Is this a risk you really want to take for CHICKENPOX?? Or for that matter measles,mumps or rubella, which you are unlikely to get and which cause mostly mild illness anyway?
HeLa cells, immortal cell lines, cell line contamination and cancer:
Information from FDA on immortal (neoplastic) cell lines, safety and testing
HeLa cells and contamination:
"While lab culturing may indicate that so called immortal cells are not immediately changing to overt tumor cells, it is now well known in the scientific community that after these cells have been repeatedly cultured a certain number of times, something causes them to convert to a cancerous state. Normal embryo cells presumably represent a state in development which is genetically unstable, rendering them considerably more susceptible to malignant transformation.
When new vaccine batches are needed, the virus is cultivated on the existing fetal cell lines, which in turn have been sub-cultured numerous times over the years. We have already discussed the absolute finite lifespan of these cell lines and we know the end of that timeline is rapidly approaching. According to the Hayflick Limit, the population can only double a limited number of times (around 50) before the cells senesce and are unable to grow any more. 
And what of the new fetal cell line, PER C6 recently introduced to the US last year and described in our previous section? PER C6 is a designer cell altered by introducing into the culture an E1 cellular gene transformed by an adenovirus type 5 (AD 5). This transformation process turns a normal cell into an immortal neoplastic cell. In other words, PER C6 is a normal cell that has been modified to resist cell senescence. And in doing so, it introduces the potential for cancer to form in the vaccine recipient."
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