The worst of the Ebola epidemic that hit Western Africa and killed thousands may be over, but flare-ups are still to be expected. If not properly contained, new cases have the potential to spiral out of control.

In a bid to prevent the highly fatal hemorrhagic virus from wreaking havoc again, hundreds of people in Guinea are being vaccinated with an experimental Merck vaccine for Ebola, the World Health Organization (WHO) revealed on April 1.

The VSV-EBOV vaccine has been found highly effective against Ebola infection based on results of a large trial conducted in 2015.

Those who received the vaccine were possible contacts of eight people who were infected in a flare-up of the disease. WHO's office in Guinea said that more than 1,000 of the possible contacts of the eight latest Ebola patients are now under medical observation.

The UN health agency said that of these contacts, 800 have already been vaccinated including 182 who were considered as high-risk contacts.

The approach called "ring vaccination" involves swiftly giving vaccine to those who have come into contact with somebody infected with Ebola as well as those who had contact with them.

"The 'ring' vaccination method adopted for the vaccine trial is based on the smallpox eradication strategy," John-Arne Røttingen, from the Norwegian Institute of Public Health explained in an earlier WHO statement about the VSV-EBOV. "The premise is that by vaccinating all people who have come into contact with an infected person you create a protective 'ring' and stop the virus from spreading further."

Ebola's re-emergence in Guinea is the first since the major outbreak was declared over for the country in December last year. Eight cases were identified in this flare-up, seven of which were fatal. Six of those who died were from one extended family. WHO has said that new clusters may occur because of the reintroduction of the virus.

WHO's chief of emergency responses Bruce Aylward said studies of Ebola survivors show that the virus could persist in semen up to 15 months, which means survivors are still capable of spreading the disease.

"WHO has continuously stressed that flare-ups like this one should be anticipated, largely due to virus persistence in some survivors, and that the three Ebola-affected countries must maintain strong capacity to prevent, detect and respond to further outbreaks," WHO said in a statement.