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Professor Adrian Hill, director of the Jenner Institute and chief investigator of the trials with the Ebola vaccine Chimp Adenovirus type 3 (ChAd3), holds a vial of the vaccine before injecting it into British volunteer Ruth Steve Parsons/AFP/Getty Images ( 533495127 )

Clinical research fellow at Oxford University, Dr Felicity Hartnell, fills a syringe with Ebola vaccine Chimp Adenovirus type 3 (ChAd3) before giving it to British volunteer Ruth Atkins at the Oxford Vaccine Group Centre for Clinical Vaccinology and Tropical Medicine (CCVTM) in Oxford on September 17, 2014. ASteve Parsons/AFP/Getty Images ( 533495116 )

Clinical research fellow at Oxford University, Dr Felicity Hartnell, injects British volunteer Ruth Atkins with Ebola vaccine Chimp Adenovirus type 3 (ChAd3) at the Oxford Vaccine Group Centre for Clinical Vaccinology and Tropical Medicine (CCVTM) in Oxford on September 17, 2014. Steve Parsons/AFP/Getty Images ( 533495110 )

Professor Adrian Hill, director of the Jenner Institute and chief investigator of the trials with the Ebola vaccine Chimp Adenovirus type 3 (ChAd3), takes a vial of the vaccine before injecting it into British volunteer Ruth Atkins at the Oxford Vaccine Group Centre for Clinical Vaccinology and Tropical Medicine (CCVTM) in Oxford on September 17, 2014. Steve Parsons/AFP/Getty Images ( 533495140 )

Clinical research fellow at Oxford University Dr Felicity Hartnell (L) looks on as nurse Raquel Lopez Roman (C) draws blood from British volunteer Ruth Atkins before getting injected with the Ebola vaccine Chimp Adenovirus type 3 (ChAd3) at the Oxford Vaccine Group Centre for Clinical Vaccinology and Tropical Medicine (CCVTM) in Oxford on September 17, 2014. Steve Parsons/AFP/Getty Images ( 533495134 )