Skipping the Measles Vaccine = Drunk Driving

There’s a lot of heat but not much light in most of the discussions around measles and whether or not to make vaccinations mandatory. It turns out that’s not the real question; all fifty states currently require the MMR (measles, mumps, and rubella) vaccine for admission into public schools.

The problem is that some schools allow exceptions based on “philosophical” and religious grounds. Even more troubling, sometimes the exception process can be pretty easy (e.g., the parent simply signs a form). Idaho’s exemption process is streamlined… and it shows. Last year, only 88% of that state’s kindergartners were vaccinated. Of the 1,540 exemptions granted, 1,304 were due to philosophical objections; only 89 were due to medical concerns. Mississippi, on the other hand, allows no exemptions due to philosophical or religious concerns. There, 99.7% of kindergartners were vaccinated.

The question of whether we should allow parents to skip mandatory MMR vaccinations on non-medical grounds isn’t as easy at it might first appear, for a couple of reasons. First, the stakes aren’t as high as the recent heated discussions might lead you to believe. In the 1960s, about 500 kids died each year from measles. At that time, there were about 64 million kids, which means that the chance of dying from measles each year was about 1 in 125,000 – slightly less than the chance of dying in a skydive. That said, the death of any child is a tragedy, and 500 preventable childhood deaths are worth our attention.

Indeed, by any reasonable measure, the benefits of the MMR vaccine far outweigh the risks. The argument against vaccination simply cannot be made based on sound, scientific evidence. Period. If you’re against vaccination, it’s not because the evidence is on your side.

This leads us to the second reason why revoking non-medical exemptions for vaccinations isn’t just a slam dunk. The US has a long and glorious history of allowing people to do all sorts of foolish things, as long as that foolish behavior doesn’t put other people at material risk.

How to Think Clearly About Making MMR Vaccines Fully Mandatory

There’s really no question about whether unvaccinated kids pose a risk of harm to others; they most certainly do. Even kids who have been fully vaccinated (i.e., the recommended two-dose MMR regimen) have a 3% chance of getting infected if they’re exposed to someone with the measles, and people who are immunocompromised can’t be vaccinated even though they’re at elevated risk.

When your behavior imposes a cost or a risk on me or others, economists say that you’re creating a “negative externality” – “negative” because it’s something we’d like to avoid, and “externality” because the costs are borne by people who didn’t engage in the behavior.

Life, of course, is full of negative externalities: people who smell bad because they don’t shower, who wear clothes that we find unattractive, who talk too loudly on their cell phones, who allow their dogs to poop on our lawns, and almost all teenagers at some point. Typically, we don’t restrict the undesirable behavior of others until the negative externality is big enough.

That’s the crux of the matter: each kid who doesn’t get vaccinated causes a risk of harm to those who have and to those who can’t. The fundamental question is whether the risk that unvaccinated kids impose on others is big enough to warrant regulating.

Risks Worth Regulating

When is a risk big enough for the government to step in? Drunk driving is a good example. When people drive drunk, they vastly increase the chance of being involved in a fatal accident. About 2,000 people – folks who weren’t the drunk driver or passengers in the drunk driver’s car and therefore had no way to avoid the risk – are killed each year due to drunk drivers. That’s a negative externality: drunk driving puts people at risk who weren’t engaging in the behavior.

Just how big is the risk imposed by drunk driving? There are about 112 million episodes of drunk driving annually, resulting in those 2,000 deaths. That means that each time someone drives drunk, he’s imposing an externality of about 1 in 56,000 (2,000 deaths per 112 million episodes) chance of death to those around him. Society has deemed that risk as big enough to prohibit.

Now let’s think about what this might mean for making the MMR vaccine truly mandatory. My back of the envelope estimate is that failure to vaccinate your kid imposes a comparable level of risk on those kids who either did get vaccinated or can’t get vaccinated for medical reasons: about 1 in 56,000. (The math and assumptions are at the end of this essay.)

What to Do About People Who Refuse to Vaccinate Their Kids

If my estimate is in the ballpark, eliminating philosophical and religious exceptions to vaccination makes a lot of sense. After all, we don’t allow drunk drivers off the hook just because they don’t believe they’re imposing a risk to others or because their religion requires that they drink and drive.

If we don’t have the political will to do away with exemptions on grounds other than medical concerns, there are other options that maintain freedom of choice while advantaging better decisions. We should borrow a couple of tools from the behavioral sciences.

