Fun with Liquid Nitrogen

I’ve got news for you: you’re one of a herd. No, I don’t mean that in the “wake up sheeple” sense that your average conspiracy nut will go to upon finding that 99% of the planet believes him to be a lunatic, but in a sense related to immunology. It’s called herd immunity and it’s all the rage.

Herd immunity states that, given a certain population, if a certain percentage – usually between 75% and 95% for diseases transmissible via molecules through the air – are immune to that disease, it cannot easily spread from the host to the rest of the unimmunized (and under-immunized) population unless it’s helped along by sinister evil government-military-Illuminati scientists in movies from the 80s. In other words, for a disease to be of very little threat to a population, only most must be immune to it, not all.

The best two examples of this are from over 400 years apart for the same disease: smallpox. The first tale comes from the expedition into the Aztec empire by Spanish conquistador Hernán Cortés in 1519. Though severely outnumbered as a small detachment of soldiers in an empire with an entire army, Cortés’s men prevailed with few casualties partly because of their weaponry and tactics but also because of the disease they brought along with them – smallpox – that some estimate quite literally decimated the population, bringing the Aztec empire to its knees at the hands of a few hundred Spaniards.

“But wait!” you say. “Shouldn’t have most of the Spaniards died from the smallpox as well?” Well, a few did, but much, much fewer by comparison than the 60-90% of the Aztec who succumbed to the disease. Why?

The Spaniards lived in standard European towns by early 16th century standards in which they were often in close contact with cattle. Since cattle carry a disease similar to smallpox but much less deadly, over time they developed antibodies that were at least somewhat effective in warding off smallpox – a form of immunity called passive immunity. Though not all of them had effective antibodies to smallpox, enough of them did to keep the disease from spreading amongst them. The Aztec, however, not only did not live amongst cattle but had never even seen cows before. They had absolutely no immunity to smallpox and so it ran rampant through their cities and towns, killing most of its victims as it went.

The later example of how smallpox teaches us about herd immunity is from much later – the 1970s – when the disease was officially eradicated from the planet. Thanks to a decade-long effort started in 1967 by the World Health Organization, by 1979 enough people had received a smallpox vaccine globally to give the disease no further avenues of transmission and the number of worldwide cases dropped to zero. Unlike the passive conference of antibodies in the case of the conquistadors, the vaccine conferred active immunity, directly exposing B- and T-cells – the major components of long-term immunity – to a weakened or “attenuated” form of the disease that could not effectively cause infection in most people but could still induce immunological memory that would last for decades.

You’ll notice I said “could not effectively cause infection in most people” up there. That’s because, as the vaccine is a weakened form of the virus, people whose immune systems work properly – that is, those who are “immunologically competent” – could keep it from causing the disease itself while still gaining memory. Those who had a poor immune system, though, due to having an immunodeficiency disease, were on immunosuppressants due to an organ transplant or were immunocompromised due to receiving radiation therapy for cancer could not receive the vaccine or it was ineffective, often not because the attenuated form would cause the disease but rather because their immune systems were too weak to gain memory from it. Thankfully, these people constituted in total only a small percentage of the population, herd immunity was reached, and smallpox ceased to take victims (of course, as I’m under 40 I am one of billions of people globally who was never vaccinated for smallpox, but that’s another issue entirely).

Sounds great, huh? Your neighbors got vaccinated so you don’t have to. Well, not so fast. As I stated above, there’s a cutoff percentage to reach herd immunity and once immunization rate drops below that percentage the disease might come right back. And, since there will always be folks who cannot accept vaccines for the reasons listed above (many vaccines are now much safer than 40 years ago but the decision to vaccinate or not depends on the immunocompromised patient, the vaccine and the disease), as Dr. Joseph Albietz noted earlier this month, voluntarily deciding not to immunize despite a lack of medically-relevant reasoning is not only silly, it’s potentially dangerous as it could lower the local immunization rate below the threshold for herd immunity.

How does that work? Well, it’s like this. Say you’ve got a population of 25 poorly-drawn blobs, 20 of which (green) are immunized and 5 of which (orange) are not:

As you can see, in this case, when exposed to the red blob infected with Disease X, the disease cannot spread because it can’t “find” another host. In this case, 80% immunization seems to be a sufficient rate for herd immunity. But what if just two fewer blobs were vaccinated against Disease X, lowering the immunization rate by only 8% to 72%? Not much of a difference, right? Wrong:

Now the disease spreads from the original host to infect 5 of the blobs – 20% of the population, up from zero. That’s a 20% increase in infection in exchange for a drop of only 8% in immunization.

Clearly this is only a hypothetical example and a drastic simplification, but the idea is sound: only a small drop below herd immunity can cause cases of diseases to spike from zero or near-zero to a surprisingly high number, increasing the exposure to the disease to those who could not be vaccinated due to legitimate concerns and increasing the health care burden for everyone to cover the cost of treating 19th century diseases such as measles, mumps and rubella in 2010.

Unfortunately, the anti-vaccine movement continues to plod along, gathering members from the extreme right and extreme left, touting patently bogus claims that vaccines contain “toxins” (a nice, ambiguous word that has absolutely zero meaning. Hell, water is toxic if you drink enough of it) or “cause autism.” The latter of those was a claim made in 1998 by Andrew Wakefield and, much like herpes, just won’t seem to go away. While it unfortunately cemented Wakefield as a leader of the anti-vaccine movement and a hero to many misguided parents of autistic children, it has completely eroded his medical reputation. As Phil Plait reported, last week Wakefield was cited in the UK for acting “dishonestly and irresponsibly” for reporting those false claims. One wonders if the resulting punishment will be anything more than a slap on the wrist, but that the decision was made to cite Wakefield should come as a good sign that science might slowly be prevailing over pseudoscience in this matter.

Though the cognitive dissonance of those in the anti-vaccine camp will prevent them from ever really understanding the extent to which their decisions cause more harm than good, hopefully, reporting on rulings such as this one against Wakefield will help revive for most people the sense of the danger in not guarding against diseases that have not gone away, but rather just slipped out of the herd. You don’t have to use the power of awesome thinking to understand how bad it will be if they slip back in.

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