By Kate Kelland

LONDON (Reuters) – British drugmaker GlaxoSmithKline <GSK.L> is giving up its work on developing three potential vaccines against the deadly Ebola and Marburg viruses, despite an ongoing Ebola outbreak in Democratic Republic of Congo.

While Ebola is a deadly and contagious disease, it is also still relatively rare, making the potential market for a vaccine sporadic and very likely unprofitable. This poses a dilemma for drug companies: With no real prospect of a financial return, can they justify the investment in expensive development and trials.

GSK’s vaccine candidates – two designed to protect against Ebola and one against the Marburg virus – will be transferred to the Sabin Vaccine Institute in Washington, D.C., the British drugmaker said in a statement. There is no financial element to the agreement, a spokesman said.

The transfer agreement will see Sabin continue to develop the candidate vaccines, one of which – a potential Ebola shot known as ChAd3 – has been through mid-stage, Phase II, trials in Africa and could possibly be used to halt or limit future Ebola epidemics.

“Enabling Sabin to build on the scientific progress GSK has delivered up to Phase II increases the likelihood these candidate vaccines may help prevent potential future outbreaks,” Thomas Breuer, chief medical officer of GSK Vaccines, said in a statement.

GSK had put its Ebola vaccine work on hold after it was unable to progress the product through final stage, or Phase III, clinical trials toward the end of the 2014-16 epidemic, due to a dwindling number of Ebola cases.

U.S. drugmakers Merck <MRK.N> and Johnson & Johnson <JNJ.N> are also developing potential vaccines against Ebola, and have made more progress with them than GSK had in clinical trials.

The Merck shot, known as VSV EBOV, is currently being used in the ongoing Ebola outbreak in Congo, which was last month declared an international health emergency by the World Health Organization (WHO).

The disease has killed more than 1,800 people in the Congo outbreak which began a year ago and has become the second-worst on record.

GSK said Sabin had agreed a collaboration deal with the Vaccine Research Center at the U.S. National Institute of Allergy and Infectious Diseases (NIAID) to further develop the vaccine candidates.

The ChAd3 shot was originally developed by NIAID in collaboration with the Swiss-based firm Okairos, which was bought by GSK in 2013. All three experimental vaccines have shown promise in safety trials after being administered to more than 5,000 adults and 600 children, GSK said.

(Reporting by Kate Kelland; editing by Jason Neely and David Evans)