We start with the principle that a person is obligated to refrain from causing harm (or significant risk of harm) to others, and obligated to prevent harm to others, when the costs to the person are relatively small and the benefits great, unless the relevant others give their consent. In saying a person is obligated, we mean that unless the person has a reasonable excuse, or there are additional reasons that outweigh the obligation, the person is blameworthy for his conduct.
Ah, yes, "significant", "relatively", "relevant", "reasonable excuse", "additional reasons that outweigh" -- with this many levels of qualification, each of which involves some degree of judgment call, you are no longer dealing with any kind of obligation at all. This is particularly true given that they will immediately go on and note that this principle, already enervated by qualifications, will certainly need further qualifications and exceptions. I've talked before about obligation creep, as well as the overmoralization of arguments, and this is a good example of both. This many qualifications is a guarantee that you are really dealing with "more reasonable than not" rather than with "obligatory"; but, of course, if you admit that it is a matter of personal judgment, even if one that will usually go one way in reasonable people, you can't use that to try to push people into it. It's the rhetorical goal that's imposing the pressure to manufacture an obligation that the argument's very structure eliminates. And there are certainly more factors involved than purely moral ones; trust in doctors, in medical research, in medical supply chains, and the difficulty of informed about the safety and effectiveness of every vaccine, to name just four that come up when dealing with the anti-vaccination movement. Vaccination decisions obviously have a moral component, which may sometimes be significant, but mostly they are just practical safety decisions, like deciding whether it is worthwhile to buy extra insurance or to learn CPR. Philosophers, focusing on the moral components of the decision, tend to treat them as if they were the only important elements at all; and, of course, overmoralizing an argument also has its rhetorical use in trying to push people to do something.
Vaccination arguments are also encouraged along both of these paths by the tendency to exaggerate the agency of those who are not vaccinating. The worst offenders are the "they are literally killing people" arguments, but there are milder cases of the same disease. Copp and Dworkin say:
We think there are five different harms or risks of harm. First, non-vaccinators create a risk that they will transmit the disease to others if they get it. Second they actually do harm to these others if they get the disease and transmit it. Third, they are weakening the community’s protective herd immunity, even if only to a small degree, which increases the risk to everyone that the disease will spread in the population. Fourth, if herd immunity has been established by others, who have gone to the trouble of getting vaccinated, the non-vaccinators are free-loading. Finally, fifth, if the non-vaccinators have children and refuse to vaccinate them, then they are creating a risk to their children.
Non-vaccinators aren't creating a risk; the risk is the default, not something created by them. Instead, they are not reducing the risk. While it's true that they do harm if they get or transmit the disease, neither getting nor transmitting the disease is usually intentional; getting it is usually an accident and transmission most likely when you don't know you have it, or when you don't know that your safeguards from transmitting it have failed. Thus, despite the way Copp and Dworkin have stated it, this falls not under the "causing harm" but the more complicated "preventing harm to others" part of the principle. Non-vaccinators are not 'weakening herd immunity'; again, lack of herd immunity is the default, and therefore they are failing to contribute to building it up. We are almost all failing to contribute to some kind of herd immunity, because most of us have not had the hundreds of shots that would be required to vaccinate against all the major diseases that have vaccines. Whether someone is free-loading cannot be determined prior to determining that they already have the obligation, so it should not even be a consideration at this stage of the argument. And, again, the risk to the children is the default; non-vaccinators are not reducing it rather than creating it.
All of these (except the free-loading one, which, as I noted, is not even determinable at this stage of the argument) are things that can be morally serious. They are morally serious as a matter of negligence, not as a matter of direct causal action. Yet over and over again, non-vaccinators are treated as if they were directly causing things that they are not causing. Failing to take basic fire prevention steps is a very serious matter, but it is not arson, and someone who kept trying to treat it as arson would quite clearly be trying to pull a fast one for rhetorical effect. The reason is obvious: moral principles apply relatively straightforwardly to direct causation, but their application to non-preventions and failures of risk-reduction can get very tortuous and complicated. This overstating of agency, too, seems to be a reason for obligation creep and overmoralization of arguments. It attempts to cut the knot. But cutting the knot is not untying it, and trying to force through a conclusion causes problems down the line.
