Monday, April 18, 2011

When Analogies Attack!

Analogies can be very useful tools for illustrating a point. Advertisers and other PR types use them in commercials, politicians use them to support their causes or denigrate their opponents'. You can encapsulate concepts that could take entire books to explain and put them into a few sentences, illuminating that which may otherwise be too difficult to understand. Penn and Teller used a visual analogy to explain herd immunity in their Bullsh!t episode on vaccines, throwing balls at two groups of little figures. One group had a shield in front, thereby blocking the majority of balls, while the other group was hit every single time. In fact, analogies are such a valuable concept that we even test students on their ability to understand them.

Sadly, analogies are often prone to misunderstanding, themselves, or are so flawed as to make them entirely inapplicable to the concept being explained. I experienced this myself just recently on Twitter, as I got into a back-and-forth with someone, trying to explain how a disease like measles has a significantly higher rate of subsequent injury than the vaccine against it. It all culminated in my interlocutor offering this analogy...

Sandy is saying that adverse reactions to a vaccine are like intentionally letting a bomb explode in one school (presumably out of many) and saying that the injuries are okay because it wasn't very many. In her mind, vaccines are like bombs. Due to the limitations of Twitter, all I could respond was, "Your analogy is wrong." But that really isn't a very satisfying reply, hence, this post.

The problem is that Sandy's analogy is not quite accurate. First, she does not take into account that both vaccinating and not vaccinating have consequences. She focuses only on one, completely ignoring the other. Second, her analogy implies that vaccines have no value, that absolutely nothing good comes of it. Third, in the scenario of a bomb, everyone within the blast radius will definitely be harmed; not everyone who receives a vaccine will definitely be harmed. Further, a bomb damages the structure, so there will be more injuries caused by indirect effects, such as a wall or part of the ceiling collapsing. With a vaccine, if an injury occurs, it occurs only to the individual who received the vaccine.

Now, I will readily acknowledge that vaccines do carry risks. In some cases, the risks may outweigh any benefits received. For example, someone with a severe egg allergy and who would very likely suffer serious injury or potentially even death if they receive an influenza vaccine probably should not receive the vaccine. There may also be unforeseen risks, thing we don't yet know, but these reactions would have to be so rare that we don't know about them. Considering we know that, for example, encephalitis (swelling of the brain) following measles vaccination occurs at a rate of about 1 per 3 million doses, any adverse reaction that is currently unknown would have to occur at an even less frequent rate than 1/3,000,000 doses.

So, we know that vaccines carry risks, but we also know that the diseases they prevent carry risks. To go back to encephalitis, that occurs following infection with measles at a rate of around 1 per 1 thousand. In other words, you are 3,000 times more likely to get encephalitis from measles than from the vaccine. Let's use a visual analogy for that:

Don't get me wrong. That 1 case of encephalitis, whether from the disease or from the vaccine, is awful. I can't begin to imagine the emotional struggle that the family must deal with, let alone the effect on the child. But keep this in mind: if 3 million people get the vaccine, we'll expect around 1 case of encephalitis. If 3 million people get measles, we'll expect around 3,000 cases of encephalitis. Given the option, it makes a lot more sense to go with the lesser risk, i.e., the vaccine.

Back to Sandy's analogy of the bomb. Can we come up with something that more accurately describes what is going on with regard to benefits and risks? We could look at it like this. The choice to get vaccinated or not is like choosing to take self defense courses or not. If you take the self defense course, there is a chance that you may suffer some minor injuries (bruises, maybe a pulled muscle). There is also a small chance that you could break a bone. Even more rarely, you might suffer a very serious injury, perhaps resulting in paralysis or even death. Unlikely, but possible, especially considering the course takes precautions to minimize such serious injuries, but accidents can happen. If you do not take the course, you are not at risk for any of those injuries.

But the risk of injury isn't the only consequence of taking the course. One day, you're put into a situation where you are attacked. Running away is, for whatever reason, not an option. All you can do is fight. Since the self-defense course was really, really good, if you took it, you will almost certainly escape unharmed. At most, you may have a minor scratch or bruise. Your attacker doesn't want anything more to do with you, so he doesn't follow you around; he knows you mean business.

In contrast, if you did not take the course, you will certainly get beaten up pretty bad. You probably have at least some natural ability to defend yourself, so you probably will spend a week or two recovering, after which, you won't have any lasting injuries. There's a decent chance, though, that your ability to defend yourself isn't much of a match for your attacker and he beats the tar out of you. In that case, there's a very good chance that you will get a concussion, broken bones or even more severe injuries. There is also a pretty good chance that you will have permanent injuries or even that you could die, since your attacker isn't holding anything back and really wants to do you a good bit of violence. But it doesn't stop there. Assuming you survive, the attacker then follows you around for about a week and attacks everyone you pass by during that time: the people on the bus, those in the waiting room at the doctor's office, your family. Age doesn't matter for this guy. He'll attack adults and infants alike, all with the same force and persistence. Close your eyes and really picture this scenario for a moment. Picture the thug beating you up, then beating up every person you pass.

Vaccinating is like taking the defense course. There's some tiny bit of personal risk, but you're prepared for the attack and are nearly guaranteed to come out unscathed. Not vaccinating is like avoiding the defense course. Sure, you miss the tiny risk of minor injuries, but when the attack comes, you're guaranteed to feel some hurt, and there's a very real chance that it'll be bad.

In the end, when choosing whether or not to vaccinate, you need to consider not only the risks of the vaccine, but also the risks of the disease and the benefits of the vaccine. When you choose not to vaccinate, you choose not only to avoid the tiny risk of injury from the vaccine, but also accept the larger risk of injury from the disease. Not only that, but your choice affects more than just yourself. You are also making a decision for those around you; you are choosing to put them at risk of being attacked by the disease.

From a safety standpoint, the final balance is strongly in favor of vaccines.


  1. Thanks for this, Todd. It has always amazed me how some really weird arguments keep being used to avoid vaccination. I can understand, though. We tend to see the world immediately around us and the time immediately before us, all instead of the big picture.

  2. Here's another analogy that a reader sent to me via e-mail:

    An analogy I thought of a while back is this - during the London Blitz, "Blackouts" were instituted. Streetlights were turned off, windows were blocked, etc., all so that the German bombers would have less light to use to target the citizens of London. Note that if you used lights yourself, you saved yourself trouble and made yourself safer from the dangers of darkness-caused injury, and the odds that the bombers would hit your house if you were less than diligent about observing blackout rules were fairly small, though you were increasing the chance that your neighborhood would be hit - and fires started by bombs would spread to houses regardless of whether the blackout rules had been observed. "Blackout" regulations are analogous to vaccines, German bombs are analogous to infections, and fires are analogous to diseases.


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