eli5: How was smallpox eradicated but other diseases are significantly harder to completely get rid of?
In: Biology
You have the question backwards, but others have already covered the answer to the proper question (“why was smallpox so easy to eliminate?”) Instead, I want to use an example of a disease we’re *close* to eradicating: Poliovirus.
Polio has three main variants (wild poliovirus 1, 2, and 3). We’ve eliminated WPV2 and WPV3. The only way those viruses still exist is that, because of the “inactivated” oral version of the vaccine, it’s possible for the inactivated viruses to mutate and regain their ability to cause harm, *if* the local population doesn’t maintain a minimum level of vaccination. (As long as ~90% of a population is vaccinated, it’s effectively impossible for the virus to spread in a way that can cause damage.) This oral vaccine is useful because it allows contact immunity–people who don’t get the oral vaccine directly can acquire it from someone else who got it, which makes vaccinating large areas significantly easier (plus, the inactivated poliovirus is much easier to make, store, distribute, and deploy than the injected version.)
We’ve phased out the WPV2 oral vaccine completely, and it’s possible we might phase out WPV3 as well. Only a few dozen confirmed polio cases occurred in 2023, and if we’re lucky, we might be able to declare all three types eradicated (outside of the rare re-activated oral vaccine cases), at which point we could switch purely to the injected vaccine.
The key difference with polio, like smallpox, is that it ONLY affects humans. This is a human-only virus. If we know nobody’s gotten it for 20 years, we can be pretty confident there isn’t any of it left out in the world. It also helps that, as stated, the oral polio vaccine is cheap to make, store, distribute, and deploy.
Latest Answers