Background. The Oka strain of live attenuated varicella virus was licensed for use in healthy children in the United States in March, 1995. We report a postmarketing evaluation of the short term safety of this vaccine within Kaiser Permanente.
Methods. After licensure varicella vaccination was introduced into the preventive care program of the Northern California Kaiser Permanente Medical Care Program. Potential adverse events after vaccination with varicella vaccine were identified from automated clinical databases of hospitalizations, emergency room visits and clinic visits. Deaths were identified from automated clinical databases at Kaiser as well as from the State death records for California. To evaluate safety, rates of diagnosis-specific events in the risk periods were compared with the rates of such diagnosis-specific events in two self control and one historical control period.
Results. During the study period of April 1, 1995, to December 31, 1996, a total of 89 753 adults and children received varicella vaccine. A total of 3200 relative risks were calculated, and of these 5 hospital diagnostic categories, 9 emergency visit diagnostic categories and 30 outpatient diagnostic categories demonstrated at least 1 relative risk with a P value of <0.05 in 1 or more age groups and in comparisons with 1 control period or more. The p value for these tests was not adjusted for multiple comparisons. Of these categories 14 demonstrated an increased risk either in more than 1 age group or against more than 1 comparison group. These categories included elective procedures, febrile seizure, febrile illness, well child, acute gastroenteritis, varicella, congenital anomaly, "rule out sepsis," trauma, viral syndrome, apnea, back pain, congenital valvular heart disease and vision evaluation for glasses. Of these the outcomes of elective procedure, congenital anomaly, congenital valvular heart disease, well child and vision evaluation for glasses were judged not to have a biologically plausible association with vaccination. A second diagnostic grouping included febrile illness, viral illness, febrile seizure and "rule out sepsis." In an analysis of these events which adjusted for the concomitant administration of M-M-RII vaccine, none of the associations was statistically associated with receipt of varicella vaccine. The diagnostic category of "rule out sepsis" still had a relative risk of 1.95 with P = 0.02. None of the children in the "rule out sepsis" category had positive bacteriologic cultures from any other normally sterile site. Because of the large number of gastroenteritis cases, we reviewed a random sample of 100 exposed and 100 unexposed cases. From this review no consistent time association or clustering of any of these events was seen in the exposed follow-up time interval. Only gastroenteritis and negative evaluations for sepsis were thought to be possibly associated with receipt of varicella vaccine. Although there was a statistically significant increased risk over the entire 30 day-period, there was no clustering of these events within the 30-day window.
Conclusion. In this study population of 89 753 children and adults, the varicella vaccine (Oka strain, Merck) appeared to have a favorable safety profile. In addition rates of varicella-like rash and of breakthrough cases were both low and consistent with the rates observed in prelicensure studies.