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 ad verse effects provide the opportunity to detect potential risks not observe d before licensure because of the relatively small sample size and other li mitations of clinical trials. Objectives To detect potential hazards, including rare events, associated w ith varicella vaccine, and to assess case reports for clinical and epidemio logical implications. Design and Setting Postlicensure case-series study of suspected vaccine adv erse events reported to the US Vaccine Adverse Event Reporting System (VAER S) 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. Approxim ately 4% of reports described serious adverse events, including 14 deaths. The most frequently reported adverse events were rashes, possible vaccine f ailures, and injection site reactions. Misinterpretation of varicella serol ogy after vaccination appeared to account for 17% of reports of possible va ccine failures. Among 251 patients with herpes tester, 14 had the vaccine s train of varicella tester virus (VZV), while 12 had the wild-type virus. No ne of 30 anaphylaxis cases was fatal, An immunodeficient patient with pneum onia had the vaccine strain of VZV in a lung biopsy, Pregnant women occasio nally received varicella vaccine through confusion with varicella tester im munoglobulin. Although the role of varicella vaccine remained unproven in m ost serious adverse event reports, there were a few positive rechallenge re ports and consistency of many cases with syndromes recognized as complicati ons of natural varicella. Conclusion Most of the reported adverse events associated with varicella va ccine 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 re quire further study to clarify whether varicella vaccine plays a role. Educ ation is needed to ensure appropriate use of varicella serologic assays and to eliminate confusion between varicella vaccine and varicella zoster immu noglobulin.