The article we suggest this week, Acute Demyelinating Events Following Vaccines: A Case-Centered Analysis, concerns case reports suggesting that vaccines may trigger transverse myelitis (TM) or acute disseminated encephalomyelitis (ADEM), but whose evidence for a causal association is inconclusive.

SUMMARY
Concerns that vaccines might trigger autoimmune demyelination have existed for many years. Most publications on the subject are anecdotal case reports, with limited scientific evidence to support these concerns, and multiple studies have found no association. The Institute of Medicine has judged the evidence to be inconclusive with respect to a causal association between specific acute demyelinating events (ADEs) and any vaccine. A significant challenge for earlier studies of ADEs, in general, is that there is limited understanding about the underlying pathophysiology or causes of these diseases. Acute transverse myelitis (TM) is a rare acquired demyelinating disorder that presents with sudden onset of neurologic deficits due to spinal cord lesions. Like TM, acute disseminated encephalomyelitis (ADEM) is a rare disease, often preceded by a respiratory or gastrointestinal illness. It has also been reported after a number of immunizations, mostly as case reports unable to support causality.
The study used a case-centered design, which compares vaccination patterns during an exposure interval prior to the outcome in cases versus the entire study population, matched by age, sex, and site. The authors used the Vaccine Safety Datalink (VSD) site's internal electronic medical record diagnostic text codes to identify first-ever TM and ADEM diagnoses in any setting, and required at least 1 diagnosis by a neurologist within 3 months from the initial diagnosis.
Over all VSD sites, 81 cases were accepted as new, acute-onset idiopathic TM: 67 of these cases had received an immunization within 9 months prior to the onset.
56 cases were accepted as an acute new diagnosis of ADEM: 47 of these cases had received an immunization within 9 months prior to the onset.
For TM, there was no statistically significant increased risk of immunization in either the 5- to 28-day or the 2- to 42-day risk interval prior to onset. For ADEM, the Tdap (adolescent and adult tetanus, reduced diphtheria, acellular pertussis) vaccine was associated with a statistically significant increase in risk in the 5- to 28-day exposure interval (P = 0.04), but not the longer 2- to 42-day interval. The excess risk for Tdap was calculated to be 0.385.
The authors concluded that for TM, they did not find evidence of a safety concern, or any association with subsequent illness, while for ADEM they found a possible association with the Tdap vaccine, although with some caveats. They speculated that, assuming that Tdap does increase the risk of ADEM, the attributable risk is only 0.4 cases per 1 million doses of vaccine, and because so many vaccines were given, the excess risk is not likely to exceed 1.16 cases of ADEM per million doses of Tdap. Finally, they highlighted that for other vaccines, the excess risk is even smaller or nonexistent.

AUTHORS: Baxter R, Lewis E, Goddard K, Fireman B, Bakshi N, DeStefano F, Gee J, Tseng HF, Naleway AL, Klein NP.