When you hear the word “polio”,  your mind might first jump to black and white images of children in iron lungs, or maybe President Franklin  Roosevelt sitting in his wheelchair.

In other words, it might feel  like a thing of the past. Poliomyelitis, the virus that  causes the condition we call polio, is on the brink of eradication.

And we have vaccines to thank for that. The first polio vaccine was  introduced in the 1950s, and over the next two decades,  cases of poliovirus paralysis dropped from roughly 15,000 per  year in the U. S., to fewer than 10!

But it’s been like 50 years since then and we still haven’t managed  to stamp it out, entirely. So in order to understand why, we need  to explain a little about the vaccines we have at our disposal, and dig a little  deeper into the history of the virus itself. [♪ INTRO] Poliovirus has probably been  around for thousands of years. Some archaeological evidence  in Egypt might suggest that it’s been causing paralysis since antiquity.

But it wasn’t until 1789 that a  description of polio paralysis showed up in the known written record. In a medical text, the English  physician Michael Underwood documented children who had developed a  fever, followed by what he called a “debility of the lower extremities.” At this time, polio was  spreading from human to human, like contagious diseases do, but the  trouble it caused societies at large didn’t really ramp up until the late  19th century…with full blown epidemics. And the reason why it happened at that point  in history is a little counterintuitive.

You see, a live poliovirus can catch a  ride out of the body via the mouth and nose or the butt, although it’s much  more likely that you’d catch polio from an infected person’s poo  than, say, like getting sneezed on. So scientists have hypothesized  that due to poor sanitation, poliovirus was so widespread people  often got their first exposure as babies. And that is actually a good thing.

Because these babies had protection. They acquired polio-targeting  antibodies from mom’s immune system while they were in the womb, and  could top up while they were nursing. In many cases, those antibodies kept a  baby from developing symptoms of polio while their immune system could  create its own antibodies, which would go on to protect  that baby later into adulthood.

But, the timing of this whole  thing was super important. If a baby wasn’t exposed to the poliovirus  in the first few months after birth, maybe because they grew up in a  time and place with great sanitation and hand washing practices,  they wouldn’t have a chance to make antibodies while under mom’s protection. And then the first time they  got exposed to poliovirus, their body would go into the encounter  with an untrained immune system.

The improvements in hygiene and sewage  systems in the mid-to-late 19th century may have done a lot of good preventing  the spread of other diseases, while unintentionally increasing  the chance of contracting polio to epidemic proportions in the early 20th century. So the question now is, what would happen if you came in contact with poliovirus with  an untrained immune system today? Well, there are actually a few possible outcomes.

Most likely, you would notice nothing at all. Around 70% of poliovirus infections  are asymptomatic in children, with similar rates in adults. But, even if you don’t have any symptoms, you can still infect other  people who could have them.

If you did get sick, you would most  likely experience mild symptoms, like a sore throat and a low fever,  and get better in about a week. Around 1 to 5% of kids also experience  headaches, stiffness and pain in the neck, back, and legs, and sometimes vomiting. Though definitely unpleasant and painful, most still recover fully in one to two weeks.

But unfortunately, none of this is why  the word polio evokes such awful memories. During these polio epidemics, a very  small percentage of infected children developed flu-like symptoms for  a short time and then got better. As a parent, you might have thought  that your child had simply had a cold.

But then, a few days later, the  child would develop a fever, severe pain in the limbs, and finally paralysis. Adults who developed this rare  kind of polio often skipped the flu-like symptoms altogether, and jumped  straight to the severe pain and paralysis. The degree of paralysis varied.

Some would develop paralysis in the lower limbs. Others would develop it in the  muscles of the face and neck, and the respiratory muscles. These people often struggled to  breathe on their own, and many died.

In temperate climates, poliovirus  seemed to spread during warmer months. So for many, summertime fun was  haunted by the specter of polio. Meanwhile, in tropical regions,  it was a year-long haunting.

But hope came in 1955 when the first  vaccine against poliovirus was introduced. Developed by Dr. Jonas Salk,  it was given as an injection.

To this day, the injectable polio vaccine  contains an inactivated form of the type of poliovirus found in nature,  known as wild type poliovirus. The virus has been inactivated  by exposing it to formaldehyde, a chemical that damages it so much  that it can no longer cause illness. But it’s still a weird lump of foreign  proteins floating around in your body, so your immune system will create antibodies to protect against any future wild  type poliovirus infections.

This vaccine has proved incredibly  effective at preventing disease, but some experts questioned whether  it would offer long-lasting immunity. So research continued, and thanks to  the efforts of Dr. Albert B.

Sabin, a second vaccine was introduced  to the public in 1961. This time, instead of getting a jab in the arm, you could take the vaccine in a drink  or even dropped into a sugar cube.   The oral vaccine was also different because it contained a live but  attenuated version of poliovirus. An attenuated, or weakened, virus is one that has been modified so it can’t cause the disease.

Basically, scientists force the virus to replicate in non-human cells in a lab, over and over again. Because the poliovirus can only infect humans, this process causes a lot of stress  on the virus, leaving it weakened. And then when it’s introduced into human  cells, it can still replicate in the gut, but basically it cannot access and  attack the central nervous system.

And because it hasn’t been outright  damaged like an inactivated virus, the oral poliovirus vaccine  elicits a stronger immune response. In other words, a person requires  fewer doses of the vaccine if they get the oral version  instead of the injection. That’s a huge advantage when you’re  trying to vaccinate groups of children who move around a lot, or who  don’t have regular access to healthcare.

And there are also other  benefits to the oral vaccine. Unlike the inactivated version,  the live attenuated poliovirus can replicate in your intestines. That means a vaccinated person  will develop localized antibodies in their intestines, which just  so happens to be the main place where wild type poliovirus replicates.

So if that person ever comes into  contact with wild type poliovirus, the local immunity will prevent the  virus from making it into their poop. Meanwhile, a person vaccinated  with the inactivated, injectable vaccine would be protected  from getting sick from poliovirus, but could potentially infect other people. Finally, the oral attenuated vaccine  can provide some passive immunization.

A person who takes the oral vaccine will shed the weakened form of the virus in  their poop for a couple of weeks. So anybody who comes in contact  with that poop…say, by accidentally swallowing contaminated water while swimming… has a chance of getting exposed  to the attenuated virus, too. Much like babies in the womb, they can  develop a temporary immunity themselves.

But despite all these  benefits, the oral vaccine is not the perfect solution to our polio problem. For one thing, despite not having  brains, viruses are pretty frickin smart. And if an attenuated poliovirus  keeps getting bounced around between a bunch of unvaccinated people,  or finds its way into someone with a weakened immune system, that can cause  problems for a lot of other people.

In fact, that’s why people  who are immunocompromised are discouraged from taking the oral polio vaccine. Their immune system can’t readily  clear out the attenuated virus. And the longer it hangs around inside  anyone who doesn’t have enough immunity, the more time it has to change back into something that can cause paralytic disease.

It’s that mutated kind of poliovirus  which can spread to other people. Scientists call this vaccine-derived poliovirus. That name can make it sound like it’s the  original vaccine that’s causing the harm.

This might sound scary enough  that people think twice about even getting vaccinated, but you  cannot get vaccine-derived poliovirus by getting either the injectable  or the oral polio vaccine. People only develop symptomatic  polio when someone else got an attenuated oral vaccine and then that  attenuated virus was able to move from person to person long enough  that it had time to evolve the ability to not just infect, but cause  symptoms in an unvaccinated person. And getting vaccinated protects you  from both the wild type and vaccine derived poliovirus, because there’s  no difference between the wild type and the vaccine derived poliovirus  in terms of how symptoms develop.

Just like an infection with wild type poliovirus, most people who come into contact  with vaccine-derived poliovirus will never get sick, while others  will have flu-like symptoms. But rarely, a few will develop paralysis. There are cases of infections from  vaccine-derived poliovirus all over the world, including one case in New  York in the summer of 2022.

A 20-year-old man who hadn’t  been vaccinated against polio contracted the mutated virus, and  went on to develop paralytic disease. In 2021, there were only six  reported cases of polio caused by the wild type poliovirus in the entire world. But there were also nearly 300  cases of vaccine-derived polio.

So, if the vast majority of  polio cases are being caused by the vaccine-derived poliovirus,  which comes from mutations of the virus contained in the oral vaccine, shouldn’t  we just stop using the oral vaccine? The short answer is “yes”. But how we get there is complicated.

Despite the risk of vaccine-derived poliovirus, the oral vaccine has several  advantages over the injectable vaccine. Not only can it prevent people  from spreading both wild and vaccine-derived strains, and  provide temporary passive immunity, it’s also much less expensive to manufacture. Still, scientists and physicians  recognize we need to move away from using the oral vaccine if we are  going to entirely eradicate polio.

The Global Polio Eradication Initiative has a plan to discontinue its use by the late 2020s. And many countries have high  enough vaccination rates that they have already stopped using it. The US did so back in 2000.

But communities with lower vaccination rates  can still benefit from the oral vaccine. So to help make it even safer,  scientists have already modified the oral vaccine to reduce how frequently  vaccine-derived poliovirus can pop up. You see, this continues to be complex.

Wild type poliovirus actually exists  as three different viral strains: type 1, type 2, and type 3. Both the inactivated and  the live attenuated vaccines can be manufactured to contain all three strains. These so-called trivalent vaccines  were in use until early this century.

However, researchers determined that we had completely eradicated type  2 poliovirus back in 1999. That meant that the only thing that the  type 2 attenuated strain was good for was potentially mutating into something  that might cause paralytic disease. In 2016, the global health  community switched the attenuated, oral vaccine from trivalent to bivalent, so it only contained the strains for  type I and type 3 wild type poliovirus.

Meanwhile, the inactivated, injectable  poliovirus vaccine still contains all three strains, and still protects against  all wild type and vaccine-derived strains. If we want to eradicate polio, and we do, it’s going to take everyone  around the world working together. So you might be wondering what  you could do to join this effort.

Many people who grew up in  developed countries receive all their poliovirus vaccinations in their  regular schedule of childhood vaccinations. But, if you didn’t, it might be wise to talk  to your doctor about getting vaccinated. And if you're a parent, talk to  your pediatrician about vaccination.

Because even though we love it when you  watch our stuff, it’s always a good idea to take any medical questions you have  to a trusted healthcare professional. Thanks for watching this episode of  SciShow and thank you especially to all of our patrons who allow us to take on  really big, complicated issues like this. You know, we have a range of difficulties  of topics to cover here on SciShow; it was definitely a trickier one.

It took a long time to put together, so  thanks to the whole team for doing it and thanks especially to our  patrons for supporting it. If you wanna become one of those  patrons and help support us making good science content on the  internet for free for everybody, you can do that at patreon.com/scishow. [♪ OUTRO]