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.
Nearly a fifth of the population of Guinea-Bissau are infected with hepatitis B, a viral infection that affects the liver.
Those infected during infancy are particularly likely to develop chronic infections, which kill up to 25% of people, mostly through liver failure or liver cancer, according to data published in 2020.
The WHO is working with the government to bring forward the birth-dose vaccine.
“The country had been expected to add it to its routine immunisation schedule in 2028, but officials are considering introducing it in the first quarter of next year,” Mulombo said.
Guinea-Bissau currently vaccinates children against hepatitis B at six weeks, which means it is less effective.
