They were part of a large clinical trial, getting either the polio shot or a saline placebo, and helped prove that the vaccine was safe and effective.
Do You Remember Sabin Sundays?
Of course, that wasn’t the end of the story though.
After the Cutter Incident, Albert Sabin soon proved that his live, oral polio vaccine was better than Salk’s inactivated polio vaccine.
And it was first given in the United States on April 24, 1960 – the first Sabin Sunday, when 20,000 children came to Cincinnati Children’s to receive his sugar cube vaccine.
“On three consecutive Sundays — “Sabin Sundays” — in 1960, millions of families lined up at churches and schools across the country to swallow a spoonful of pink syrup or a sugar cube treated with a life-saving polio vaccine, developed by UC researcher Albert Sabin.”
Sabin Sunday, 1960
Sabin Oral Sunday immunization programs continued over the next few years all over the country as kids got caught up on their polio vaccines.
Can you imagine taking your kids to school to get them vaccinated on a Sunday?
Millions of parents did it!
They lined up to get their kids vaccinated and protected.
It’s true, polio has been eliminated in the United States.
But that doesn’t mean that we can stop vaccinating kids against polio yet.
Why Do We Still Vaccinate If Polio Has Been Eliminated?
For one thing, the last polio case in the United States was a lot more recent than 40 years ago.
What happened 40 years ago?
That was when we had the last endemic case of polio in the United States, in 1979. After that, in addition to cases of VAPP, there were at least 6 cases of imported paralytic poliomyelitis. In fact, the last case of wild polio in the United States was in 1993, just 26 years ago.
And just ten years ago, in 2009, was the very last case of VAPP, a patient with a long-standing combined immunodeficiency who was probably infected in the late 1990s, even though she didn’t develop paralysis until years later.
But still, why couldn’t we stop vaccinating against polio in the United States, even though polio isn’t eradicated yet? After all, we stopped using the smallpox vaccine in 1972, before smallpox was declared eradicated (1980).
While that is true, smallpox isn’t as contagious as polio and there hadn’t been a case of smallpox in the United States for over 30 years when we stopped using the vaccine.
Until wild polio is eradicated and the oral polio vaccine isn’t used anymore (OPV switch), we must continue to vaccinate against polio to prevent new outbreaks.
That is the polio eradication and endgame strategic plan.
Over the next few years, the world will hopefully switch to using just the injectable form of the polio vaccines, which eliminates the risk of VAPP.
But if we are so close, why not just stop vaccinating in those parts of the world that don’t have polio?
Because we are so close to eradicating polio.
Why take the risk of polio spreading from one of the remaining endemic countries, paralyzing kids, and putting eradication efforts further behind?
There is a lot more interest in polio these days, but not because we are close to eradicating this deadly disease, but rather because of the emergence of cases of acute flaccid myelitis (AFM).
Although the cases have a different cause, the symptoms of AFM are the same that we used to see during the outbreaks of polio that used to hit each summer in the pre-vaccine era.
Surprisingly, in most people, the poliovirus doesn’t actually cause any symptoms. They simply have an asymptomatic infection.
In some others, the poliovirus causes flu-like symptoms, including a fever, sore throat, nausea, and a headache – symptoms that last about 3 to 5 days.
Much more rarely, the poliovirus causes meningitis or paralysis.
It is these cases of paralytic polio that most people are aware of and that panicked parents during summers in the 1940s and 50s.
After having flu-like symptoms, those kids who would develop paralytic polio can develop pain and then flaccid paralysis.
“The most severe form, paralytic poliomyelitis, which is seen in less than 1% of patients, presents as excruciating episodes of pain in back and lower limbs. In children, the disease may present in biphasic form—a period of prodrome followed by a brief symptom-free period of 7 to 10 days and then appearance of asymmetrical paralysis of limbs. Flaccid paralysis is the hallmark with loss of deep tendon reflexes eventually.”
Mehndirattta et al on Poliomyelitis Historical Facts, Epidemiology, and Current Challenges in Eradication
Of course, polio wasn’t always called polio.
Other names have included infantile spinal paralysis, infantile paralysis, Heine-Medin disease, poliomyelitis anterior acuta, and acute anterior poliomyelitis.
