Vaccines and Geniuses

As Orac points out today, Vox Day, one of the most obnoxious assholes that I've encountered on the net, is an anti-vaccine guy, who's been using the recent outbreak of mumps in Iowa (discussed by Tara here) to argue that vaccines must not work.

It's quite sad in its way: Vox is a self-proclaimed genius and member of Mensa. And yet, this is typical of his idea of argument. The "math" is so poor that it's laughable, and the rest of the reasoning is no better.

So, here's Vox:

From the Star & Sickle:

A mumps vaccine was introduced in 1967. Iowa law requires schoolchildren to be vaccinated against measles and rubella, and the mumps vaccine is included in the same shot. The state's last major outbreak was in 1987, when 476 people were infected.

Of the 245 patients this year, at least 66 percent had had the recommended two-shot vaccination, while 14 percent had received one dose, the Public Health Department said.

"The vaccine is working," Quinlisk said. "The vaccine certainly was made to cover this particular strain, because it's a fairly common strain of mumps." Quinlisk said the vaccine overall is considered about 95 percent effective.

So, the question is this: if the vaccine, not improved hygiene or some other factor, is primarily responsible for preventing transmission of the disease it is supposed to prevent, how is it possible that 80 percent of the infected in this latest outbreak are at least partially vaccinated against it?

And isn't it at least somewhat doubt-inspiring that the health authorities continue to insist that the vaccine is working in the face of direct evidence that, at least in some cases, it is not?

So, Vox wants to argue that vaccines don't really work, because there's a mumps outbreak in Iowa. And 80% of the cases are people who received at least one dose of the MMR vaccine.

Let's look at some numbers, to get an idea of what the infection rates mean. We'll be charitable, and stack all of the numbers in Vox's favor - so whatever we come up with is going to make the infection rates seem much higher than they are.

So - let's start by coming up with an estimate of the number of people in the pool of potentially infectable. Playing the numbers in Vox's favor, we'll assume that all of the mumps cases came from people born in the same year. The birth rate in Iowa is is pretty consistent, acording to the official statistics: around 38,000/year. Of those, the number vaccinated is around 81%, again according to the official statistics. So, the pool of vaccinated people born in that year is around 30,700.

Now, the cited effectiveness rate of the MMR against mumps is 95%. What that number means is that 95% of the people who receive the vaccine will develop an effective immunity to mumps. So - the expected number of vaccinated people who would not be immune would be around 1500 people (rounding down).

There are 245 infections this year, according to the article Vox cited. That would mean that 15% of the non-immune population of vaccinated people had become infected. That's a damned high number, but not completely beyond reason for something as contagious as mumps.

But then, remember that all of the numbers are stacked heavily in favor of making the vaccines look bad: in particular, these numbers are based on reducing the pool of potential infectables by several orders of magnitude (because we assume all cases were born in the same year, and all of them were born in Iowa) - we find that the number of infections with mumps is entirely possible. It makes for a pretty serious outbreak for sure - but not something that calls into question the effectiveness of vaccines.

Now, to put the nail in the coffin: back to some official statistics. How many cases of mumps were there before vaccines became mandatory in Iowa? Official statistics are here: during the decade before mumps vaccines became mandatory in Iowa, the number of infections per year ranged from 2000 to 12000 per year. Now, during a dramatic outbreak, described as an epidemic, we've got a couple of hundred cases. Does that look to you like the vaccines are not working?

Nah. Me neither.