Background: A recent publication reporting the presence of low levels of reverse transcriptase (RT) activity in certain vaccines for human use necessitated that regulatory agencies address the issue of whether this RT activity presented a risk to humans. Detection of low levels of RT activity corresponding to fewer than ten virions became possible with the development of highly-sensitive polymerase chain reaction (PCR)-based RT (PBRT) assays. Variations of the PBRT assay were developed in three laboratories. These assays were reported as being at least one million-fold more sensitive than conventional RT assays.
Objective: To ascertain the sensitivity and reliability of PBRT assays in different laboratories and to determine which vaccine samples possessed RT activity.
Study design: Coded panels of licensed vaccines together with positive and negative controls was assembled at the Center for Biologics Evaluation and Research (CBER) of the Food and Drug Administration (FDA) and distributed to five cooperating laboratories as well as to our laboratory at CBER. Each laboratory carried out their version of the PBRT assay and submitted the results to the coordinator at CBER.
Results: Results of the PBRT analyses carried out in the six laboratories are presented. Five of the six laboratories reported results that were highly consistent. RT activity was detected in live attenuated vaccines that were prepared in chick embryo cells (mumps, measles and yellow fever), but very low or undetectable RT activity was found in vaccines produced in mammalian cells (rabies and rubella). Influenza vaccines from several manufacturers included in the panel displayed the most variability, with different products of this inactivated vaccine having differing amounts of RT activity.
Conclusions: Only vaccines produced in chick embryo cells had significant RT activity. Because RT activity was present in the allantoic fluid of uninfected chick embryos and culture medium from chick embryo fibroblasts, the RT activity arises from the cell substrate used for vaccine production. The PBRT assays were reliably able to detect the low levels of RT activity in chicken-derived vaccines.