At my last visit to urgent care with one of my kids, the doctor who saw us mentioned that there is currently an epidemic of pertussis (whooping cough) in California, one that presents serious danger for the very young children (among others) hanging out in the waiting area. We double-checked that both my kids are current on their pertussis vaccinations (they are). I checked that I was current on my own pertussis vaccination back in December when I got my flu shot.

Sharing a world with vulnerable little kids, it’s just the responsible thing to do.

You’re already on the internet reading about science and health, so it will probably come as no surprise to you that California’s pertussis epidemic is a result of the downturn in vaccination in recent years, nor that this downturn has been driven in large part by parents worried that childhood vaccinations might lead to their kids getting autism, or asthma, or some other chronic disease. Never mind that study after study has failed to uncover evidence of such a link; these parents are weighing the risks and benefits (at least as they understand them) of vaccinating or opting out and trying to make the best decision they can for their children.

The problem is that the other children with which their children are sharing a world get ignored in the calculation.

Of course, parents are accountable to the kids they are raising. They have a duty to do what is best for them, as well as they can determine what that is. They probably also have a duty to put some effort into making a sensible determination of what’s best for their kids (which may involve seeking out expert advice, and evaluating who has the expertise to be offering trustworthy advice).

?But parents and kids are also part of a community, and arguably they are accountable to other members of that community. I’d argue that members of a community may have an obligation to share relevant information with each other — and, to avoid spreading misinformation, not to represent themselves as experts when they are not. Moreover, when parents make choices with the potential to impact not only themselves and their kids but also other members of the community, they have a duty to do what is necessary to minimize bad impacts on others. Among other things, this might mean keeping your unvaccinated-by-choice kids isolated from kids who haven’t been vaccinated because of their age, because of compromised immune function, or because they are allergic to a vaccine ingredient. If you’re not willing to do your part for herd immunity, you need to take responsibility for staying out of the herd.

?Otherwise, you are a free-rider on the sacrifices of the other members of the community, and you are breaking trust with them.

I know from experience that this claim upsets non-vaccinating parents a lot. They imagine that I am declaring them bad people, guilty of making a conscious choice to hurt others. I am not. However, I do think they are making a choice that has the potential to cause great harm to others. If I didn’t think that pointing out the potential consequences might be valuable to these non-vaccinating parents, at least in helping them understand more fully what they’re choosing, I wouldn’t bother.

So here, let’s take a careful look at my claim that vaccination refuseniks are free-riders.

?First, what’s a free-rider?

?In the simplest terms, a free-rider is someone who accepts a benefit without paying for it. The free-rider is able to partake of this benefit because others have assumed the costs necessary to bring it about. But if no one was willing to assume those costs (or indeed, in some cases, if there is not a critical mass of people assuming those costs), then that benefit would not be available, either.

?Thus, when I claim that people who opt out of vaccination are free-riders on society, what I’m saying is that they are receiving benefits for which they haven’t paid their fair share — and that they receive these benefits only because other members of society have assumed the costs by being vaccinated.

?Before we go any further, let’s acknowledge that people who choose to vaccinate and those who do not probably have very different understandings of the risks and benefits, and especially of their magnitudes and likelihoods. Ideally, we’d be starting this discussion about the ethics of opting out of vaccination with some agreement about what the likely outcomes are, what the unlikely outcomes are, what the unfortunate-but-tolerable outcomes are, and what the to-be-avoided-at-all-costs outcomes are.

?That’s not likely to happen. People don’t even accept the same facts (regardless of scientific consensus), let alone the same weightings of them in decision making.

?But ethical decision making is supposed to help us get along even in a world where people have different values and interests than our own. So, plausibly, we can talk about whether certain kinds of choices fit the pattern of free-riding even if we can’t come to agreement on probabilities and a hierarchy of really bad outcomes.

?So, let’s say all the folks in my community are vaccinated against measles except me. Within this community (assuming I’m not wandering off to exotic and unvaccinated lands, and that people from exotic and unvaccinated lands don’t come wandering through), my chances of getting measles are extremely low. Indeed, they are as low as they are because everyone else in the community has been vaccinated against measles — none of my neighbors can serve as a host where the virus can hang out and then get transmitted to me. (By the way, the NIH has a nifty Disease Transmission Simulator that you can play around with to get a feel for how infectious diseases and populations whose members have differing levels of immunity interact.)

?I get a benefit (freedom from measles) that I didn’t pay for. The other folks in my community who got the vaccine paid for it.

?In fact, it usually doesn’t require that everyone else in the community be vaccinated against measles for me to be reasonably safe from it. Owing to “herd immunity,” measles is unlikely to run through the community if the people without immunity are relatively few and well interspersed with the vaccinated people. This is a good thing, since babies in the U.S. don’t get their first vaccination against measles until 12 months, and some people are unable to get vaccinated even if they’re willing to bear the cost (e.g., because they have compromised immune systems or are allergic to an ingredient of the vaccine). And, in other cases, people may get vaccinated but the vaccines might not be fully effective — if exposed, they might still get the disease. Herd immunity tends to protect these folks from the disease — at least as long as enough of the herd is vaccinated.

?If too few members of the herd are vaccinated, even some of those who have borne the costs of being vaccinated (because even very good vaccines can’t deliver 100% protection to 100% of the people who get them), or who would bear those costs were they able (owing to their age or health or access to medical care), may miss out on the benefit. Too many free-riders can spoil things even for those who are paying their fair share.

