The eight most recent Ebola cases reported during the outbreak in Uganda have no known links to current patients, the World Health Organization said on Wednesday, raising concern over the spread of the disease.

In a briefing note, the WHO said that initial investigations of the cases by the Ugandan Ministry of Health had shown that the new cases were not contacts of people already known to have Ebola.

“We remain concerned that there may be more chains of transmission and more contacts than we know of in affected communities,” WHO Director-General Tedros Adhanom Ghebreyesus told reporters.

Since the start of the outbreak last month, 60 cases have been confirmed and 20 probable, and there have been 44 deaths, according to the WHO.

The strain that is spreading in Uganda is Sudan, and existing vaccines and therapies do not work against it.

However, the Ugandan government is collaborating with the WHO to launch a trial of two vaccines under development that do target the Sudanese strain: one developed by the University of Oxford and the Serum Institute, and another manufactured by the Sabin Institute in the United States, confirmed the WHO.

The United States has also sent experimental therapies to help stem the outbreak.