The real reason polio is so dangerous

13,626 views ・ 2024-11-05

TED-Ed


Please double-click on the English subtitles below to play the video.

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In 1952, polio was everywhere.
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For years, this virus killed or paralyzed roughly half a million people annually,
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leaving survivors reliant on crutches, wheelchairs, and respirators.
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Yet just 10 years later, paralytic polio cases in the US dropped by 96%.
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Soon, similar trends spread worldwide,
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and it looked like we were on track to eradicate polio for good.
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But in the 21st century, the virus started striking back.
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So, what’s the source of these recent spikes,
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and how can researchers fighting polio finally get rid of this deadly disease?
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To answer these questions, we first need to understand the true danger of polio.
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Most infected individuals only present mild flu symptoms or no symptoms at all,
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with paralysis occurring in less than 1% of cases.
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Which is why the real danger of polio is how infectious it is.
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Historically, there have been three strains of poliovirus,
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all of which typically enter our mouths from airborne droplets
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or contact with infected fecal matter.
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This means polio runs rampant in communities with poor sanitation.
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And once infected, individuals remain contagious for 3 to 6 weeks,
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spreading a silent outbreak with few trackable symptoms.
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This is what made polio unstoppable
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until US physician Jonas Salk found a solution in the early 1950s.
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He created an inactivated version of the virus that, when injected,
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prevented all three strains from causing paralysis.
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However, this inactivated poliovirus vaccine, or IPV,
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didn’t stop poliovirus from living in our bodies and spreading to others.
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Fortunately, Polish American microbiologist Albert Sabin
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was creating the oral polio vaccine, or OPV, at the same time.
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This even cheaper and easier to administer treatment
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contained weakened variants of each poliovirus strain,
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known as attenuated viruses,
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And once ingested, these prevented polio from settling in the body altogether.
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Over the next several decades,
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IPV and OPV eliminated polio in country after country.
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But near the turn of the century,
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this arms race between medical ingenuity and viral evolution took a turn.
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Scientists discovered variant strains of polio—
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each almost identical to one of the three existing strains.
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And their source was even more troubling.
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Up to this point, one of OPV’s greatest advantages was that its attenuated viruses
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could spread just like wild polio,
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moving through the air and immunizing unvaccinated individuals.
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But researchers discovered that if these weakened viruses circulated
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for several weeks in under-vaccinated populations,
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they could mutate into new vaccine-derived strains.
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To be clear, this didn’t mean the attenuated vaccine
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was inherently dangerous.
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OPV had already been used to stop wild poliovirus type 2 worldwide
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and eradicate all wild poliovirus
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from the Americas, West Pacific, and countless other countries.
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The issue was ensuring populations met a certain vaccination threshold—
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specifically, at least 80% of every community needed to be vaccinated
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to prevent strains from mutating.
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So, in our current fight against polio,
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we've taken two approaches to this problem.
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First, vaccine developers have made even safer forms of OPV.
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Monovalent and bivalent OPVs
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respectively contain one or two types of attenuated poliovirus,
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reducing the number of strains that could possibly mutate.
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These vaccines helped eradicate wild poliovirus types 2 and 3,
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and today they’re tackling the last remnants of type 1 poliovirus
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in Pakistan and Afghanistan.
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Vaccine researchers also developed OPVs targeting vaccine-derived polio.
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Currently, most polio cases are caused by vaccine-derived poliovirus
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type 2 outbreaks in Africa and the Middle East.
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So researchers created a new vaccine specifically to tackle this strain
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with minimal risk of mutating.
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Second, medical workers on the front lines are using cutting-edge technology
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to ensure every population meets that 80% threshold.
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Digital tools like geospatial imaging and analysis
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help them locate and immunize remote communities.
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Extensive monitoring systems ensure they don’t miss a single child.
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And waste surveillance systems employed across the globe
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can alert us to potential outbreaks.
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Today, the fight against polio is at a critical moment.
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We’re primed to eradicate wild poliovirus in the near future,
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and with our new vaccines,
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eradicating vaccine-derived strains might not be much further off.
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But doctors still struggle to reach areas
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experiencing military conflict and civil unrest.
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And without keeping vaccination rates high,
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polio’s silent outbreaks could easily surge.
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So it’s essential that we keep up the pressure
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to finally finish what we started over 70 years ago.
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