The first use of the name “polio” came from Adolph Kussmaul, with his use of the term poliomyelitis anterior acuta, which was derived from the Greek polios “grey” and myelos “marrow” and itis “inflammation.” It was because he knew that it was caused by inflammation of the spinal cord gray matter, even if he didn’t know why.
Polio didn’t just suddenly appear in the middle of the 20th century though, it was likely around for ages.
In addition to an Egyptian funeral stele (a stone slab used as a monument) portraying Roma the Doorkeeper from 1500 BCE that suggests he had paralytic polio, archeologists have found evidence of polio in skeletons as far back as the Neolithic period.
Still, we don’t really know how long polio has been around and we don’t know why we began to see more cases in the mid-20th Century, although there are theories, including, ironically, about hygiene. While we often credit improved sanitation and hygiene for helping to reduce mortality from many diseases, some think that this actually set us up for polio outbreaks, as we were no longer exposed as infants, when we still had some maternal immunity.
The one thing that we do know is that we are on the verge of eradicating polio, as there are very cases now, in just a few countries.
Polio Timeline and Milestones
In addition to the more ancient discoveries about polio, there is a lot to learn about vaccines and vaccine-preventable diseases if we look at the major milestones of this important disease.
Although in the end it is a success story, the road to figuring out what caused polio symptoms and how polio could be prevented was very long.
Michael Underwood describes what is thought to be paralytic polio in his book A Treatise on the Diseases of Children, with General Directions for the Management of Infants from Birth in a section on “Debility of the Lower Extremities” (1789)
first reported outbreak of polio in Worksop, England (1835)
Jacob von Heine, head of an orthopedic hospital in Germany, publishes a monograph that describes 29 cases of paralytic polio, and actually attributes the condition to inflammation of the anterior horns of the spinal cord, although the cause was still not known (1840)
first use of the term poliomyelitis by Adolph Kussmaul (1874)
Nils August Bergenholtz reports on an outbreak of paralytic polio in Sweden (1881)
Karl-Oskar Medin, a pediatrician who reported on a polio epidemic in Sweden (1887), later presents his findings at the Tenth International Conference in Berlin (1890)
the first major outbreak in the United States is documented in Rutland County, Vermont and causes 132 cases of paralysis and 18 deaths (1894)
Ivar Wickman tracks cases of polio during an epidemic in Sweden in 1905 and was the first to suggest that polio was contagious and that you could get it from “those afflicted with the abortive type” (1907)
although they don’t actually identify the poliovirus, Dr. Karl Landsteiner and Dr. Erwin Popper identify that a virus causes polio when they inject material from the spinal cord of a child who had recently died with polio into the peritoneum of two monkeys, both of which soon developed paralytic polio (1908)
Simon Flexner, first discovers polio antibodies (1911), but unlike other researchers at the time, pushes the theory that polio was spread by the olfactory route, instead of the fecal-oral route, which was why we saw the development of nose sprays, etc., to try and prevent polio, none of which worked of course
a large polio epidemic in the United States causes at least 27,000 cases and 6,000 deaths (1916)
Philip Drinker and Louis Agassiz Shaw invent the first iron lung, the Drinker respirator (1929)
Frank M. Burnet and Jean Macnamara proposed that there were antigenically different strains of poliovirus (1931)
John R. Paul and James D. Trask help figure out how polio was spread by identifying the polio virus in human waste and sewage samples (1932)
Maurice Brodie and John Kolmer have unsuccessful field trials of early polio vaccines, including allergic reactions and vaccine induced polio because of poor attenuation (1935)
Sister Elizabeth Kenny establishes a clinic in Australia to treat polio survivors (1932) and later publishes her treatment recommendations, Infantile Paralysis and Cerebral Diplegia (1937)
the National Foundation for Infantile Paralysis is founded by FDR to stop polio (1937)
Carl Kling found traces of the poliovirus in the Stockholm sewage system (1942)
the Sister Kenny Institute is built in Minneapolis, as her treatment methods become widely accepted after years of controversy (1942)
the U.S. Army Neurotropic Virus Commission, including Albert Sabin, gets a grant from the NFIP to study polio in North Africa (1943)
Isabel Morgan actually developed the first inactivated polio vaccine, but only tested it on monkeys (1949)
John Enders, with T. H. Weller and F. C. Robbins, received the Nobel Prize in 1954 for their work on the cultivation of the poliomyelitis viruses (1949)
