Sudden sensorineural hearing loss (SSNHL) is defined as sensorineural hearing loss (SNHL) of at least 30 decibels (dB) over at least 3 contiguous audiometric frequencies that occurs within a 72-hour period. The hearing loss is most commonly unilateral, with bilateral involvement occurring less commonly. This condition often occurs without an identifiable etiology, although some proposed mechanisms, such as microvascular, immune, or infectious causes, have been identified. Approximately 10% of patients will have a specific cause identified, but the majority of patients are diagnosed with idiopathic SSNHL.

This condition has an incidence of approximately 5-30 cases per 100 000 per year. It affects males and females equally, with individuals aged 50-60 years most commonly affected. Potential risk factors include cardiovascular disease and autoimmune conditions.

Prognosis for these patients is generally good. Complete or partial recovery occurs in approximately two-thirds of affected patients. However, those without any changes over a 2-week period are less likely to recover. The presence of vestibular dysfunction, more severe hearing loss, and increased time between onset and treatment are all factors that can indicate a worse prognosis.

Limited reports of hearing loss, including sudden cases, have been documented in association with Pfizer-BioNTech, Moderna, and AstraZeneca shots, as well as COVID-19. These reports suffer from a number of limitations including recall bias, nocebo effect, and survey methodology. Causality has not been determined and studies that include control groups, audiometric data, and more accurate patient reporting are needed. A plausible mechanism of action involving the vestibulocochlear nerve has been suggested if causality is ultimately proven.