Ebola spreads easily through bodily fluids and the medical strategy involves vaccinating all the people a patient may have infected and then vaccinating a second “ring” of contacts around each of those potential sufferers.

That would include family members but also people who may have come into contact with a sufferer in church or on public transport, each a potential Ebola time-bomb who must be found and vaccinated by virus-hunting experts.

The VSV-EBOV vaccine, developed by Merck, has been administered to 1,112 people, including 567 in the northwestern city. That covers all known contacts of confirmed Ebola cases in the city as well as those people’s contacts, the ministry said in a statement late on Sunday.

There have been no new deaths from Ebola since May 25 and the last confirmed case was recorded on May 29, although health officials say it is too soon to make any definitive pronouncements about the outbreak’s course.

The latest data from the health ministry shows 53 cases of Ebola in the outbreak, including 37 confirmed, 13 probable and three suspected cases.

One new suspected case was recorded on Monday in the rural community of Iboko and five suspected cases came back negative, the health ministry said.

This is the ninth outbreak of Ebola in Congo since the disease was first detected in the country in 1976. Health officials have moved aggressively to head off a repeat of the 2013-16 outbreak in West Africa that killed over 11,300 people.

The vaccine was first rolled out in Mbandaka on May 21 and hailed as a paradigm shift in the fight against Ebola by the World Health Organization (WHO).

The WHO said last week that it was cautiously optimistic about the progress of the response yet Mbandaka’s location directly upstream the Congo River of the capital Kinshasa, a city of more than 10 million people, remains a concern.