IMPORTANCE: As part of postauthorization safety surveillance, the US Food and 
Drug Administration (FDA) has identified a potential safety concern for 
Guillain-Barré syndrome (GBS) following receipt of the Ad26.COV2.S 
(Janssen/Johnson & Johnson) COVID-19 vaccine.
OBJECTIVE: To assess reports of GBS received in the Vaccine Adverse Event 
Reporting System (VAERS) following Ad26.COV2.S vaccination.
DESIGN, SETTING, AND PARTICIPANTS: Reports of presumptive GBS were identified in 
a US passive reporting system (VAERS) February-July 2021 and characterized, 
including demographics, clinical characteristics, and relevant medical history.
EXPOSURES: Receipt of the Ad26.COV2.S vaccine; the comparator was the background 
rate of GBS in the general (unvaccinated) population that had been estimated and 
published based on a standardized case definition.
MAIN OUTCOMES AND MEASURES: Presumptive GBS; the reporting rate was analyzed, 
including calculation of the observed to expected ratio based on background 
rates and vaccine administration data. Because of limited availability of 
medical records, cases were not assessed according to the Brighton Collaboration 
criteria for GBS.
RESULTS: As of July 24, 2021, 130 reports of presumptive GBS were identified in 
VAERS following Ad26.COV2.S vaccination (median age, 56 years; IQR, 45-62 years; 
111 individuals [86.0%] were < 65 years; 77 men [59.7%]). The median time to 
onset of GBS following vaccination was 13 days (IQR, 10-18 days), with 105 cases 
(81.4%) beginning within 21 days and 123 (95.3%) within 42 days. One hundred 
twenty-one reports (93.1%) were serious, including 1 death. With approximately 
13 209 858 doses of vaccine administered to adults in the US, the estimated 
crude reporting rate was 1 case of GBS per 100 000 doses administered. The 
overall estimated observed to expected rate ratio was 4.18 (95% CI, 3.47-4.98) 
for the 42-day window, and in the worst-case scenario analysis for adults 18 
years or older, corresponded to an estimated absolute rate increase of 6.36 per 
100 000 person-years (based on a rate of approximately 8.36 cases per 100 000 
person-years [123 cases per 1 472 162 person-years] compared with a background 
rate of approximately 2 cases per 100 000 person-years). For both risk windows, 
the observed to expected rate ratio was elevated in all age groups except 
individuals aged 18 through 29 years.
CONCLUSIONS AND RELEVANCE: These findings suggest a potential small but 
statistically significant safety concern for Guillain-Barré syndrome following 
receipt of the Ad26.COV2.S vaccine. However, the findings are subject to the 
limitations of passive reporting systems and presumptive case definition, and 
they must be considered preliminary pending analysis of medical records to 
establish a definitive diagnosis.