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Home » Polio-like acute flaccid myelitis – research points to a virus, not vaccines

Polio-like acute flaccid myelitis – research points to a virus, not vaccines

Since 2014, I have been writing about acute flaccid myelitis (AFM), a mysterious viral disease that presents with polio-like symptoms. Since my initial report, the CDC has stated that a total of 590 individuals have been stricken by acute flaccid myelitis from 2014 through 2019.

Each report of AFM has brought out zombie memes pushed by the anti-vaccine religion, they blame acute flaccid myelitis on the polio vaccine. Of course, these bogus claims aren’t based on any scientific evidence, but that’s never stopped the anti-vaccine zealots.

These outbreaks have caused the public health sleuths to search for the actual causes of this polio-like syndrome. And there just isn’t any robust or valid evidence that the polio vaccine is anyway related to acute flaccid myelitis.

As we know, polio can be a crippling and potentially deadly infectious disease caused by the poliovirus, a human enterovirus, that spreads from person to person invading the brain and spinal cord and causing paralysis. Because polio has no cure, the polio vaccine is the best way to protect ourselves from the crippling disease.

The United States last experienced a polio epidemic in the 1950sprior to the introduction of the polio vaccine 60 years ago. Today, polio has been eradicated from most of the planet, as the number of worldwide polio cases has fallen from an estimated 350,000 in 1988 to 32 in 2018 – a decline of more than 99% in reported cases.

Because real scientists wanted to know what caused this acute flaccid myelitis outbreak, they tried to hunt down the actual cause. A recent study of most of the individuals who have contracted the disease seems to be narrowing down on a couple of culprits.

Acute flaccid myelitis – enteroviruses

Enteroviruses are a genus of several RNA viruses that are associated with numerous diseases that afflict humans and other mammals. As of today, researchers have identified 84 different serotypes of human enteroviruses, although there is some variability in antigens within each of the different serotypes.

Early in the research on these viruses, they were given separate names, such as poliovirusesCoxsackie A virusesCoxsackie B viruses, and echoviruses. Today, the viruses, as they are identified, are given names in a system of consecutive numbers. Enterovirus 68 (known as EV68, EV-D68, or HEV68) is the 68th in the list of these viruses.

Although many people claim that enteroviruses are only transmitted from the stool of an infected person (hence the “entero” portion of the name of the virus), it also can be found in respiratory secretions.

Probably, the most famous enterovirus is the poliovirus which is responsible for poliomyelitis. Poliovirus is an enterovirus, but not all enteroviruses cause polio.

There are 81 non-polio and 3 polio enteroviruses that can cause disease in humans. Of the 81 non-polio types, there are 22 Coxsackie A viruses, 6 Coxsackie B viruses, 28 echoviruses, and 25 other enteroviruses. Several of these viruses, such as poliovirus, are transmitted through the oral-fecal route, but others can be transmitted through oral-oral routes like simple sneezing.

According to the CDC, there have been many non-polio enterovirus outbreaks across the world recently:

  • Coxsackievirus A16 is the most common cause of hand, foot, and mouth disease (HFMD) in the United States.
  • Coxsackievirus A6 was the most commonly reported type of enterovirus in this country from 2009 to 2013, mostly due to a large outbreak in 2012 of severe hand, foot, and mouth disease. Some of the infected people developed symptoms that were more severe than usual.
  • Coxsackievirus A24 and enterovirus 70 have been associated with outbreaks of conjunctivitis.
  • Echoviruses 13, 18, and 30 have caused outbreaks of viral meningitis in the United States.
  • Enterovirus 71 has caused large outbreaks of HFMD worldwide, especially in children in Asia. Some infections from this virus have been associated with severe neurologic disease, such as brainstem encephalitis.
  • Enterovirus 68 caused a nationwide outbreak in 2014 of severe respiratory illness in the United States.

So, enteroviruses are quite common and have been associated with numerous outbreaks across the world.

Enterovirus D-68.

Enterovirus 68

I first became aware of EV68 when I read a 2014 report from the CDC which described results from testing of 23 children in California who had presented with the polio-like syndrome or acute flaccid myelitis. The syndrome includes muscle weakness or paralysis along with an infection of the grey matter of the spinal cord. In general, these are similar to the symptoms of paralytic polio, though many other viruses can present with the same symptoms (See note 1).

