Skeptical Raptor's Blog hunting pseudoscience in the internet jungle

Hey antivaccination gang–it’s really simple math

As you may be aware, there is a relatively large measles outbreak in New York City, hitting 26 individuals according to the most recent report (pdf) from the New York City Department of Health. An outbreak of 26 cases of measles may seem small, but compared to the historical average of around 60 measles cases per year for the whole United States, it really is a relatively large outbreak.

©2014, RubellaMeaslesInitiative

©2014, RubellaMeaslesInitiative

According to the most recent data, 12 of the cases are children and 14 are adults, and nine of 12 children were unvaccinated (2 were because parents got an exemption, and the other 7 because they were too young to be vaccinated with the MMR vaccine). In addition, it was difficult to determine the vaccination status of the adults, but we’ll focus on the children.

If you read the most obnoxious antivaccination websites (and I did it for you), you’d see claims that only 2 of the 26 were unvaccinated (simply not true or an ignorant misreading of the actual data), implying that 90% of those who caught measles were vaccinated. In fact, it’s at least 9 who were unvaccinated.

So let’s go with some simple math, just based on this small sample. But if the antivaccination lunatics are going to invent numbers, it is my job to obtain real numbers that show factually what is happening.

The outbreak is centered in Upper Manhattan and The Bronx areas of Manhattan, a total population of just under 2 million individuals. Now the outbreak is actually more focused than those areas, but it makes the math easier for the anti-science crowd.

I’m going to vastly oversimplify the risk of the measles outbreak, because I am aware that the antivaccine crowd is math challenged. If I used real epidemiological data, measure the risk in the exact geographical areas of the outbreak, data that I don’t have, the incidence rate would be much higher. If I did this by actual age, say 0-18 months, the risk would absolutely frighten you. But we’ll keep it simple.

  • Total risk for measles for vaccinated children, 3/496,850 or 6 out of one million
  • Total risk for measles for unvaccinated children, 9/53,372 or  169 out of one million

So, despite what you’ve heard from the antivaccination squad, the risk for contracting measles in an outbreak is nearly 30X higher in the unvaccinated group. Again, my numbers here vastly underestimate the risk, because the actual calculation would be done using data from the small area that this outbreak occurred, with the risk for the unvaccinated group probably being 10,000X higher than stated here. And I’m not even getting to risk reducing strategies like increasing the vaccination uptake rate to 95 or 96%, and the herd effect would have stopped this outbreak in its tracks.

It really is simple math. So simple that a caveman could do it.

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