The World Health Organisation (WHO) said almost all the people it considered at immediate risk from an Ebola outbreak in the Democratic Republic of Congo (DRC) city of Mbandaka have been vaccinated against the disease.
WHO Deputy Director-General for Emergency Preparedness and Response, Peter Salama, said more than 400 potential Ebola contacts and frontline staff have received the vaccine, representing 90 per cent of those considered at risk.
"We can't conclude that we've safeguarded the city of Mbandaka, but we can say that so far there has not been an explosive increase in cases. So that's a positive sign and a positive development," he said. "We have reason to be cautiously optimistic."
The virus has infected 54 people and killed 25 in the latest outbreak.
Médecins sans Frontières (MSF) emergency coordinator in Bikoro, Luis Encinas, fears the worst might be yet to come.
"We don't have a true picture of what's going on. We only see the tip of the iceberg," he said. "So what is very worrying is that we do not see all the transmission chains. There is not yet a 100 per cent follow-up of contacts. This means that we are still discovering new cases which have no link to the confirmed cases, and that is quite worrying."
WHO hopes to get government approval within days to use five experimental drugs to treat Ebola patients, in a clinical trial which could lay the foundation for rapid treatment in any future outbreaks.
"If we don't use the opportunity to learn in this situation we will never be able to know which is better than the other," Mr Salama said.
But the trial would be complicated, with some drugs requiring six to 12 hours of daily intravenous infusions and daily assessment of liver and kidney function.
A WHO spokesman said the drugs were the antivirals Remdesivir developed by Gilead Sciences and Favipiravir developed by Toyama Chemical Co, the monoclonal antibody mAB114, and two monoclonal antibody cocktails: ZMapp and a drug developed by Regeneron Pharmaceuticals known as RECN3470-3471-3479.
Ebola, which spreads easily through bodily fluids, claimed more than 11,000 lives in the region during an outbreak that reached its peak in 2014.