David Bodian creates the monkey model using field isolates of poliovirus and with Jonas Salk, identifies the three poliovirus serotyes (1950s)
Hilary Koprowki develops the first oral, live polio vaccine, (1950) although Sabin’s vaccine eventually gets licensed because it is thought to be less neurovirulent in monkeys and undergoes more testing
there are 58,000 cases of paralytic polio in the United States (1952)
Renato Dulbecco, with Marguerite Vogt, successfully grows and purifies polio virus (1952)
the Polio Pioneers vaccine field trial, led by Thomas Francis Jr., that proves that Jonas Salk’s polio vaccine is safe and effective begins (1954)
last imported case of polio in the United States (1993)
the WHO Region of the Americas is declared polio free (1994)
“last” case of VAPP that was acquired in the United States (1999)
last case of wild poliovirus type 2 (1999)
the WHO Western Pacific Region is declared polio free (2000)
the United States switches back to using the an inactivated polio vaccine because of concerns over VAPP (2000)
the WHO European Region is declared polio free (2002)
outbreak of vaccine derived polio among a group of unvaccinated Amish in Minnesota (2005)
last case of VAPP that was acquired outside the United States, an unvaccinated 22-year-old U.S. college student who became infected with polio vaccine virus while traveling in Costa Rica in a university-sponsored study-abroad program (2005)
last case of VAPP in the United States, a patient with a long-standing combined immunodeficiency who was probably infected in the late 1990s (2009)
Bob Sears says that it is okay to delay the polio vaccine on his alternative vaccine schedule because “we don’t have polio in the United States” (2015)
a global switch from trivalent OPV to bivalent OPV in routine immunization programs (2016)
polio remains endemic in just two countries, Afghanistan and Pakistan (2018)
So it should be clear, that despite what some folks think, polio wasn’t conquered overnight. And Salk and Sabin obviously had a lot of help, although those are the names we most commonly hear connected with polio eradication.
“Until poliovirus transmission is interrupted in these countries, all countries remain at risk of importation of polio, especially vulnerable countries with weak public health and immunization services and travel or trade links to endemic countries.”
Global Polio Eradication Initiative on Endemic Countries
It is well known that you can very rarely develop polio after being vaccinated with the oral polio vaccine.
VAPP or vaccine-associated paralytic polio are cases of polio that are actually caused by the polio vaccine. That’s why many countries switch over to the inactivated form of the polio vaccine once polio is under good control.
But can you get polio after an injection?
What is Provocation Polio?
You are probably thinking, sure, if the injection is full of live polio virus, right?
But this is actually the idea behind provocation polio.
No, the injection doesn’t give you polio, but if you are already infected with polio, the idea is that getting an injection could be a risk factor for developing paralytic polio.
“Provocation poliomyelitis describes the enhanced risk of paralytic manifestations that follows injection in the 30 days preceding paralysis onset.”
Remember, most people with polio don’t actually have any symptoms, although some do have flu-like symptoms. And fewer than 1% develop paralysis or weakness when they have polio. Although that doesn’t sound like a lot, during a polio epidemic, when a lot of kids are getting polio, the cases of paralytic polio quickly add up.
What else can provoke paralytic polio?
strenuous exercise (paralytic polio)
tonsillectomy (bulbar polio)
So how does an injection provoke paralytic polio?
“Skeletal muscle injury induces retrograde axonal transport of poliovirus and thereby facilitates viral invasion of the central nervous system and the progression of spinal cord damage.”
Gromeier et al on Mechanism of Injury-Provoked Poliomyelitis
Injury to a muscle by the needle is thought to have allowed the polio virus to move through the nerves in the area to the spinal cord, as long as the polio virus was already in their blood. How do we know it was the needle and not the vaccine itself? In experiments, they injected saline, and not an actual vaccine.
Is this how everyone developed paralytic polio?
Remember, kids didn’t get many vaccines around the time we were seeing polio outbreaks in the 1940s and 50s, although other injections, like penicillin were also thought to provoke paralytic polio.
Why were they getting penicillin? Often to treat congenital syphilis.
And although they went so far as to delay vaccines during outbreaks and to not do tonsillectomies during the summer, when polio outbreaks were more common, kids still got paralytic polio.
Could Provocation AFM Be a Thing?
Have you guessed why some folks are talking about provocation polio again, even though we are on the verge of eradicating polio?
