Considerations for causality assessment of neurological and neuropsychiatric complications of SARS-CoV-2 vaccines: from cerebral venous sinus thrombosis to functional neurological disorder

Worldwide SARS-CoV-2 vaccination campaigns for prevention of COVID-19 are currently underway. In clinical trials and in open-label monitoring the vaccines have shown efficacy against COVID-19 and there have been limited and transient side effects. Previous vaccination campaigns have taught us that neurological adverse events to vaccinations can occur. In this review, we summarise what is already known about neurological and neuropsychiatric adverse events of COVID-19 vaccinations, and place this in the context of historical vaccination campaigns. There have been a number of neurological and neuropsychiatric adverse events following immunisation (AEFI) in association with SARS-CoV-2 vaccinations, however in each case there is either no definitive evidence currently to support causality or recognised adverse events are extremely rare. Causality assessment aids such as the Causality Assessment of an Adverse Event Following Immunization from the World Health Organisation and the Bradford Hill criteria may help us better understand potential neurological and neuropsychiatric adverse events to COVID-19 vaccinations. Functional neurological disorder (FND) can be precipitated by the process of vaccination and has previously been noted to potentially spread between individuals, particularly in younger communities. Importantly FND does not implicate the vaccine constituents and therefore should not hamper ongoing vaccination campaigns. Although neurological and neuropsychiatric AEFI may occur after SARS-CoV-2 vaccinations, at present there are no common causally associated neurological adverse events. It is likely that some patients will develop FND in response to vaccination, although this does not implicate vaccine constituents. In cases of future serious neurological or neuropsychiatric AEFIs, judicious and rapid assessment of causality must occur. In general, the benefits of ARS-CoV-2 vaccination at present outweigh the risks from a neurological standpoint, although in specific situations the risk-benefit ratio will vary depending on geographic and demographic factors as well as population risk factors. Ensuring as minimal disruption as possible to ongoing swift worldwide vaccination campaigns is essential to establish the herd immunity required to end the COVID-19 pandemic.