EV68 almost always causes respiratory illnesses, which can vary from mild to severe. However, the symptoms can range from almost asymptomatic to flu-like to polio-like. Generally, the initial symptoms are like the common cold, which include a runny nose, sore throat, cough, and fever. As the infection progresses, more serious symptoms may arise up to and including flaccid paralysis.

The majority of people who are exposed to EV68 may not realize that they’ve contracted the disease. In healthy adults, the disease is mostly asymptomatic, and they may not even know that they had contracted the virus. However, EV68 is disproportionately debilitating to very young children. This article focuses on the effects on children.

The outbreak of acute flaccid myelitis

As I mentioned previously, the CDC examined the early data on 23 patients who had contracted AFM. They determined that none of the patients had any form of the poliovirus. They also couldn’t find any evidence of a recent infection with any other infectious agent that could possibly be linked to AFM. In addition, they found that apparently only two of the patients had an EV68 infection.

In another study, published in Lancet Infectious Diseases, the researchers closely examined samples from 14 patients with AFM with an EV68 infection – they could not find any other infectious disease, other than EV68. The authors concluded that:

These findings strengthen the putative association between enterovirus D68< and acute flaccid myelitis and the contention that acute flaccid myelitis is a rare yet severe clinical manifestation of enterovirus D68 infection in susceptible hosts.

By 2014, there was a wider outbreak of enterovirus 68 in the USA. The CDC reported that,

In summer and fall 2014, the United States experienced a nationwide outbreak of EV-D68 associated with severe respiratory illness. From mid-August 2014 to January 15, 2015, CDC or state public health laboratories confirmed a total of 1,153 people in 49 states and the District of Columbia with respiratory illness caused by EV-D68. Almost all of the confirmed cases were among children, many whom had asthma or a history of wheezing. Additionally, there were likely many thousands of mild EV-D68 infections for which people did not seek medical treatment and/or get tested.

CDC received about 2,600 specimens for enterovirus testing during 2014, which was substantially more than usual. About 36% of those tested positive for EV-D68. About 33% tested positive for an enterovirus or rhinovirus other than EV-D68.

EV-D68 was detected in specimens from 14 patients who died in the U.S. in 2014. State and local officials have the authority to determine and release information about the cause of these deaths.

After all of this data and evidence, what can we conclude?

First of all, acute flaccid myelitis is extraordinarily rare, so rare that we might only see 1.4 cases per 100,000 population. Because it is rare, it is extraordinarily difficult to establish causality between a virus, like EV68, and the outbreak of AFM. But like good scientists everywhere, researchers provided evidence that helped us link the AFM outbreak to EV68.

Second, polio was ruled out. This was not polio. These diseases are unrelated to polio. And the polio vaccine had nothing to do with it (given that 2 out of 23 people were unvaccinated, making the risk of AFM technically higher for unvaccinated individuals than vaccinated ones based on this limited sample).

What we know about acute flaccid myelitis cases

Dr. Vincent Iannelli, writing at Vaxopedia, reviewed the nearly 350 cases of the acute flaccid myelitis, or the subtype of a polio-like syndrome, that we have been seeing since 2014. Dr. Iannelli wrote:

  • most cases occur in children
  • they have a magnetic resonance image (MRI) showing spinal cord lesion largely restricted to gray matter and spanning one or more vertebral segments
  • AFM can be caused by viruses, environmental toxins, and genetic disorders, although no common etiology has been found in recent cases
  • cases are occurring sporadically – after 120 cases in 34 states in 2014, there were only 24 cases in 17 states in 2015, but then 149 cases in 39 states in 2016 and 33 cases in 16 states in 2017. And there have been about 38 cases in 16 states in 2018.
  • most cases occur in the late summer and early fall
  • most had symptoms of a viral illness, including respiratory symptoms or diarrhea
  • some were unvaccinated
  • the ages of affected children ranged from 5 months to 20 years
  • some, but not all, were positive for enterovirus D68

Causality in acute flaccid myelitis outbreak

In a recent study published in the respected journal Eurosurveillance, a journal that specifically focuses on infectious diseases and epidemiology, Australian researchers focused on this 2014 outbreak of acute flaccid myelitis. EV68 is a rare virus, and initially, it was never considered to cause the polio-like syndrome.

