ATLANTA — Postlicensure safety monitoring by three national vaccine safety systems indicates that, in the more than 2 years since the human papillomavirus vaccine Gardasil was licensed, serious adverse events have been rare and have not been specifically linked with the product.
More than 20 million doses of Gardasil, manufactured by Merck & Co., had been distributed and are under passive surveillance, and more than 375,000 doses are under active surveillance as of Aug. 31, the Centers for Disease Control and Prevention officials reported at the fall meeting of the CDC's Advisory Committee on Immunization Practices.
Of the more than 10,300 adverse events reported thus far to the Vaccine Adverse Event Reporting System (VAERS), 94% have been nonserious. Most of the adverse event reports to VAERS were consistent with prelicensure trial data. Among the adverse events reported are syncope, dizziness, nausea, Guillain Barré syndrome, venous thromboembolism, and death, said Dr. Angela Calugar of the CDC.
About 51 reports, 3 of which were serious, have been made per 100,000 doses given. About half of all reports are in those aged 11–18 years, and 25% are in those aged 19–26 years, Dr. Calugar said, noting that this likely reflects the proportion of patients in these age groups who are receiving HPV vaccine.
Serious adverse events—including syncope, venous thromboembolism (VTE), and Guillain Barré syndrome—occurred mostly in individuals with other contributing factors. For example, of 18 reports of VTE, 14 were in patients who also used hormonal contraceptives, which are known to increase VTE risk, she noted.
As for the 17 deaths reported in association with HPV vaccination, no clusters based on patient age, timing of vaccination, or other factors were identified that might hint at a causal relationship, she said.
Furthermore, the Clinical Immunization Safety Assessment (CISA) Network—a collaboration of six U.S. academic centers that conduct research on adverse events associated with immunizations—has found insufficient evidence to support a causal relationship between HPV vaccination and the reported serious adverse events, reported Dr. Barbara Slade, also of the CDC.
Likewise, findings of the Vaccine Safety Datalink project—which was established in 1990 to improve the evaluation of vaccine safety through active surveillance and epidemiologic studies—showed that among more than 375,000 doses administered and monitored, no statistically significant risk for any prespecified adverse event (Guillain Barré syndrome, seizures, syncope, anaphylaxis, other allergic reactions, appendicitis, stroke, and VTE) occurred after vaccination in either youth (ages 9–18 years) or adults (ages 19–26 years), Julianne Gee of the CDC reported.