The Peaks That Didn't Come Back: Acute Flaccid Myelitis in the U.S., 2020–2025

Acute flaccid myelitis (AFM) is rare, serious and mostly strikes children: sudden, floppy weakness in one or more limbs, with lesions in the gray matter of the spinal cord. It looks exactly like paralytic polio — clinically and on scans. In 2014, 2016 and 2018, AFM surged to 120–238 cases a year, each time alongside the circulation of enterovirus D68 (EV-D68), a common cause. Then the every-other-year peaks stopped.

Six quiet years

About 75% of patients each year were under 18, with median ages of 7 to 12.

Confirmed AFM cases, August 2014–December 2025, and EV-D68 detections in children's respiratory specimens from the New Vaccine Surveillance Network, 2017–2025. CDC.

EV-D68 came back; AFM didn't

Pandemic measures in 2020–2021 likely suppressed EV-D68 along with other respiratory viruses. The New Vaccine Surveillance Network — which since 2017 has systematically tested children with acute respiratory illness at sentinel hospitals — saw EV-D68 return in 2022, above 2018 levels, and rise again in 2024 and, unexpectedly, 2025, when it began climbing in July and reached 9% of weekly respiratory cases by September. Yet AFM cases did not rise with it, and only one AFM patient in six years — a respiratory specimen in 2025 — had EV-D68 detected.

Why is unclear. One hypothesis: the strains now circulating are less able to attack nerves. A recent study found the dominant EV-D68 strains lack some amino acid changes in the viral capsid protein VP1 that produce neurovirulence in mouse and neuron models — an idea that needs direct testing. The virus may also have been missed: specimens are usually taken after weakness begins, not during the illness up to 7 days before, when the virus is easier to find, and many enterovirus-positive samples are never typed.

The patients

• 98–100% were hospitalized, 58–85% within a day of weakness beginning; 62–94% first went to an emergency department.
• 47% (2023) to 76% (2025) went to intensive care; 18–47% of those hospitalized needed breathing support, and 15–41% a mechanical ventilator.
• The most common treatments were steroids and intravenous immunoglobulin, given to 41–59%.
• No one died.

AFM in 2022 and 2024 resembled the EV-D68-driven cases of 2018: more patients had a preceding respiratory illness or fever (72–79%, against 42–62% in other years) and inflammatory cells in spinal fluid (69–72%). In 2022, arms were affected more often than legs (75% vs 65%) — the reverse of every other year.

Of the 163 patients tested (95%), 26% (2023) to 55% (2024) had an enterovirus or rhinovirus, most often in respiratory specimens; enterovirus A71 turned up in stool in a few.

The polio question

Because AFM and polio are indistinguishable, every AFM case is checked for poliovirus — above all through stool testing. That worked in 2022: an unvaccinated young adult in New York, first evaluated for possible AFM, was found through the AFM protocol to have paralytic polio. With no travel history, and the last indigenous U.S. case in 1979, polio hadn't been suspected. Poliovirus can circulate in the environment for months after being shed in stool, and vaccination is what prevents paralytic disease.

Yet only about half of AFM patients had stool tested — rising to 74% in 2023, perhaps after the New York case, and 76% in 2025 — below the 80% recommended internationally for paralysis surveillance. Most AFM patients were at low polio risk: 75% (2021) to 100% (2025) of those with documented doses had had 3 or more, and only 2% were completely unvaccinated. Patients with unknown status were more often 18 or older, and may simply lack records.

What CDC asks

• Clinicians: stay alert for AFM, especially after 2025's unexpected EV-D68 rise, and report suspected cases to the health department.
• Collect stool from every patient suspected of AFM, and record polio vaccination status.
• Everyone: stay up to date with polio vaccination.

Limits. Reporting is passive, so cases may be undercounted; records may be incomplete; and missing vaccination data may understate how many patients were vaccinated.

Sources

Based on Lopez AS, English R, Rogers SL, et al., "Acute Flaccid Myelitis Surveillance — United States, January 2020–December 2025," MMWR Morbidity and Mortality Weekly Report volume 75, number 26, Centers for Disease Control and Prevention; a work of the United States government in the public domain.