However, using the Bradford Hill criteria, a method that includes a checklist of nine points that can be used to determine whether epidemiological evidence can indicate a causal relationship between a presumed cause and an observed effect, the researchers examined all of the data that related to this outbreak of AFM to determine if EV68 was the causal factor.

They drew on 20 published studies that had focused on the polio-like syndrome outbreak and EV68. Using the Bradford Hill criteria, the researchers quantitatively analyzed each of the points that establish causality, the authors concluded that:

In summary, the application of the Bradford Hill criteria suggests that EV-D68 causes AFM. AFM has not previously been associated with EV-D68, and a mouse model shows that the original Fermon strain does not cause AFM, whereas the 2014 outbreak strain does. It appears that the incidence of this infection and the clade-specific epidemiology have changed. Phylogeographic epidemiology will further our understanding of the temporal and spatial spread of increasingly neurovirulent clades and improve risk analysis. Further investigation into this relationship is important because of the severity of AFM, ongoing outbreaks of AFM and because there is currently no treatment for AFM related to EV-D68, and no vaccine to prevent infection.

That’s a lot of solid evidence that EV68 probably was the infectious agent that caused the outbreak of polio-like syndrome, acute flaccid myelitis. And we have pretty solid evidence that it was not polio or vaccine-derived poliovirus (VDPV), a very rare virus caused by the mutation or recombination of the attenuated viruses used in the oral polio vaccine (OPV).

Nevertheless, the anti-vaccine pseudoscience crowd claims that the polio vaccine created EV68, which led to the acute flaccid myelitis outbreak. As I wrote above, EV68 is related to polioviruses – they are in the same genus, Enterovirus. But, saying that EV68 is the same as poliovirus is scientifically incorrect.

It is nearly impossible to determine when the common ancestor of the poliovirus and EV68 diverged during the evolution of enteroviruses, but they are so different that EV68 RNA cannot recombine with poliovirus RNA. They have completely different protein coats that means when EV68 meets up with poliovirus, they’re as different as a dog and cat – they’re both furry and all, but they would rather not have much to do with each other.

VDPV is extremely rare, and generally, it is only a concern in populations that have low levels of vaccination against polio. In populations with high levels of polio vaccination, people would be immune to the VDPV, since its antigens are the same as wild-type poliovirus.

It is important to note that the AFM outbreak occurred mostly in countries, like the USA and Canada, where the typical vaccine used is the injectable polio vaccine (IPV) which uses inactivated poliovirus and cannot replicate or mutate.

In other words, if you’re trying to blame the polio-like syndrome outbreak on the polio vaccine, at least in most of the world which uses IPV, those claims are implausible if not impossible.

acute flaccid myelitis

CDC Update – 13 November 2018

The CDC published, in a recent issue of Morbidity and Mortality Weekly Report, an analysis of 2018 acute flaccid myelitis cases in the United States. Some of the key points are:

  • From 1 January through 2 November 2018, the researchers found 80 confirmed acute flaccid myelitis cases in 25 states. Most of the cases occurred during September and October (though there’s probably no correlation between time of year and AFM).
  • There were 3X as many AFM cases so far in 2018 compared to 2017.
  • The median age of the patients was 4 years old.
  • Almost all of the cases reported fever and/or respiratory illnesses in the 3-10 days prior to the onset of limb weakness.
  • None of the patients died.
  • There are about 250 additional cases that are still under investigation.

However, the most important data was that 11 patients (29%) were positive for enterovirus EV-A71 and 14 (37%) for EV-D68. The remaining patients with AFM had other viruses. In addition, all stool specimens tested negative for poliovirus.

The CDC is still investigating the causes of the disease. It could be a new enterovirus. It could be a combination of other viruses. It could be some other factor.

But we know it’s not caused by the polio vaccine. Or any vaccine, for that matter. Although this is a concerning new disease, the numbers are tiny. In 1952, prior to the launch of the polio vaccine, there were 57,628 cases reported while 3,145 died of the disease and 21,269 were left with mild to disabling paralysis. Let’s hope that acute flaccid myelitis doesn’t become an infectious disease that terrible – of course, once we know what the pathogen is, we can develop a vaccine to stop it.

