I still can’t believe I’m writing this.
Last time, which was sadly much longer ago than I intended, I was writing about whether or not vaccines work. Which, of course, they do. Every disease we have a vaccine for has seen the rate of infection (and the death rate) decline by several orders of magnitude since the vaccine was introduced. Really, this isn’t even a discussion we should be having anymore.
Except that we still have memes—like the one above—floating around. Because we still have people deliberately pushing an anti-science agenda, who are trying to put their mad ideology above human life. Why? I genuinely don’t understand. But rather than dwell on that, let’s move to the other half of that meme:
“How are unvaccinated children a danger to vaccinated children?”
“Old fool! Why, there is a large patch in the hollow of his left breast as bare as a snail out of its shell!”
That’s a quote from The Hobbit—specifically, the scene where Smaug the dragon is boasting about how he is utterly indestructible. “Your information is antiquated,” Smaug declares. “I am armored above and below with iron scales and hard gems. No blade can pierce me.” But Bilbo Baggins, with his keen eyes, spots a weakness: an unarmored patch on his chest, leaving him open and vulnerable.
Uhm. Wait, what? I thought we were talking about vaccines?
We are. Really.
See, there’s a thing called community immunity, more often referred to as “herd immunity.” And, as you might imagine from the name, it is the ability of the community as a whole to defend itself from infection in the same way that a herd of animals protects itself from predators. Having enough tough, strong members can help the herd fight off the predators and keep most of the group alive.
Here’s how it works, in a nutshell: diseases are caused by living things (generally bacteria or viruses, though there some debate about whether viruses are “really” alive). Human diseases use humans as a breeding ground and food source, and they need humans to pass the disease organisms on to other hosts before our immune system kills them completely or they kill us (limiting their ability to be transmitted). In order to maximize their chances of survival, the infecting organism needs to balance out how long it lives, how easily it can come into contact with other hosts, and how likely it is to infect another host.
Smart people who study this kind of thing have come up with a figure called the Basic Reproductive Rate (referred to as R0), which is the average number of people an infected individual will subsequently infect as s/he comes into contact with them over the life of the infection. If R0 is greater than 1, it will continue to spread; if it is less than 1 it will burn out. And if R0 = 1? Then the rate of disease remains constant in the community.
If you want to see how R0 is calculated, let me refer you to “Notes On R0” by James Holland Jones, available on the website of Stanford University. But here’s the R0 for some of the disease we vaccinate against:
- Diphtheria: 5–7 people
- Measles: 12–18 people
- Mumps: 4–7 people
- Pertussis: 12–17 people
- Polio: 5–7 people
- Rubella: 6–7 people
- Smallpox: 5–7 people
- Chickenpox: 7–10 people
So, where does herd immunity fit into this?
R0 assumes that there is nobody immune in the population. Herd immunity is an actual rate that can be calculated from R0, indicating the minimum percentage of the population that needs to be immune to a disease to reduce R0 to less than 1 (because we don’t want a constant level of sick people). Those figures are generally estimated as follows:
- Diphtheria: 85%
- Measles: 83–94%
- Mumps: 75–86%
- Pertussis: 92–94%
- Polio: 80–86%
- Rubella: 83–85%
- Smallpox: 80–85%
- Chickenpox: I didn’t actually find a herd immunity level for this, but a rough calculation shows 86–91%.
Now, the herd immunity level will never reach 100% (sadly). Some people simply can’t get vaccinated for legitimate medical reasons—most commonly an allergy to some component of the vaccine, or they’re too young, or they have some medical condition that interacts poorly with the vaccine (such as an immunodeficiency, or pregnancy). Other people might get a bad batch that doesn’t work, or it simply might not take. Nothing’s perfect, after all. The problem is, herd immunity has a fairly narrow range for a population. Dip below 83% immunity, and measles will slowly spread from susceptible individual to susceptible individual. Dip below 75% immunity, and whooping cough will inexorably spread. Sure, the immune people are likely to be fine, but the people who need herd immunity because they can’t be immunized are at risk.
In other words, the unvaccinated child isn’t a threat to any specific individual child. My son probably won’t get chickenpox because your daughter never got vaccinated. But your hypothetical unvaccinated child is a potential threat to the entire community, taking the community one notch closer to falling below that critical immunity threshold that helps keep everyone safe.
Your hypothetical unvaccinated child is a chink in the armor of the community. And disease organisms are evolved to be really, really good at taking advantage of those weaknesses.
Now look, I clearly got hot under the collar in my last article. I don’t regret that. Eradicating disease, and the suffering and death it causes, is worth a little passion. Because vaccines are demonstrably the single greatest medical discovery in the history of man, and they continue to work and they continue to benefit us despite a long history of people opposing them, often for no good reason at all. Fear. Ignorance. Rumor-mongering. Hucksters selling snake oil to profit off misery. And despite that, we’ve brought these disease to their knees, only to see them resurge as anti-intellectual frauds gain more and more publicity and find greater and greater outlets for their lies.
So, yeah. I get worked up. Because this is important. Sit down and talk to someone who lived through the polio epidemics. Talk to someone who’s nursed a child through whooping cough. Understand why this is important. And then do the right thing.