(CNN) In a "worst-case scenario," the current Ebola outbreak in the Democratic Republic of Congo may take up to two years to end, a World Health Organization official said Thursday.

The outbreak, which began on August 1, is "not under control," Dr. Mike Ryan, executive director of WHO Health Emergencies Programme, said during a press briefing. "We may end up dealing with this outbreak for a long time."

As of Wednesday, the cumulative number of probable cases of the viral disease -- which causes fever, severe headache and in some cases hemorrhaging -- is 2,025, according to Congo's Ministry of Health. Of the 1,931 confirmed Ebola patients, 1,357 have died while 552 have made a full recovery. On average, Ebola kills about half of those infected, yet the current outbreak has caused fatalities in 70% of those infected.

Ryan said that numbers have stabilized and even fallen in the last two weeks, yet he also said there's still "substantial transmission" in some health zones. While there is a smaller geographic footprint, the spread of disease is rampant within affected zones, he added.

Still, he is hopeful: "If we make the right improvements, and we put the right effort into this, and we put the resources into this, and the funding into this, and the people into this," there is the possibility the outbreak ends in a much shorter time, Ryan said. "At this point, I think the outcome is effort related."

North Kivu and neighboring Ituri province remain the epicenter of the outbreak. The two provinces are among the most populated in the nation and border Uganda, Rwanda and South Sudan.

Not all Ebola cases detected by health workers

Ryan said "we're probably detecting in excess of 75% of cases -- we may be missing up to a quarter of cases."

A week ago, 20,000 patient contacts required follow-up; On Wednesday, that number dropped to 15,000, he said. More than a fifth of cases seek health care in a different health zone from where they live, so it's "quite difficult" to trace contacts and this behavior has become a "major driver" of the epidemic, he said."It can easily flare up again."

While it took more than 220 days to reach 1,000 confirmed cases on March 24, it has taken less than 75 days to reach a total of 2,000 confirmed cases in the current outbreak.

Earlier this week, Tariq Riebl, emergency response director at the International Rescue Committee, stated that "the fact that we have hit 2,000 persons infected with Ebola so quickly demonstrates that this outbreak is spreading faster when it should be slowing. We are now seeing eight to 20 cases recorded each day, a number that is very likely an underestimate. Just a few months ago, we were only seeing three to five cases a day." The IRC, which has been working in Congo since 1996, provides humanitarian assistance and post-conflict development services.

A major stumbling block to ending this outbreak has been that "community members do not trust medical staff and aid organizations, and are not coming in to health facilities when they show Ebola symptoms," said Riebl.

Treating an outbreak amid conflict

One key reason for this mistrust is long-term conflict has smoldered in the northeastern region of the country for decades, with armed groups causing intermittent violence. More than a million refugees and internally displaced people are traveling through and out of North Kivu and Ituri provinces, and this movement is a potential risk factor for the spread of Ebola, according to WHO.

US Ambassador Mike Hammer, who has traveled to Ebola hot spots over the past couple of months, previously told CNN that "there are over 100 armed groups" operating in the conflict zone of eastern Congo, which encompasses the outbreak-affected region, and that one group, known as the Alliance Defense Force, has become "increasingly associated with ISIS."

ISIS has taken credit for the most recent attack that butchered 13 people in the town of Beni earlier this week, according to the United Nations.

As of May 21, there have been 131 attacks on health care personnel in Ebola-affected areas , with 40 injuries and five deaths of health workers and patients, according to WHO.

Médecins Sans Frontières (MSF or Doctors without Borders) pulled out of Congo following separate attacks on its Ebola treatment centers in Butembo and Katwa earlier this year.

While MSF is not currently running treatment centers, it remains engaged in the response. MSF said in a statement that it is "helping existing health care structures in North Kivu and Ituri provinces to prepare for and handle potential Ebola cases and managing transit centers where patients with suspected Ebola are tested and sent to Ebola Treatment Centers if they test positive."

WHO's Ryan said at the briefing that "we've seen a period of calm over the last 6 weeks."

"We've definitely seen in the last four weeks a decrease in security incidents," yet he immediately added that the situation is "extremely volatile." He said a health center staff member described the calm as "eerie" and evoked "a sense of trepidation."

Perceptions about who is being protected

Ryan explained that there are two ways to mitigate risks to health personnel: Increase the general level of security or provide point security. When you reduce the general security threat, you can reduce the number of teams that require an escort, he said

"When you have too much point security, it's very visible to communities," he said, noting that general area security is "better" and added that WHO will try to "avoid any perception that we're securitizing the response. The police are not being used as an instrument of the public health operation."

In addition to deaths among the general population, 110 health workers, including 37 who died, have succumbed to the viral disease. Meanwhile, a total 130,254 people, including health care staff, have been vaccinated with the rVSV-ZEBOV vaccine, manufactured by Merck, following approval by Congo's health ministry ethics committee in May 2018.

"We'd be in a lot worse situation without this vaccine right now," said Ryan. "But the vaccine can't protect you if you're not vaccinated." He said about 90% of the people who are offered the vaccine take it and the 10% who don't are still spreading the infection.

He added that the response effort is also "not vaccinating everyone who needs to be vaccinated."

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Late detection of cases is another issue because it leads to community deaths -- and fear: "We need to make sure when people are sick they need to get to isolation quickly."

This is the 10th Ebola outbreak since 1976 in Congo, where the virus is endemic. It follows closely on the heels of a previous 2018 outbreak in Eastern Congo. The deadliest outbreak occurred in West Africa in 2014, when the disease killed more than 11,000 people.