“Seizing on a 2014 historical perspective piece on a phenomenon known as “polio provocation” in the highly respected medical journal, The Lancet, anti-vaccine forces have attempted to link the recent AFM cases (as they attempt to do with many other medical occurrences) to childhood vaccinations.”
Dr. Amesh Adalja on Clusters of polio-like illness in the US not a cause for panic
“…is there any relationship between vaccination status and a developing acute flaccid myelitis? Meaning, are vaccines a risk factor? And the data so far says no, the overwhelming number of children who have gotten AFM have had no recent vaccination of any kind or vaccine exposure. These cases over these years have been happening before flu season and flu vaccination starts, which is one of the questions that comes up, and there hasn’t been any pattern to vaccine exposure of any kind in developing AFM. So far, we have not found a link between the two.”
Benjamin Greenberg, MD on 2018 Podcast on Acute Flaccid Myelitis
For vaccines to provoke AFM, you would have to have gotten a recent vaccine.
We aren’t seeing that and anything else all of the kids with AFM had in common that might provoke paralysis, like acupuncture, cupping, or dry needling, would likely have come out in epidemiological reports.
“Before the vaccine was developed, the diagnosis of polio required 24 or more hours of paralysis. After the vaccine release, the diagnosis changed to at least 60 days of paralysis. As you can imagine, cases of polio dropped significantly.”
The Myth That Polio Went Away Because They Changed the Diagnostic Criteria
In 1952, there were 21,000 cases of paralytic polio in the United States.
But were there really?
Didn’t they change the way they diagnosed polio a few years later, right after the first polio vaccines came out, making it less likely that folks would be diagnosed with polio?
The original diagnostic criteria for polio came from the World Health Organization and included:
“Signs and symptoms of nonparalytic poliomyelitis with the addition of partial or complete paralysis of one or more muscle groups, detected on two examinations at least 24 hours apart.”
It changed in 1955 to include residual paralysis 10 to 20 days after onset of illness and again 50 to 70 days after onset.
“In the past children’s paralysis was often not correctly diagnosed as polio. Stool samples need to be analyzed to be able to distinguish paralytic symptoms from Guillain-Barré Syndrome, transverse myelitis, or traumatic neuritis.”
Polio – Data Quality and Measurement
But you coulld’t just use stool samples, as many kids might have recently had non-paralytic polio, and could test positive for polio (false positive test), but have another reason to have paralysis.
“Isolation of poliovirus is helpful but not necessary to confirm a case of paralytic poliomyelitis, and isolation of poliovirus itself does not confirm diagnosis.”
Alexander et al. on Vaccine Policy Changes and Epidemiology of Poliomyelitis in the United States
Since polio causes residual paralysis, the new diagnostic criteria helped to make sure that kids were diagnosed correctly.
Did We Overestimate the Number of Kids with Polio?
Some folks think that since we changed the criteria, we overestimated the number of kids with polio in the years before the vaccine came out.
Most of this idea seems to come from a panel discussion in 1960 by critics of the original polio vaccine, The Present Status of Polio Vaccines, including two, Dr. Herald R. Cox and Dr. Herman Kleinman, who were working on a competing live-virus vaccine.
None in the group were arguing against vaccines, or even really, that the Salk polio vaccine didn’t work at all though. They just didn’t think that it was effective as some folks thought.
“I’ve talked long enough. The only other thing I can say is that the live poliovirus vaccine is coming. It takes time. The one thing I am sure of in this life is that the truth always wins out.”
Dr. Herald R. Cox on The Present Status of Polio Vaccines
Dr. Cox did talk a lot about the oral polio vaccine. He talked about successful trials in Minneapolis, Nicaragua, Finland, West Germany, France, Spain, Canada, Japan, and Costa Rica, etc.
When anti-vaccine folks cherry pick quotes from The Present Status of Polio Vaccines discussion panel, they seem to leave out all of the stuff about how well the oral polio vaccine works.
“Since nothing is available, there seems to be no alternative but to push the use of it. I don’t think we should do so in ignorance, nor too complacently, believing that as long as we have something partially effective there is no need to have something better.”
Dr. Bernard Greenberg on The Present Status of Polio Vaccines
And of course, they did, fairly soon, switch to something better – the Sabin live-virus oral polio vaccine.
Interestingly, using the idea that we changed the diagnostic criteria to make polio go away in an argument about vaccines is known as the Greenberg Gambit.
