THE PROVENANCE of the RA 27/3 attenuated rubella strain has already been described in print.1,2 Detailed information was given at the London Conference just three months ago.3 Therefore we shall only outline the history of this strain, before proceeding to examine its in vitro characteristics and its behavior when inoculated in man.
In order to avoid the problem of passenger viruses, the RA 27/3 strain was isolated directly from naturally infected material in WI-38 human diploid fibroblasts.4
Explant cultures were made of the dissected organs of a particular fetus aborted because of rubella, the 27th in our series of fetuses aborted during the 1964 epidemic. The third explant, which happened to be from kidney, was selected arbitrarily for further study. Fibroblast cells that grew out from this explant could be subcultivated after several weeks. The presence of rubella virus in the supernatant fluids was confirmed. After four