It may be impossible to use a vaccine to tackle Democratic Republic of Congo’s new Ebola outbreak that has seen four confirmed cases, including two health workers, a senior World Health Organisation official has said.

It remains unclear whether the virus is the Zaire, Sudan or Bundibugyo strain. If it is the Zaire strain, the Merck vaccine used in the last Ebola outbreak may be an option, otherwise the situation will be much more complex, WHO says.