It tells you something about anti-vaccine arguments, that these folks are misinterpreting something someone said about vaccines almost 60 years ago.
In pushing the idea that polio hasn’t been eliminated, but rather just redefined, they also miss that:
But isn’t polio still around and just renamed as transverse myelitis, Guillain-Barré syndrome (GBS), and aseptic meningitis?
Let’s do the math.
Using the adjusted numbers in the The Present Status of Polio Vaccines discussion, there were at about 6,000 cases of paralytic polio in the United States in 1959.
While 3,000 to 6,000 people in the United States develop Guillain-Barré syndrome each year, the risk increases with age, and it is rare in young kids. Remember, paralytic polio mostly affected younger children, typically those under age 5 years.
“Transverse myelitis can affect people of any age, gender, or race. It does not appear to be genetic or run in families. A peak in incidence rates (the number of new cases per year) appears to occur between 10 and 19 years and 30 and 39 years.”
Transverse Myelitis Fact Sheet
Similarly, transverse myelitis is uncommon in younger children, and there are even fewer cases, about 1,400 a year.
What about aseptic meningitis? That doesn’t usually cause paralysis.
So do the math.
You aren’t going to find that many kids (remember, the incidence was 5-7 per 1,000) under age 5 years who really have “polio,” but instead, because of a worldwide conspiracy about vaccines, are getting diagnosed with transverse myelitis, Guillain-Barré syndrome (GBS), or aseptic meningitis instead.
Anyway, kids with acute flaccid paralysis are thoroughly tested to make sure they don’t have polio. And both transverse myelitis and Guillain-Barré syndrome have different signs and symptoms from paralytic polio. Unlike polio, which as asymmetric muscle atrophy, the atrophy in transverse myelitis and Guillain-Barré syndrome is symmetrical. Also, unlike those other conditions that cause AFP, with polio, nerve conduction velocity tests and electromyography testing will be abnormal. Plus, polio typically starts with a fever. The other conditions don’t. So while these conditions might all be included in a differential diagnosis for someone with AFP, they are not usually that hard to distinguish.
“Each case of AFP should be followed by a diagnosis to find its cause. Within 14 days of the onset of AFP two stool samples should be collected 24 to 48 hours apart and need to be sent to a GPEI accredited laboratory to be tested for the poliovirus.”
Polio – Data Quality and Measurement
But why be so strict on following up on every case of AFP?
It’s very simple.
If you miss a case of polio, then it could lead to many more cases of polio. And that would tmake it very hard to eradicate polio in an area.
If anything, until the establishment of the Global Polio Eradication Initiative (GPEI) in 1988, it is thought that cases of polio and paralytic polio were greatly underestimated in many parts of the world!
And now polio is almost eradicated.
“DR. SABIN: Let us agree, at least, that things are not being brushed aside. Let us say that we might disagree on the extent to which certain things have received study. But I hope that Dr. Bodian realizes that nobody is brushing things aside. I would not have taken the trouble of spending several months studying viremia with different strains in chimpanzees and human volunteers, and viremia produced by certain low temperature mutants to correlate it with their invasive capacity, if I were merely brushing it aside.”
Live Polio Vaccines – Papers Presented and Discussions Held at the First International Conference on Live Poliovirus Vaccines
If they redefined how paralytic polio was diagnosed in 1955 as part of a conspiracy to make it look like the polio vaccines were working, then why did the number of cases continue to drop into the 1960s?
Shouldn’t they have just dropped in 1955 and then stayed at the same lower level?
And why don’t any of the folks with other conditions that cause paralysis, like transverse myelitis and Guillain-Barré syndrome (GBS) ever have polio virus in their system when they are tested?
Also, if the renaming theory explains why the polio vaccine didn’t work, then why do anti-vaccine folks also need to push misinformation about DDT and polio?
What to Know About Polio Myths and Conspiracies
The near eradication of polio from the world is one of the big success stories of the modern era, just as those who push the idea that has all been faked is a snapshot of society at one of our low points.
Polio is caused by one of three wild-type polio viruses.
Of course, anti-vaccine folks like to push misinformation about polio being caused by a lot of other things, from poor hygiene and eating too much white bread to having a tonsillectomy or being exposed to pesticides, like DDT.
