The trial of a potential Ebola drug in Liberia has been stopped because the case numbers in the country are now so low that it is unlikely to get a clear result.

Chimerix, the US manufacturer of the drug brincidofovir, said on Friday it was pulling out of the trial, run by Oxford University at a Médecins Sans Frontières treatment centre in Monrovia. On Tuesday the Wellcome Trust, which funded the trial, announced it had ended.

Last week there were four new cases of Ebola in Liberia. Without larger numbers it is impossible to assess whether fewer people die after treatment with the daily pill than were dying before the trial started.

The ending of the trial will raise questions about the viability of a vaccine trial that began in Liberia this week. The British pharmaceutical company GlaxoSmithKline, which makes the vaccine, announced the immunisation of the first frontline healthcare workers in the trial on Monday. But if few people are falling ill in Liberia, there will not be enough cases of disease in the vaccinated group or the control group to make a valid comparison and establish whether the vaccine works.

New trials are likely to focus on Sierra Leone, which has the highest case numbers, although those too are falling rapidly.

Prof Peter Horby, of the University of Oxford, who was leading the brincidofovir trial, said: “The past weeks have brought the extremely positive news that Ebola infections are falling across west Africa, including in Liberia where our trial of brincidofovir was based. We’re delighted that infections are falling, but fewer patients makes it more difficult to carry out the robust scientific studies needed to ensure a new treatment will be safe and effective.

“On Friday Chimerix took the decision to withdraw from the trial, following discussions with the FDA [the US drugs regulator]. It is now clear that we will not be able to complete our study of brincidofovir, leaving us no option but to stop enrolling to the study with immediate effect.”

Dr Jeremy Farrar, director of the Wellcome Trust, which is funding a multimillion-pound therapeutics platform for experimental Ebola treatments, including the brincidofovir study, said: “The WHO recently confirmed that new cases of Ebola had fallen below 100 in a week for the first time since the crisis reached its peak. This is testament to the extraordinary efforts of organisations like MSF and the doctors, nurses and volunteers who have worked in west Africa from the outset to bring this dreadful epidemic under control.

“We’re now at the stage when new interventions that could help to finally end the current outbreak, and prevent future ones, are ready to be tested in Africa. It’s therefore disappointing that the trial of this therapy cannot continue, but it is essential that other studies of potential treatments and vaccines continue and hopefully will still be able to deliver meaningful results for this and the inevitable future epidemics of Ebola. We must never be in a position again when we do not have diagnostics, treatments and vaccines for this terrible infection.”