Who’s Your Herd?

I’m not an epidemiologist. This morning, I had decided I was going to write a post about how I am not an epidemiologist, but that was before I saw this WIRED opinion piece, in which Sarah Fallon makes the point, essentially, that herd immunity is high enough in California and people can’t sue vaccine manufacturers (not true), so California doesn’t need a new vaccine law.

So that this post doesn’t come back to bite me at some point, let me also be clear that I am not in California, and I have no idea what kind of law California needs. No one in Minnesota is seeking to end all non-medical exemptions as they are in California, so the debate in California is not personal for me. (Very few of you will see what I just did there.) But many Californians whom I respect say that now is the time to end the personal belief exemption in California, and I trust them.

I’m also not a lawyer, so I will simply link to the estimable Dorit Reiss’ post about the fallacies of “we can’t sue vaccine makers so evil evil something” here.

Back to the meat of the post. I do try to avoid writing about scientific matters too often because I am not a scientist and because manner scientists are already doing the wonderful work of writing about vaccines. However, a few sentences in the aforementioned WIRED piece irked me:

Remember, vaccination rates have not suddenly plummeted. They are, and remain, high. And we remain at the upper end of the 83 to 94 percent “herd immunity” threshold that keeps measles at bay. To know how many kids in California are vaccinated against what diseases, we need to use the data-crunching tools and reporting methods that are available to us to count all the vaccines that have actually been administered. And I think we would be happy with what we found.

I do not have to be an epidemiologist or a scientist to understand that the description above is not how herd immunity works. Measles does not spread in a statewide manner, blanketing all of California with contagion or not based on the statewide vaccination rates. Measles is spread from in person, in a room or an elevator. Make no mistake, measles is crazy contagious. A measles outbreak in Minnesota illustrates this perfectly:

Lynfield said the virus can spread through the air, as it did in 1991 when 16 people at a Special Olympics event at the Metrodome were infected. A track and field athlete from Argentina was the original case in that outbreak. Interestingly, two of the infected people were sitting in the upper deck of the Dome — more than 30 meters above the athletes.

We definitely should be concerned about those large-view herd immunity issues. If California had an immunization rate of 60%, we would all be concerned.

But for measles or any other vaccine-preventable disease, we need to ask who is our herd? Where are our children most at risk for catching a disease?

A child in preschool or Kindergarten spends a great deal of time playing with toys his or her classmates play with, holding hands with friends, sitting side-by-side in group time. They may only have 15 or 20 classmates, but these children constitute their most important herd because of the amount of time spent with them and the proximity to them.

A college student living in a dorm has a different herd (and different vaccine requirements because of this different herd behavior). Those people sharing a bathroom with a college student or sharing a kitchenette in the floor lounge make up one herd. Classmates in a lab or a lecture hall or a study group are another herd.

If a preschooler is in a classroom with classmates who are all fully immunized, the chance that this child will contract a vaccine-preventable disease are greatly reduced. And if the child we send off to college lives in a dorm and attends classes with people who are all fully vaccinated, they, too, are at greatly reduced risk for catching a vaccine-preventable disease. And this is true whether or not our preschooler or college student in question is vaccinated. That’s the power of herd immunity.

The issue in California is not how many children in the state have parents who are making the choice to contribute to herd immunity. The issue is that too many communities have too many children who are unvaccinated and leaving those community-level herds at-risk. Even a classroom-level herd that has a high vaccine opt-out rate can be at risk if a disease is introduced into that classroom.

Those unvaccinated children who become infected then venture out into the greater community. Is their classroom a fluke, powered by an alpha-mom who has spread anti-vaccine misinformation like a contagion and frightened the classmates out of vaccinating? (See why it is so important not to remain silent during vaccine discussions?) Or is this classroom part of a greater community trend, waiting for disease to take over? Is the community made of tendrils that reach into other communities? How far can an outbreak spread?

“Our state vaccination rates are high enough” is never a cogent argument. Every community deserves protection, and every child in every classroom deserves the safety net of a highly vaccinated herd. I’m not an epidemiologist, but I am a mother with children, and I want them to have the protection of a highly vaccinated classroom. Until anti-vaccine misinformation is vanquished once and for all, we will need better laws to protect our children.