated.

The math used by the authors took into consideration vaccine coverage in the United States as reported by CDC. If they dropped vaccine coverage by 1%, the average infection chain becomes 2.8 people. Bring that coverage down to 93.9% from 95.9% (a decrease of two percentage points), and the chain jumps to a whopping 4.3 people on average. Let that sink in a little. A simple drop in two percentage points in our current MMR coverage pretty much doubles the number of people infected with measles from people who bring it from overseas, according to the mathematical model presented by the authors. (It’s a good model, by the way. I’m not an ace in biostatistics, but their methodology is sound.) The authors couldn’t have said it better:

“Our projections for the impact of a decrease in population immunity are dramatic. Although there are many caveats to the analysis, the possibility that the average size of chains can more than double with a 2% drop in population immunity motivates a closer examination of the data to ensure that transmission is being maintained at low levels.”

I would take this one step further and say that we need to work very, very hard to keep that 2% drop from happening, and prepare those communities where this drop is being seen to properly allocate the resources for when it happens. It’s going to be a rough ride if it does, with each case of measles costing a lot of money to investigate, follow-up, and, let’s hope not, hospitalize and treat.

The only good news in this analysis is that the transmissibility of measles appears to remain well below 1.0. This means that one case of measles doesn’t give it to more than one other person on average. However, if it does reach 1.0, then we have a problem. A big problem. Measles becomes endemic again and, once again, we have to push the MMR vaccine on a public whose mistrust of vaccines is pretty high. How can it reach 1.0? Through a combination of less immune people, more importation, or both. (The very, very unlikely possibility of a virus mutation exists, but the virus is a pretty stable one.) Heck, even if the magic number is not reached, we’re trending toward bigger outbreaks that are more sustained.

And that’s just measles. We have other diseases making a comeback, like mumps, chickenpox, and whooping cough. But that’s all for a different post.

Here’s the citation of the paper:

Practice of Epidemiology:
Seth Blumberg, Wayne T. A. Enanoria, James O. Lloyd-Smith, Thomas M. Lietman, and Travis C. Porco
Identifying Postelimination Trends for the Introduction and Transmissibility of Measles in the United States
Am. J. Epidemiol. first published online April 30, 2014 doi:10.1093/aje/kwu068

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René F. Najera, DrPH

I'm a Doctor of Public Health, having studied at the Johns Hopkins University Bloomberg School of Public Health.
All opinions are my own and in no way represent anyone else or any of the organizations for which I work.
About History of Vaccines: I am the editor of the History of Vaccines site, a project of the College of Physicians of Philadelphia. Please read the About page on the site for more information.
About Epidemiological: I am the sole contributor to Epidemiological, my personal blog to discuss all sorts of issues. It also has an About page you should check out.

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