A year-long Ebola outbreak in eastern Democratic Republic of Congo has killed at least 1,800, the second biggest toll ever, and efforts to contain the virus have been hobbled by militia violence and some local resistance to outside interference.

“Ongoing vaccination activities have reached the majority (98%) of eligible contacts, and 1,314 contacts, contacts of contacts and frontline workers (have been) vaccinated to date,” the WHO said in a statement on Sunday.

The use of an experimental Ebola vaccine, developed by Merck, has proven to be a key weapon against the hemorrhagic fever, although reaching contacts in rural areas beset by violence has proven difficult.

The vaccine’s success has been most obvious in cities where contacts can be easier to trace, helping avoid the widespread havoc seen in densely populated areas during a 2013-2016 outbreak in West Africa that killed over 11,000 people.