Diary

Of Herds and Harlots, Measles is Back

Come, let us reason together.  Mark Twain famously quoted British PM Benjamin Disraeli, “there are three kinds of lies:  lies, damned lies, and statistics.”  Generally, this is true, but sometimes statistics don’t lie.  In the case of measles, the statistics speak truth.

The measles vaccine was developed in 1963, and combined with the mumps and rubella vaccines in 1971, known as the MMR vaccine.  Children are given the first shot between 12-15 months old, and another between 4 and 6 years old.  Thus has it been for well over 40 years.  In the 52 years since the measles vaccine as developed, the number of cases in America went from 400,000 to under 100 (from .2% to .00003%), asymptotic to zero, as statisticians would say.  That’s the story of an effective vaccine.

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Yet people are herd animals, and we follow the rear end of whatever we’ve got our nose behind.  A whole bunch of Americans have chosen to follow anti-vaccine activists like former Playboy bunny Jenny McCarthy, believing that they can be exceptions to the rule.  They believe a pernicious and discredited report that links MMR to autism.  The honest answer is that we don’t know what causes autism.  We do know, however, what causes measles, and we also know exactly what defends against it.  In poorer countries without adequate health services, measles remains a killer, especially of children.  According to WHO:

The disease remains one of the leading causes of death among young children globally, despite the availability of a safe and effective vaccine. Approximately 145,700 people died from measles in 2013 – mostly children under the age of 5.

Yet in California, like 19 states which allow vaccination opt-out without a reason, vaccination rates can drop significantly below the U.S. national rate of 92%.  At 90-95%, what epidemiologists call “herd immunity” kicks in, and it’s the most important protection we have for our children, for two reasons.  First, babies under a year old aren’t immunized, and are at maximum risk of contracting the disease.  Second, the vaccine is not 100% effective, which is why two shots are given.  After the first shot, only 93-95% have sufficient immunity, and after the second, that number rises to 97%.  So 3% of vaccinated children and even adults remain susceptible to measles.  The key is having enough immunity as a whole that there are less people who contract and spread the disease to those who are vulnerable.

The math just works out.  It always has.

Consider Disneyland.  It’s in California, where the vaccinated rate is well under herd immunity levels, at a small space with lots of foreign visitors, who may not be vaccinated, and populated by children of all ages.  If researchers designed a large-scale epidemiological experiment station to test the measles statistical model, Disneyland is the perfect fit.  And in December, visitors to the Happiest Place on Earth became unwitting participants in the experiment—totally by chance—when 52 of them contracted measles.

So how does that explain what happened in Disneyland? If you have a group of 1,000 people concentrated in a small space—like oh, say, hypothetically, an amusement park—about 90 percent of them will be vaccinated (hopefully). One person, maybe someone who contracted measles on a recent trip to the Philippines, moves around, spreading the virus. Measles is crazy contagious, so of the 100 people who aren’t vaccinated, about 90 will get infected. Then, of the 900 people who are vaccinated, 3 percent—27 people—get infected because they don’t have full immunity.

Now the Disneyland numbers—six vaccinated infections out of the 34 cases with known records—start to make more sense. (And considering the 16 million or so visitors the park gets every year, we might reasonably expect that number to go up.) Once vaccination levels dip below 90 or 95 percent, there aren’t enough protected people to keep the disease in check—the herd immunity that epidemiologists like to talk about so much. In the US, we’ve been doing pretty well keeping those numbers up. “But there are some fluctuations,” says Cristina Cassetti, program officer at the National Institute of Allergy and Infectious Diseases, “and if vaccination levels dip down a little, you get a situation like Disneyland.”

Herd behavior matters.  If we allow our vaccination rates to fall below the herd immunity level, measles cases go up.  And not just up, but up exponentially, because measles is virulent to the max.  In 2014, measles infected between 635 and 644 in the U.S., a 400-fold increase over previous years.  This has profound health and societal consequences.  In Palm Springs, 66 high school students were sent home until February 9th unless they confirm they’ve received MMR vaccines or show they are immune to measles.  Does this make sense?  Let’s do the numbers:  if up to 3% of the high school students are susceptible, and 1,980 students attend, then one infected student could yield up to 60 cases.  And those 60 cases could infect children and babies who lack immunity.

This isn’t the first time an advanced western democracy with a solid healthcare system has failed to protect against measles either.  The United Kingdom let its immunization rate fall to under 80%, and between 2012 and 2013, they’ve seen 1,200 children infected—a threefold increase.

If you’re one of the holdouts on vaccination, reconsider where you’re leading, and who you’re following.  Because following the herd behind a harlot, whose qualifications to set health policy are exactly zero, into the dark night of daftness is no way to live in a first-world society.

(images credit Shutterstock, The Economist)

(crossposted from sgberman.com)