GENEVA, May 29 (Xinhua) -- The World Health Organization (WHO) said Tuesday that although most high-risk people have been vaccinated in the Democratic Republic of Congo (DR Congo), they remain "cautiously optimistic" about the development of Ebola in the country.

According to Peter Salam, WHO Deputy Director-General for Emergency Preparedness and Response, more than 400 Congo people who are potential Ebola contacts have been vaccinated, representing 90 percent of those considered at risk.

The vaccination in the northwestern city of Mbanadaka, a port city with over one million residents and one of the infected areas, went smoothly, the WHO official said to reporters.

So far there has not been an explosive increase in Ebola cases, which is "a positive sign and a positive development" but they "have reason to be cautiously optimistic," Salama added, noting the remote regions of Iboko and Bikoro as the next focus of the vaccination strategy.

It's been over two weeks since the DR Congo health authority declared an outbreak of the Ebola virus disease on May 8. As of May 21, a total of 58 cases, including 27 deaths, have been reported in the country, according to the latest WHO numbers.