{HEAD ~ Viruses} produced by this method are advantageously free of any infectious agents comprised in {TAIL ~ animal sera} 
{ HEAD ~ Viruses } produced by this method are advantageously free of any infectious agents comprised in { TAIL ~ animal sera }
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL