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