Frequently Asked Questions

What can I do if I suspect AFM?
  • Seek medical care immediately if you or your child develops any of these symptoms: weakness or loss of muscle tone or reflexes in the arms or legs, facial droop or weakness, difficulty moving the eyes, drooping eyelids, difficulty with swallowing, or slurred speech.
  • If your clinician suspects you or your child has AFM, they should hospitalize immediately. AFM can progress rapidly and sometimes require machines to help patients breathe.

Possible Causes

Why does CDC believe that viruses play a role in AFM?

  • CDC has been tracking AFM since 2014. This has contributed important evidence to support a role for viruses, including enteroviruses, in AFM. For example,
    • We’ve seen a seasonal pattern to AFM. Most patients develop AFM in late summer or early fall. At this same time of year, many viruses, including enteroviruses, commonly circulate in the U.S.
    • Most patients had respiratory symptoms or fever consistent with a viral infection less than a week before limb weakness began.
  • Our work will help lead to more answers about this complex illness, such as why some people develop AFM, and why outbreaks happen.

Recent research has indicated that enteroviruses are the likely cause of AFM. Does CDC believe this as well?

  • AFM can be caused by viruses, including enteroviruses.
  • Studies have shown that AFM patients had antibodies specific for enteroviruses in their spinal fluid more often than persons without AFM. Having antibodies for enterovirus means that a person was previously infected with the virus. This is an important step toward understanding how AFM develops and why some children are severely affected.
  • CDC’s partnerships with other institutions, such as universities, federal agencies, and health departments, are important to better understand AFM and these outbreaks.

How is AFM different from polio?

  • Poliomyelitis is the term used to describe the syndrome of acute flaccid limb weakness and lesions in the grey matter of the spinal cord. Poliomyelitis caused by poliovirus no longer occurs in the United States. In 2014, the term AFM was adopted to describe poliomyelitis without a known cause and not caused by poliovirus.
  • We now have stronger evidence that points to AFM being caused by non-polio enteroviruses, such as EV-D68 and EV-A71. Poliovirus can be prevented by a vaccine. However, there is no vaccine yet available for EV-A71 in the United States, and no vaccine for EV-D68 or for the other enteroviruses that are believed to cause AFM.
  • All the stool specimens from AFM patients that we received tested negative for poliovirus. We continue to test all specimens from AFM patients to look for viruses, including poliovirus and non-polio enteroviruses.

Is there an immune response that could trigger AFM?

  • CDC continues to work toward learning why some people develop AFM, including if a virus leads to an inflammatory or immune response directed toward motor neurons (nerves that make the muscles move).

Is there a link between vaccinations and AFM?

  • CDC has no evidence to suggest that vaccinations cause AFM.
  • Of the AFM cases reviewed in 2018, about 85% had no recorded vaccinations in the 30 days prior to the beginning of their limb weakness.
  • We continue to review past medical history of all cases, including past vaccinations, as part of our work.
    • In 2018, AFM occurred in children who had received vaccines, and also in children who had received no vaccines. CDC will conduct further research using medical and vaccination histories to examine this question.
    • In a 2016 study of 10 confirmed cases of AFM in children, 9 had received all childhood vaccines recommended by the Advisory Council on Immunization Practices. When vaccine records were examined, the average amount of time between when they got their last vaccine and when they started to experience AFM symptoms (i.e., limb weakness), was almost 2 years.

Health Outcomes

Has anyone died from AFM?

  • Two patients with confirmed AFM died soon after developing AFM,  one in 2017 and one in 2020. We have also learned of deaths in cases confirmed in previous years.
  • CDC is gathering information on deaths reported to us from health departments. We are also gathering information as part of our patient outcome data collection. We ask that health departments send information about every death of an AFM case to CDC.

When will CDC or state health departments follow up with all patients?

  • CDC has implemented a systematic process for determining the patients’ outcomes at 2, 6, and 12 months after limb weakness began.
  • We are currently interviewing confirmed and probable cases from 2018 and will be interviewing these cases in subsequent years moving forward. We are also going to be reaching out to confirmed cases from 2014 – 2017.


Why was there no outbreak of AFM in 2020?

  • There have been increases in AFM cases in 2014, 2016, and 2018 across the U.S. Based on this pattern we anticipated an increase in cases in 2020, but we did not see that. CDC believes prevention measures such as social distancing, mask wearing, and increased hand washing may have reduced the circulation of enteroviruses that can cause AFM.
  • We continue to monitor reported AFM cases closely and stay prepared to respond to any increase in cases.

How are AFM cases reported to CDC?

All health departments have some type of requirement for clinicians to report suspected AFM cases. The steps for reporting are:

  1. The health department works with healthcare providers to collect medical information and spinal cord MRI images.
  2. The health department then sends this information to CDC.
  3. Once we receive a report from the health department, the patient summary form and spinal cord images are reviewed by experts and given a case classification based on the AFM surveillance case definition.
  4. Once we’ve made a final case classification, we send that information to the health department who then shares with the clinician, and the clinician shares it with the patient and their family. Clinicians should not wait for CDC’s case classification to diagnose or treat their patient.

To protect patient privacy, CDC does not receive patient information such as names, so we are not able to tell whether a specific patient’s records have been sent to us. We strongly encourage you to reach out to your state health department if you have questions about whether your child has been reported to CDC. If you are unable to reach your health department, please contact us at

How do we know every case of AFM is being recognized?

  • AFM is not nationally notifiable, but all jurisdictions have some type of requirement for clinicians to report patients under investigation (PUIs) for AFM cases. Some have made AFM reportable, some require reporting of acute onset of flaccid paralysis without other apparent cause, and some require reporting of new or emerging conditions of public health importance, like AFM.
  • CDC does not decide which diseases are nationally notifiable, but we do advise the Council of State and Territorial Epidemiologistsexternal icon and make recommendations about which diseases should be nationally notifiable.
  • In June 2019, it was proposed that AFM be made nationally notifiable at the Council of State and Territorial Epidemiologists (CSTE) annual meeting. The proposal did not pass.
  • State and local health departments, CSTE, and CDC, will continue to collaborate and respond to AFM.

Why is CDC’s case definition not counting all patients with AFM?

  • CDC has case definitions for AFM with specific criteria that we use to classify patients as having a confirmed or probable case of this illness. If a patient does not meet these criteria, they are classified as “not a case” of AFM.
  • By focusing on patients with the most similar symptoms (confirmed cases), we can better compare them to understand possible risk factors and causes of AFM.
  • CDC case classifications do not change treatment or patient care.
  • Even though some patients may not meet our case definitions, the information provided about these patients still helps us understand the full spectrum of illness.
  • As we continue to learn more about AFM and its initial symptoms, we are working with our partners to refine our case definitions so that we can better understand AFM.

Page last reviewed: December 29, 2020