Real scientists doing real research on acute flaccid myelitis.

CDC Update – 12 July 2019

In a recent report in the peer-reviewed Morbidity and Mortality Weekly Report (MMWR), CDC researchers updated clinical, laboratory, and outcome data for the 233 confirmed cases of acute flaccid myelitis that were reported in 2018.

As in previous outbreaks of AFM, the average age of the patients was about 5 years. Most of these patients presented with respiratory symptoms or fever within four weeks of developing limb weakness. 

On average, these patients were hospitalized with 1 day of onset of limb weakness. Blood, sputum, and stool samples were collected within 2-7 days. Unfortunately, there was a significant delay, from 18 to 36 days after symptoms onset, in reporting to the CDC. This hampered the agency’s ability to determine the causes of AFM.

According to the CDC report:

  • 98% of patients were hospitalized,
  • 60% required care in the intensive care unit,
  • 27% required mechanical ventilation. 
  • 44% of the confirmed cases exhibited several enterovirus types, including EV-D68 and EV-A71.
  • Of the 74 cases for which there were cerebrospinal fluid specimens, only two were found positive for enteroviruses – one with EV-D68 and one with EV-A71.
  • No stool specimens tested positive for poliovirus – once again this eliminates the negligible possibility that a polio vaccine was the cause of these cases of AFM

The CDC is continuing to monitor trends and clinical presentations of acute flaccid myelitis throughout the country. They are investigating possible risk factors for the disease to understand how and which viruses may lead to AFM. They are also tracking the long-term outcomes of these AFM patients.

Approximately 70-80% (possibly more) of the children who exhibited AFM have experienced ongoing limb weakness several months after onset. This is why the CDC has taken an aggressive approach to research on this condition. The CDC website now includes preliminary recommendations for clinical management of acute flaccid myelitis.

Acute flaccid myelitis – October 2019 update

Tracy Ayers, Ph.D., and colleagues from the Centers for Disease Control and Prevention, published in the respected journal Pediatrics, examined children with clinical case criteria of acute flaccid limb weakness from January 1, 2015, through December 31, 2017. They found the following:

  • 79% reported a respiratory or febrile illness from 2-7 days before the onset of limb weakness.
  • Cases tended to cluster in the late summer or fall.
  • Viral pathogens were found in almost half of the patients. 
  • Enterovirus D68 was found in 24% of cases.
  • The authors stated that these findings “strongly suggested a viral etiology including…enteroviruses.”
  • The authors also stated that enterovirus D68 appears to be the most likely driving force behind acute flaccid myelitis. However, because enteroviruses circulate widely, it is difficult to establish causality especially when samples are taken from non-sterile sites. In other words, the “background noise” of enteroviruses makes it very difficult to identify causal agents. 
  • Polioviruses were not found in any of the samples.
  • AFM is a dangerous disease. At the time of limb weakness onset, 33% of patients exhibited cranial nerve issues, 36% exhibited quadriplegia, and 33% required mechanical ventilation. 
  • Critically, AFM may be an under-reported disease, since there are no national reporting standards for it. The CDC may look into doing this.
Obligatory cute puppy photo. Photo by Andrew Schultz on Unsplash.


We know that the polio vaccine is unrelated to acute flaccid myelitis. It would take special pleading or magic to create some biologically implausible link between the vaccine and any polio-like syndrome.

Although the CDC hasn’t firmly established that EV68 (or possibly EV71) are linked to the acute flaccid myelitis outbreak, there are some powerful data that it might be. Nevertheless, as data pours into real scientists, the CDC will narrow in on the guilty virus, whether it’s one or more enteroviruses or a completely new virus. And then we can create a new vaccine.


  1. There appear to be several interchangeable medical terms for the same disease. For the purposes of this article, I will stick with either polio-like syndrome (some people call it a polio-like illness) or acute flaccid myelitis, even though they aren’t completely interchangeable. In addition, acute flaccid paralysis with anterior myelitis has been used to describe the polio-like syndrome, although acute flaccid myelitis is now the most common term.

This article was first written in January 2018 and republished with revisions in October 2018. It has been further updated as a result of new research in November 2018, July 2019, and October 2019. All include more data that shows that acute flaccid myelitis is unrelated to the polio vaccine.

Key citations:


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