To investigate the association between Gardasil vaccination and encephalopathy.

Encephalopathy after Gardasil vaccination has been reported in prior case reports.

We used data derived from the Vaccine Adverse Event Reporting System (VAERS) from 2006 to 2019. Cases of encephalopathy, encephalitis, and encephalomyelitis (henceforth, “encephalopathy”) after Gardasil vaccination were identified, rated by the Brighton criteria, and characterized by their temporal relationship with Gardasil vaccination. Case-centered analysis using a logistic regression model was utilized. The initial 6 weeks after vaccination were defined as the risk period for possible cause-effect between vaccination and encephalopathy.

561 cases of encephalopathy after vaccinations from 2006-2020 were identified. 123 (21.9%) of these cases were associated with Gardasil vaccination. Of these cases, 6 (4.87%) were confirmed, 17 (3.03%) were probable, and 32 (5.7%) were possible encephalopathy cases as determined by the Brighton criteria. The mean age was 14.31 ± 2.65 years and 91.1% of patients were female. There was an unbalanced distribution of post-vaccination encephalopathy cases: 62% of these subjects developed encephalopathy within 6 weeks, of which 59.7% developed encephalopathy within 2 weeks. 4% developed encephalopathy between 6 to 10 weeks, and 17% had encephalopathy after more than 10 weeks post-vaccination. 42% made a visit to the ER or doctor’s office for encephalopathy post-vaccination. Hospitalization occurred for 78%. Disability occurred in 12%. Death occurred in 6%. The median interval for onset of encephalopathy cases after vaccination with Gardasil (20 days) was longer than median interval of onset of non-encephalopathy events (1 day).

This study demonstrates an unbalanced distribution of encephalopathy cases in the first 6 weeks after Gardasil vaccination, suggesting the need for close monitoring of patients recently vaccinated with Gardasil.