“Williams describes the many blind alleys and false leads of the early days of polio research, when doctors, scientists, and public health officials were convinced that the disease was transmitted by bedbugs, budgies, cats, and flies, or caused by seafood, cow’s milk, jimson weed, fruit, vegetables, and DDT…”
If the polio virus doesn’t cause polio (germ theory denialism), then you can’t really expect the polio vaccine to prevent polio, now can you?
The DDT-Polio Connection?
There actually is a bit of a connection between polio and DDT, but not the one anti-vax folks think.
No, DDT didn’t cause polio.
“Between the end of World War II and the early 1950s, researchers, municipal officials, and individuals from Georgia to California employed DDT to stop polio by killing flies, a suspected but debated actor in the disease’s transmission.”
Conis on Polio, DDT, and Disease Risk in the United States after World War II
Yes, many towns would routinely spray with DDT after a polio epidemic came to town because they didn’t yet know what did cause polio.
For example, in May 1946, “sections of the city were blanketed” with DDT as they sought to stop the source of a polio epidemic in San Antonio, which they thought might be a “tropical mosquito.”
See the connection now?
Polio first. DDT spraying after.
This idea is especially easy to see when you understand that there were many polio outbreaks and epidemics in the late 19th and early 20th century, well before DDT was discovered to be an effective insecticide in the early 1940s.
And the spraying mostly stopped before the polio outbreaks stopped.
In 1951, although he wasn’t yet sure how the polio virus spread, Dr. Sabin did know it came from “human feces derived from patients and healthy carriers,” and he declared that there was “general agreement that there is no justification for initiating emergency insect control measures in the hope of stopping a poliomyelitis epidemic.”
“It is perhaps an established epidemiological principle that epidemiological probability must be compatible with bacteriologic (or virologic) possibility, particularly when the epidemiological probabilities lend themselves to several alternative explanations.”
Albert B Sabin, MD on Transmission of Poliomyelitis Virus
And even before that, the Editorial Board for the American Journal of Public Health, in 1946, said that “While municipal cleanliness and sanitation are always highly desirable, there is no reason to believe that improved methods of sewage treatment and disposal, more rigid standards for the purification of water supplies, or the dusting of DDT over a city from aeroplanes will have any measurable effect on the incidence of infantile paralysis.”
Also remember the other big reason that we saw DDT spraying in the United States – the elimination of malaria.
“The National Malaria Eradication Program, a cooperative undertaking by state and local health agencies of 13 southeastern states and the CDC, originally proposed by Louis Laval Williams, commenced operations on July 1, 1947. By the end of 1949, over 4,650,000 housespray applications had been made.”
CDC on Elimination of Malaria in the United States (1947 — 1951)
Did the spraying of DDT to eliminate the flies that transmit malaria in the southeastern United States correlate with extra cases of polio?
There were big outbreaks in New York, Indiana, Ohio, and many other parts of the country that didn’t spray DDT to help fight malaria.
“The peak year for use in the United States was 1959 when nearly 80 million pounds were applied. From that high point, usage declined steadily to about 13 million pounds in 1971, most of it applied to cotton.”
EPA on DDT Ban Takes Effect
Did we stop spraying with DDT in the early 1950s because it was banned and is that why we stopped seeing so much polio?
The peak year for DDT use was in 1959. Surprisingly, we don’t see that peak on any anti-vaccine graphs in 1959…
What was the peak year for polio cases? It wasn’t 1959 or 1960, as you would expect if there was a link between DDT and polio.
Although the use of DDT decreased after 1959, it was used until it was “banned” in 1972, and even then, there were exceptions for public health uses.
The polio virus causes polio.
Or at least why did we start seeing so many more cases in the late 18th through the mid 19th century, until it was controlled with our polio vaccines?
“…contrary to the prevailing “disease of development” hypothesis, our analyses demonstrate that polio’s historical expansion was straightforwardly explained by demographic trends rather than improvements in sanitation and hygiene…”
Martinez-Baker et all on Unraveling the Transmission Ecology of Polio
One rather simple and elegant explanation is that we started to get too clean, the “disease of development” hypothesis.
Improved hygiene and sanitation helped delay when kids would get polio. Remember, polio is spread by contaminated food and water through fecal-oral transmission.
So instead of routinely getting it when they were newborn babies or young infants, when they still had some protection from maternal antibodies, they got it later when they had no immunity. So polio essentially changed from an endemic disease, or something that everything got, to an epidemic form.