First, we should make it as easy as possible for parents who want to get their kids vaccinated to do so. For example, schools could pre-schedule kids for free or low-cost vaccinations prior to the start of the school year. Next, we could require every parent to listen to a two-minute presentation of the facts about vaccination, followed by an active decision about whether or not to have their kid vaccinated (i.e., active choice). Those who elect not to vaccinate their kids would be required to pay a fee (the proceeds of which could be used to subsidize the cost of vaccinations for those who choose vaccination). In addition, those who choose not to vaccinate their kids could be required to sign a form stating that they understand that, based on consensus assessment of the scientific evidence, unvaccinated children pose a risk of illness and death to other children. Finally, on request, any parent of a child at the school would be given a list of the children who have not been vaccinated due to non-medical reasons.

Of course, such schemes incur some costs and lots of hassles. But for heaven’s sake whatever we do, it should take more effort opt out of vaccinating your kid than it does to get her vaccinated. It’s bad enough that some parents choose not to vaccinate their kids against the measles, mumps and rubella. It’s worse that they’re allowed to put other kids at risk because of their “philosophy.” It’s crazy that we would make it too easy to for them to do that. That just makes no sense at all.

 *  * *

Estimating the Risk Imposed by Unvaccinated Kids on Other KIDS

Let’s suppose that we’re looking at a group of 100 children (a frightening thought itself) who are going to spend all of their school years together (i.e., kindergarten through high school). We’ll assume that 10 of the kids haven’t been vaccinated, 89 have, and 1 is immunocompromised. Now suppose one of those kids catch the measles (e.g., by traveling abroad), exposing all of the other kids to the virus. We’ll assume that 9 of the 10 unvaccinated kids get the measles (i.e., 90% chance of infection among those who are unvaccinated), that 2 of the 89 get the measles (i.e., 3% chance of infection among those who get the recommended two doses of the vaccine), and that the one immunocompromised kid gets it as well. That means that the 10 kids who didn’t get vaccinated caused three cases of measles that wouldn’t have happened otherwise.

So what? Well, each of those three kids now faces 0.2% chance of death. (It’s actually higher than that because the fatality rate for the immunocompromised kid is probably higher than average.) We’ll call that a net risk of 0.6% (= 3 kids times 0.2% per kid).

All of this numeric acrobatics assumes that there’s been a measles outbreak in the group. That’s not a sure thing, so let’s say that only happens 3% of the time. That is, we will assume that the chance of an outbreak in this group of 100 kids is 3% for the entire 12 or more years that they are in school together. (Prior to the vaccine’s availability, more than 90% of kids got the measles by the time they were 15.) If there’s a 3% chance of an outbreak, then those 10 unvaccinated kids impose on average a 0.018% chance of death (3% x 0.6%). If we divide that burden among the 10 of them, they each impose a 0.0018% chance of death on someone else. That’s a 1 in 56,000 chance of death.

Bottom line: if 10% of kids don’t get vaccinated, the analysis above (given its assumptions) estimates that each unvaccinated child imposes a risk of death on those who did get vaccinated (or who cannot) of 1 in 56,000. This may be an underestimate of the risk imposed because the MMR vaccine covers not just measles, but also mumps and rubella. Importantly, it also excludes the risk posed to the child whose parent declined to have them vaccinated.


Idaho vs. Mississippi: See

500 measles deaths per year in 1960: See Strebel PM, Papania MJ, Dayan GH, Halsey NA. Measles vaccine. In: Plotkin SA, Orenstein WA, Offit PA eds. Vaccines. 5th ed. Philadelphia, Pa: WB Saunders; 2008: 358

64.5 million kids ages 0 – 17 yrs in 1960: See

1,980 deaths imposed by drunk drivers among occupants of other vehicles and non-occupants (2012 data): See Dept of Transportation (US), National Highway Traffic Safety Administration (NHTSA). Traffic Safety Facts 2012: Alcohol-Impaired Driving. Washington (DC): NHTSA; 2014 [cited 2014 Sep 30]. Available at URL:

112M episodes of drunk driving per year (2010 data): See Vital Signs: Alcohol-Impaired Driving Among Adults — United States, 2010 [Internet]. [cited 2015 Feb 13];Available from:

3% chance of infection given exposure to measles, w/ two rounds of vaccine:

0.2% case fatality rate for measles:

Prior to the vaccine, 90% of kids had the measles by age 15: See

4M cases of measles per year in 1960: Langmuir AD. Medical importance of measles. Am J Dis Child 1962;103:54-56

90% chance of infection given exposure, w/o vaccination:

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