Acute flaccid myelitis outbreaks in the US: what the data showed
Acute flaccid myelitis outbreaks struck the United States in 2014, 2016 and 2018, a two-year rhythm that public health experts tracked closely. Each wave hit young children hardest, mostly in late summer and early fall.
This update pulls together what CDC surveillance found, with confirmed case counts through October 2026. For a plain guide to the illness itself, its signs and its causes, read AFM: a polio-like illness explained.
Quick facts
- AFM was first defined as a condition to track in 2014.
- The CDC now counts 153 confirmed cases in 2016 and 238 in 2018.
- Since 2019, yearly counts have stayed between 17 and 48.
- The typical child in 2018 was about 5 years old.
- Most cases appeared in late summer and early fall.
Sources: CDC MMWR: Vital Signs, surveillance for acute flaccid myelitis, United States, 2018; CDC: AFM cases and outbreaks; CDC: previous AFM cases and outbreaks; CDC MMWR: Vital Signs, 2018 AFM surveillance.
How the outbreaks began
Doctors began tracking AFM as its own condition in 2014. That year, the US had a large outbreak of severe breathing illness caused by enterovirus D68, often written EV-D68. Clinicians then noticed a cluster of children with sudden, floppy weakness in their arms or legs.
The CDC built a case definition so that health departments could count these children the same way across the country. That case count is what made the later pattern visible.
Sources: CDC MMWR: Vital Signs, surveillance for acute flaccid myelitis, United States, 2018.
The every-other-year pattern
After 2014, case counts spiked again in 2016 and 2018, with quieter years between. The CDC's 2019 surveillance report described nationwide AFM outbreaks every two years since 2014, and said the reason for that rhythm was still under study.
| Year | Confirmed cases (CDC) | Note |
|---|---|---|
| 2014 | 120 | First outbreak, during a national wave of EV-D68 breathing illness |
| 2015 | 22 | |
| 2016 | 153 | Second outbreak |
| 2017 | 38 | |
| 2018 | 238 | Third and largest outbreak, cases in 42 states |
| 2019 | 47 | No outbreak since |
| 2020 | 34 | |
| 2021 | 29 | |
| 2022 | 48 | |
| 2023 | 19 | |
| 2024 | 25 | |
| 2025 | 17 | Cases in 10 states |
| 2026 | 11 so far | Cases in 9 states, as of October 2, 2026 |
The CDC updates these counts as health departments send in and classify reports, so recent years can still change. Its cases page links the peak years to enteroviruses.
Sources: CDC: AFM cases and outbreaks; CDC: previous AFM cases and outbreaks; CDC MMWR: Vital Signs, 2018 AFM surveillance.
Who was affected in 2018
The CDC's 2019 report on the 2018 outbreak gave the clearest picture of the children behind the numbers. Its figures come from the cases counted when it was written, so they differ slightly from today's totals. The median age of children with confirmed AFM was 5.3 years. Nearly all of them had been sick shortly before the weakness began.
| Finding from the 2018 cases | What it means |
|---|---|
| 96 percent had an illness in the 4 weeks before weakness | AFM usually follows another illness |
| 92 percent had fever or breathing symptoms in that window | The illness often looked like a cold or flu |
| Median of 5 days from that illness to weakness | Weakness can follow quickly |
| 60 percent were admitted to intensive care | AFM can become serious fast |
| 27 percent needed help with breathing | Weak breathing muscles are the biggest danger |
These numbers describe the children counted in one year. They do not predict what will happen to any one child.
Sources: CDC MMWR: Vital Signs, 2018 AFM surveillance.
Why fast reporting matters
The 2018 report pointed to a gap: clinicians were slow to report suspected cases to public health officials. Its message was that prompt recognition, early specimen collection and rapid reporting speed up the public health work.
Speed matters for the child too. The CDC says tests that help confirm AFM work best when they are done as soon as possible after symptoms appear. Samples taken late can miss a virus that was there early.
Sources: CDC MMWR: Vital Signs, 2018 AFM surveillance; CDC: AFM diagnosis.
Where things stand now
The big two-year waves have not come back since 2018. CDC counts from 2019 through 2025 stay between 17 and 48 confirmed cases a year, and 2026 had 11 confirmed cases by early October. The CDC says it continues to receive and look into reports of suspected cases sent in by health departments. It still describes AFM as uncommon but serious, and still lists no specific way to prevent it.
For families, the practical lesson from the outbreak years has not changed. If a child suddenly cannot move an arm or leg normally, especially after a recent cold or fever, get medical care the same day. If breathing seems weak, call 911.
Sources: CDC: AFM cases and outbreaks; CDC: what CDC is doing about AFM; CDC: about AFM.
More on related conditions: transverse myelitis, pediatric neuroimmunology and the NICC overview.
Who this page is for
This update is for parents who remember the AFM headlines from 2014 to 2018 and want to know what the data actually showed. It also helps teachers, coaches and school nurses who see many children with colds each fall and want to know which warning sign matters. The one thing to remember: sudden weakness in a limb is never something to watch and wait on.
Questions people ask
Why did AFM cases rise every two years?
The CDC has not given a final answer. Its 2019 report said outbreaks came every two years from 2014 and that the cause of this pattern was still under study. The outbreak years lined up with waves of enterovirus activity, especially EV-D68, which the CDC links to the spikes in AFM cases.
What time of year does AFM usually happen?
Most cases in the outbreak years appeared in late summer and early fall, according to the CDC's 2018 surveillance report. A child can still get AFM at other times of year, so the season should never be a reason to wait. Sudden weakness in an arm or leg always needs prompt care, whatever the month.
Is acute flaccid myelitis still happening?
Yes, though far less often than in the outbreak years. The CDC counted 17 confirmed cases in 2025 and 11 in 2026 as of October 2. It still describes AFM as a serious condition with no specific prevention, and still urges fast medical care for sudden limb weakness.
What is the life expectancy for someone with acute flaccid myelitis?
There is no simple figure for this. In 2018, many children needed intensive care and about a quarter needed help breathing, which shows how serious the early illness can be. Long-term outlook depends on how much strength returns. Some children recover a great deal, while others keep lasting weakness and need ongoing rehabilitation.