Abstract
* Abbreviations: EZ = : Edmonston-Zagreb (vaccine strain) • SW = : Schwarz (vaccine) The high-titered (doses >4.7 log10 infectious units1) measles vaccine era was curtailed abruptly by safety concerns in 1992.2 No new safety issues relevant to measles vaccines have surfaced in the interim, but misunderstandings about the overall safety of one of the vaccines involved in these events, the Edmonston-Zagreb (EZ) strain, appear to us to have persisted and become sufficiently widespread to merit an attempt at clarification. A brief historical review of events may help to disclose the likely origins of this problem. The high-titer measles vaccine trials involved mainly EZ and Schwarz (SW) vaccines given to young infants. Both SW and EZ are derived from the original Edmonston strain, and differ only in their passage levels and cell substrate—chick embryo fibroblasts and WI-38, respectively.3 EZ was rather consistently and significantly more immunogenic than SW in infants with maternal antibodies, and produced seroconversions in high frequencies. Consequently, on …
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