Postlicensure safety surveillance for varicella vaccine

Context Since its licensure in 1995, the extensive use of varicella vaccine and close surveillance of the associated anecdotal reports of suspected adverse effects provide the opportunity to detect potential risks not observed before licensure because of the relatively small sample size and other limitations of clinical trials. Objectives To detect potential hazards, including rare events, associated with varicella vaccine, and to assess case reports for clinical and epidemiological implications. Design and Setting Postlicensure case-series study of suspected vaccine adverse events reported to the US Vaccine Adverse Event Reporting System (VAERS) from March 17, 1995, through July 25, 1998. Main Outcome Measures Numbers of reported adverse events, proportions, and reporting rates (reports per 100 000 doses distributed). Results VAERS received 6574 case reports of adverse events in recipients of varicella vaccine, a rate of 67.5 reports per 100 000 doses sold. Approximately 4% of reports described serious adverse events, including 14 deaths. The most frequently reported adverse events were rashes, possible vaccine failures, and injection site reactions. Misinterpretation of varicella serology after vaccination appeared to account for 17% of reports of possible vaccine failures. Among 251 patients with herpes tester, 14 had the vaccine strain of varicella tester virus (VZV), while 12 had the wild-type virus. None of 30 anaphylaxis cases was fatal, An immunodeficient patient with pneumonia had the vaccine strain of VZV in a lung biopsy, Pregnant women occasionally received varicella vaccine through confusion with varicella tester immunoglobulin. Although the role of varicella vaccine remained unproven in most serious adverse event reports, there were a few positive rechallenge reports and consistency of many cases with syndromes recognized as complications of natural varicella. Conclusion Most of the reported adverse events associated with varicella vaccine are minor, and serious risks appear to be rare. We could not confirm a vaccine etiology for most of the reported serious events; several will require further study to clarify whether varicella vaccine plays a role. Education is needed to ensure appropriate use of varicella serologic assays and to eliminate confusion between varicella vaccine and varicella zoster immunoglobulin.