I do not understand the decision not to vaccinate one’s children under the best of circumstances.

Even when the community in which a family is living is healthy and the surrounding herd immunity is robust, the evidence in favor of protecting one’s kids is weighted so heavily in favor of immunization that opting out is nothing short of baffling to me. The best I can do is chalk it up to a fear that medical science is powerless to allay, and accept that even my best arguments fall short for some people.

But you’d think that perhaps the unfounded and inchoate fear of vague harm from vaccines might be overcome by a more imminent fear of actual disease.

After all, with outbreaks of vaccine-preventable illness cropping up all over the place, even famous vaccination opponents are trying to toss their past statements against the practice down the memory hole.

It stands to reason that, were some illness spreading within a family’s local area, surely they would err on the side of protecting their own children from it if an effective preventive measure were available. If whooping cough were knocking at parents’ doors, wouldn’t that be enough to sway them toward vaccinating their kids against it?

The answer, as it happens, is no.

According to a study presented this week at the annual meeting of the Pediatric Academic Societies, vaccine rates in Washington state did not increase despite an epidemic of pertussis (whooping cough) there. That epidemic sickened thousands of individuals from October 2011 through December 2012. Over a similar span of time the Centers for Disease Control received reports of 20 pertussis-related deaths nationwide, the majority of which were in infants.

What didn’t happen in Washington during that span of time was more people choosing to vaccinate their kids against pertussis. The study’s authors had hypothesized that the threat of a potentially fatal disease nearby would lead hesitant parents to have their children immunized, and were surprised to discover no change in statewide immunization rates.

If, as Ralph Waldo Emerson said famously, a foolish consistency is the hobgoblin of little minds, then this kind of recalcitrance is a rampaging balrog. Relying on the good decisions of other parents to protect your own kids is bad enough, but refusing to revise your own bad decisions in the face of a public health crisis in your very community (and to which your refusal is contributing) is utterly stupefying.

You will convince me of the literal existence of actual balrogs before you make sense of it to me.

Unfortunately, this new study is in keeping with an earlier study that showed parents who refuse vaccines for their children only grow more resistant when given evidence of their safety. Rather than leading to a change of heart, being presented with information about immunizations’ safety and effectiveness just makes denialists dig in their heels more. For my part, I now decline to bash my head against that particular wall any longer.

If recognized and treated early enough, the effects of infection with pertussis can be somewhat mitigated. Antibiotics are usually (but not always) effective in the treatment of certain other vaccine-preventable illnesses, as well. But given the powerful regimens often required to cure infected patients and the spreading crisis of worldwide antibiotic resistance, preventive measures like comprehensive vaccination programs will only become more important, not less.

However, these programs are only as effective as people’s participation in them will allow. Refusing to vaccinate your kids, especially in the midst of an epidemic, makes everyone less safe—and is everyone’s problem. Unfortunately, this new study shows how intractable that problem truly is. If the real threat of life-threatening disease isn’t enough to change some people’s minds, I have no idea what will be.