Caution in Interpreting Facial Paralysis Data to Understand COVID-19 Vaccination Risks

To the Editor Since the rollout of the COVID-19 vaccines, there have been multiple cases of postvaccination facial paralysis, raising concerns for a possible association between COVID-19 vaccines and facial paralysis. Renoud and colleagues investigated this possible association by performing a disproportionality analysis on a large pharmacovigilance database.¹ The authors did not find any suggestion of an association, given that the rate of facial paralysis after mRNA COVID-19 vaccination was similar to rates after other viral vaccines.¹ While we appreciate the usefulness of Bayesian data mining in sieving out potential adverse events from a large data set, we would like to express caution regarding the clinical interpretation of such pharmacoepidemiologic studies in the context of facial paralysis.

The most common cause of isolated facial paralysis is Bell palsy, the cause of which remains unclear. Numerous mechanisms for Bell palsy have been proposed, including anatomical narrowing of the facial canal, viral infection, microvascular-ischemia of the facial nerve, and immune-mediated inflammatory responses.² These mechanisms may contribute to risk factors for Bell palsy, such as diabetes, hypertension, obesity, and pregnancy.³ Current pharmacoepidemiologic studies lack information on the detailed presenting history, medical records, comorbidities, and examination and investigation findings of the patients with facial paralysis. Thus, it is difficult to ascertain the accuracy of diagnoses, risk of facial paralysis in subgroups of individuals who may have comorbidities, existing predisposing factors to Bell palsy, and whether mild and transient cases were unreported in these studies.

Recent analysis of the mRNA COVID-19 vaccine trials has shown that the incidence of Bell palsy among vaccinated individuals was greater than among the general population.⁴ In addition, reports of Bell palsy cases with a temporal relationship to mRNA COVID-19 vaccination have also emerged outside of the vaccine trials.⁵

Thus, despite the lack of an apparent signal suggesting an association between the mRNA COVID-19 vaccines and facial paralysis in the study by Renoud and colleagues,¹ we need to be cognizant of the caveats and limitations of individual studies. While current data suggest that the risk of facial paralysis after mRNA COVID-19 vaccination appears low, further studies to analyze and compare the clinical histories, comorbidities, and neuroimaging and electrophysiological findings of patients with conventional Bell palsy and those who develop it after COVID-19 vaccination may help to provide further information on the possibility and risk of mRNA COVID-19 vaccine-related facial paralysis.

Corresponding Author: Eng King Tan, MD, Duke University–National University of Singapore Medical School, 8 College Rd, Singapore 169857, Singapore (gnrtek@sgh.com.sg).

Published Online: August 16, 2021. doi:10.1001/jamainternmed.2021.4333

Conflict of Interest Disclosures: None reported.

Additional Contributions: We would like to thank the National Medical Research Council (Singapore) for their support.