A cluster of Ebola cases in the Democratic Republic of Congo — just one week after it declared an end to an outbreak on the other side of the country — is especially worrying because the new infections are in a war zone.

Moreover, a new Ebola vaccine that helped defeat the earlier outbreak may not work this time because the culprit strain of Ebola virus may be different, health officials and aid groups said Thursday.

At least 20 people are believed to have died from the latest outbreak since mid-July, and four of six samples taken from survivors tested positive for Ebola, officials said Wednesday.

The virus, which causes fevers and potentially fatal hemorrhaging, can often spread out of control, as it did during a 2015 outbreak in West Africa.