Surveillance for Acute Flaccid Myelitis ― United States, 2018–2022

Discussion

The biannual peak pattern of AFM cases observed during 2014–2018 did not persist in 2020 or 2022. In 2020, nonpharmaceutical interventions for prevention of COVID-19 likely reduced the number of EV-D68 and other respiratory infections, which could have led to fewer cases of AFM (5–8). However, during the summer of 2022, sentinel surveillance among persons aged <18 years with acute respiratory illness detected increases in EV/RV and EV-D68 respiratory infections at levels not seen since 2018, suggesting that EV-D68 was widely circulating and causing respiratory illness in the United States during 2022 (7).

None of the patients with confirmed AFM since 2019 has received a positive EV-D68 test result, and only 39%–50% received a positive EV/RV test result. Diagnosing EV/RV infection among patients with AFM is challenging for several reasons. Respiratory specimens have the highest yield for detecting EV-D68, but because samples are typically collected at hospitalization several days to weeks after the start of a prodromal respiratory illness, the virus might no longer be present at the time of specimen collection (1–3). In addition, although most laboratories can test for EV/RV, further characterization (e.g., typing) is not available in most settings. CDC routinely performs EV/RV testing and, if results are positive, performs EV typing on specimens from patients with suspected AFM. Only 71% of confirmed cases during 2018–2022 had at least one specimen (respiratory, serum, cerebrospinal fluid, or stool) sent to CDC (CDC, unpublished data, 2018–2022); EV-D68 or other specific EVs might have been present in specimens that were not tested.

Historically, the clinical characteristics of confirmed AFM cases have varied among peak years (2016 and 2018) and nonpeak years (2015 and 2017), suggesting that AFM caused by EV-D68 might have a different clinical profile than AFM of other etiologies (9). Cases reported during 2019–2021 appeared similar to those reported during nonpeak years, with a lower proportion of antecedent respiratory illness or fever, upper limb involvement, and CSF pleocytosis, and a higher proportion of lower limb involvement, compared with cases in 2018. However, cases reported during 2022, when EV-D68 was circulating, did not follow this pattern: 2022 cases had a higher proportion of antecedent respiratory illness or fever, upper limb involvement, and CSF pleocytosis compared with cases during nonpeak years (2019 and 2021) and a lower proportion compared with cases during 2018.

Despite apparently increased EV-D68 circulation and EV-D68–associated respiratory disease among children, the reason why an increase in AFM cases did not occur in 2022 is unclear; possibly, EV-D68 viruses circulating in 2022 were less neurotropic or less likely to cause neurologic disease than were viruses circulating during 2014, 2016, and 2018. Another possibility is that infection with respiratory viruses including other RV/EVs, SARS-CoV-2, or respiratory syncytial virus that were frequently circulating in 2022 affected immune responses to EV-D68 and provided protection against neurologic disease (6). Data to support either of these hypotheses are lacking, and investigations are ongoing.

Limitations

The findings in this report are subject to at least three limitations. First, this analysis was based on AFM cases reported to CDC and might underestimate the actual number of AFM cases in the United States. Second, clinical information is collected from a patient summary form typically completed by a health department and clinical records, which might contain incomplete data. Finally, 29% of cases did not have any specimens sent to CDC on which EV typing could be performed, limiting the ability to identify the specific EV associated with AFM.

Implications for Public Health Practice

Current trends do not indicate when the next increase of AFM might be expected. Nonetheless, clinicians should be alert to the possibility of AFM among children with acute flaccid limb weakness and report to health departments when they suspect cases. In addition, to better understand causes for AFM, including the role of EVs and EV-D68, it is important that sufficient laboratory samples be collected to facilitate testing and typing of EVs.