?A standard reply from non-vaccinating parents is that their unvaccinated kids are not free-riders on the vaccinated mass of society because they actually get diseases like chicken pox, pertussis, and measles (and are not counting on avoiding the other diseases against which people are routinely vaccinated). In other words, they argue, they didn’t pay the cost, but they didn’t get the benefit, either.

?Does this argument work?

?I’m not convinced that it does. First off, even though unvaccinated kids may get a number of diseases that their vaccinated neighbors do not, it is still unlikely that they will catch everything against which we routinely vaccinate. By opting out of vaccination but living in the midst of a herd that is mostly vaccinated, non-vaccinating parents significantly reduce the chances of their kids getting many diseases compared to what the chances would be if they lived in a completely unvaccinated herd. That statistical reduction in disease is a benefit, and the people who got vaccinated are the ones paying for it.

?Now, one might reply that unvaccinated kids are actually incurring harm from their vaccinated neighbors, for example if they contract measles from a recently vaccinated kid shedding the live virus from the vaccine. However, the measles virus in the MMR vaccine is an attenuated virus — which is to say, it’s quite likely that unvaccinated kids contacting measles from vaccinated kids will have a milder bout of measles than they might have if they had been exposed to a full-strength measles virus out in the wild.? A milder case of measles is a benefit, at least when the alternative is a severe case of measles. Again, it’s a benefit that is available because other people bore the cost of being vaccinated.

?Indeed, even if they were to catch every single disease against which we vaccinate, unvaccinated kids would still reap further benefits by living in a society with a high vaccination rate. The fact that most members of society are vaccinated means that there is much less chance that epidemic diseases will shut down schools, industries, or government offices, much more chance that hospitals and medical offices will not be completely overwhelmed when outbreaks happen, much more chance that economic productivity will not be crippled and that people will be able to work and pay the taxes that support all manner of public services we take for granted.
?The people who vaccinate are assuming the costs that bring us a largely epidemic-free way of life. Those who opt out of vaccinating are taking that benefit for free.

?I understand that the decision not to vaccinate is often driven by concerns about what costs those who receive the vaccines might bear, and whether those costs might be worse than the benefits secured by vaccination. Set aside for the moment the issue of whether these concerns are well grounded in fact. Instead, let’s look at the parallel me might draw: ?If I vaccinate my kids, no matter what your views about the etiology of autism and asthma, you are not going to claim that my kids getting their shots raise your kids’ odds of getting autism or asthma. But if you don’t vaccinate your kids, even if I vaccinate mine, your decision does raise my kids’ chance of catching preventable infectious diseases. My decision to vaccinate doesn’t hurt you (and probably helps you in the ways discussed above). Your decision not to vaccinate could well hurt me.

?The asymmetry of these choices is pretty unavoidable.

?Here, it’s possible that a non-vaccinating parent might reply by saying that it ought to be possible for her to prioritize protecting her kids from whatever harms vaccination might bring to them without being accused of violating a social contract.

?The herd immunity thing works for us because of an implicit social contract of sorts: those who are medically able to be vaccinated get vaccinated. Obviously, this is a social contract that views the potential harms of the diseases as more significant than the potential harms of vaccination. I would argue that under such a social contract, we as a society have an obligation to take care of those who end up paying a higher cost to achieve the shared benefit.

?But if a significant number of people disagree, and think the potential harms of vaccination outweigh the potential harms of the diseases, shouldn’t they be able to opt out of this social contract?

?The only way to do this without being a free-rider is to opt out of the herd altogether — or to ensure that your actions do not bring additional costs to the folks who are abiding by the social contract. If you’re planning on getting those diseases naturally, this would mean taking responsibility for keeping the germs contained and away from the herd (which, after all, contains members who are vulnerable owing to age, medical reasons they could not be vaccinated, or the chance of less than complete immunity from the vaccines). No work, no school, no supermarkets, no playgrounds, no municipal swimming pools, no doctor’s office waiting rooms, nothing while you might be able to transmit the germs. The whole time you’re able to transmit the germs, you need to isolate yourself from the members of society whose default assumption is vaccination. Otherwise, you endanger members of the herd who bore the costs of achieving herd immunity while reaping benefits (of generally disease-free work, school, supermarkets, playgrounds, municipal swimming pools, doctor’s office waiting rooms, and so forth, for which you opted out of paying your fair share).

?Since you’ll generally be able to transmit these diseases before the first symptoms appear — even before you know for sure that you’re infected — you will not be able to take regular contact with the vaccinators for granted.

?And if you’re traveling to someplace where the diseases whose vaccines you’re opting out of are endemic, you have a duty not to bring the germs back with you to the herd of vaccinators. Does this mean quarantining yourself for some minimum number of days before your return? It probably does. Would this be a terrible inconvenience for you? Probably so, but the 10-month-old who catches the measles you bring back might also be terrible inconvenienced. Or worse.

Here, I don’t think I’m alone in judging the harm of a vaccine refusenik giving an infant pertussis as worse than the harm in making a vaccine refusenik feel bad about violating a social contract.

?An alternative, one which would admittedly required some serious logistical work, might be to join a geographically isolated herd of other people opting out of vaccination, and to commit to staying isolated from the vaccinated herd. Indeed, if the unvaccinated herd showed a lower incidence of asthma and autism after a few generations, perhaps the choices of the members of the non-vaccinating herd would be vindicated.

?In the meantime, however, opting out of vaccines but sharing a society with those who get vaccinated is taking advantage of benefits that others have paid for and even threatening those benefits. Like it or not, that makes you a free-rider.
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An earlier version of this essay originally appeared on my other blog.