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Why the two polio vaccines?

Something that I had known for a while but never really understood why is the two-vaccine approach to polio immunization. I knew that Salk and Sabin had developed the two vaccines in the 50’s. Salk’s vaccine is injected, made up of three dead (inactivated) viruses. Sabin’s vaccine is ingested, made up of three live but attenuated viruses. But why do some places in the world use one vaccine over the other. I seriously had never thought about it.

I recently attended a talk by one of the world’s leading epidemiologist and the guy who is in charge of eradicating polio. It’s a big job, for sure, but he explained why the two vaccines are needed in different circumstances and why Israel, a place where only the injected vaccine is being used right now, needs to go back to the attenuated, ingested vaccine. (Something that has the anti-vaccine forces all in a fuss.) The man’s job is daunting, but something that all epidemiologists dream of. Imagine being the one who played a key role in eradicating a virus.

A goddamned virus!

Alright, so why two vaccines and why one over the other? To answer this question, you have to understand how polio works. Polio is a virus (there are three main strains) that is transmitted from person-to-person through the fecal-oral route. If you’re sick, you shed the virus in your poop and then someone comes into contact with your poop and gets infected themselves. This happens mostly when children who play with each other and don’t wash their hands, people who don’t wash their hands and cook food for you, and when water becomes contaminated with sewage. Polio is also spread through mouth secretions, by the way.

When you ingest the virus, it infects the cells in your intestine and then the cells in your tonsils. From there, the virus spreads into the bloodstream. Once that happens, you get a flu-like illness. In time, your body reacts and gets to work to get rid of the virus. You eventually recover and become immune only to the strain of virus that you contracted. This means that you can get sick three times with polio if you are exposed to all three strains.

Now, the polio that we all know and dread is paralytic polio. It’s called paralytic because the virus destroys your nerves as it travels through them. This doesn’t happen in all cases of polio, only in about 1%. Still, 1% of 1,000 children is 10 children who end up like this:

girl with polio
Or worse.

Or worse, the children (and adults) may die from the nerves that operate the respiratory system being damaged beyond repair.

To take out polio, you have to attack it in several ways. You can clean the water and sewage and keep kids away from each other and make sure that 100% of people wash their hands. But, because we don’t live in a perfect world, we try to do those things and immunize. You start with the oral vaccine to give the body immunity in the intestinal tract. (Oral vaccine virus also spreads to other people via the fecal-oral and oral-oral route.) That way, it doesn’t matter if the virus comes along again, the people who get immunized will be immune. This buys you time to get your water clean for drinking and hand washing. Furthermore, it slowly gets rid of the virus in the environment since it will run out of hosts in which to replicate.

Once the virus is gone from the environment around you, you turn over to the injected vaccine. The injected vaccine keeps paralytic polio at bay should your population ever encounter polio again. (Remember, it’s not eradicated yet, and, as Syria and Israel have shown us polio can come back if you let your guard down.) If it does come back again, you need to get back to the oral vaccination to, again, get rid of it in the environment. After all, we’re just big incubators for these bugs.

And that, dear reader (or two) is why we have two vaccines and why we use the injected one in the United States. There is a very, very small chance of getting polio in the US, so we do the injected in case we go “over there” or people with polio come over here and it starts spreading. Once it is completely eradicated from the planet, then we can stop both vaccines, an anti-vaxxer’s dream, though it’s the anti-vaxxers who are giving these diseases legs. But that is for another post at a later time.

If I have time.

children with polio
“At least I wasn’t exposed to the horrible preservatives and chemicals in vaccines,” said no child with polio ever.

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René F. Najera, DrPH

I'm a Doctor of Public Health, having studied at the Johns Hopkins University Bloomberg School of Public Health.
All opinions are my own and in no way represent anyone else or any of the organizations for which I work.
About History of Vaccines: I am the editor of the History of Vaccines site, a project of the College of Physicians of Philadelphia. Please read the About page on the site for more information.
About Epidemiological: I am the sole contributor to Epidemiological, my personal blog to discuss all sorts of issues. It also has an About page you should check out.

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