Why a US-funded vaccine trial on babies was cancelled in Guinea-Bissau

Walter Kazadi Mulombo, the WHO representative in Guinea-Bissau, told the BBC they became concerned after receiving questions about the study and finding no publicly available protocol.

He said the main ethical concern was that the trial would withhold an intervention already proven to protect newborns against hepatitis B.

"The vaccine has been used for more than 30 years in more than 115 countries," he said. "A study that will withhold that intervention from other newborns is unethical."

He warned that babies could be exposed to the virus during childbirth if their mothers were infected, as well as later in life.