Herd immunity: the proof in the pudding

A recent article in the US News talked about a really cool thing happening with human papillomavirus (HPV). It’s the phenomenon called herd immunity, and it seems to be working.

First, some basics on herd immunity. Many of you may have a good grasp on it already. For those who don’t, PBS’s NOVA gives a great description of it. In short, the more people get vaccinated against a disease, the more difficult it is for the disease to spread throughout a community. This means even unvaccinated people are protected — but only if enough people do get their shots.

Let’s say one person, Patient Zero, gets infected with a disease. The disease wants to continue infecting surrounding people. If everyone around Patient Zero gets vaccinated, the disease won’t have any good “hosts” to spread to. So, the infection chain stops there. “Herd immunity” happens when unvaccinated people are protected simply because it’s less likely they’ll encounter someone with the disease.

Check out my all-time favorite illustration of herd immunity below. (Yes, I have a favorite.)

Herd Immunity Gif
Gif created by Reddit user /u/theotheredmund

The red color is a hypothetical disease. Yellow dots got their shots, blue dots did not. (Bet you didn’t think this blog would include slam poetry.)

You can see that a pretty high percentage of people — between 75% and 90% — need to be vaccinated before the infection chain is significantly slowed down. The more contagious the disease, the higher percentage of people need to be vaccinated to protect the rest of the community. Turns out, these numbers are actually pretty realistic.

Some diseases — polio, for example — are not quite as contagious as others. For these diseases, 75-80% of people may get vaccinated, and that may be enough to keep infection from spreading through the community. Measles, on the other hand, is super contagious. According to NOVA, 95% of people need to be vaccinated to protect the rest of the population from measles. Any lower, and we start to see outbreaks. The CDC has a list of measles outbreaks between 2008 and 2015. You might notice that outbreaks are the worst among unvaccinated populations.


Another example is the varicella zoster virusSome of us may know it by its more common name: chickenpox. When I was growing up, it seemed almost everyone I knew had gotten chickenpox at one time or another. According to the CDC, during the early 90’s, an average of 4 million people in the US got it every year. The vaccine for chickenpox was introduced to the US in 1995 (of course, I’d already had it twice by then… bastards*).

Question Everything: “You had chicken pox twice? I thought that wasn’t possible??” It is, but it’s rare! In almost every case, people who get chickenpox twice are actually experiencing shingles. It’s caused by the same virus, which “hides out” in nerve cells and can get re-activated for unknown reasons. This happens to about 1 in 3 people, and about half of them are 60 or older.

Over the next several years after the vaccine came out, chickenpox infections went down like crazy; anywhere between 72 and 98%. Chickenpox-related deaths (there were 100 to 150 every year) decreased by up to a whopping 97%.

And here’s the kicker: among infants, who can’t get the vaccine, chickenpox cases declined by 90%. Coincidence? (Hint: No.)*

Back to the US News article I linked at the start of this post: between 2009 and 2014, women aged 18 to 26 have seen a 65% decrease in HPV infection. Unvaccinated women of the same age have seen over a 50% decrease. Pretty cool, huh?

Question Everything: “So, how do we know these aren’t coincidences?” This is a great question. It’s also one that comes up in research constantly. And it’s absolutely true: correlation ≠ causation. In other words, just because two things are happening at the same time, it doesn’t mean one is causing the other. However, this is an in-depth subject, and I want to give you a solid answer. So look for it in a future post, which I will link to here!

So who exactly are we protecting? There are a lot of people who can’t get vaccinated, even if they have access to the vaccine. A common reason is that people might have allergic reactions to some ingredient in the vaccine. Others might have immune disorders; maybe their body doesn’t do very well fighting against foreign substances. In these people, the vaccine can send their immune system into overdrive and make them sick. Unfortunately, these people are also the ones most severely affected by a disease like influenza.*

Question Everything: “Can’t the flu vaccine also give you the flu?” Nope! It’s true that some vaccines contain a form of the virus. But their ability to cause disease has been taken away. For now, take some time to read these key facts from the CDC. We’ll talk more about this in a later post!

If you like weak metaphors, think of the vaccine as zombie-proof armor. If more people are wearing it, they can help protect the people who can’t wear it because of, say, a life-threatening allergy to chain mail.

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