The first health care workers lined up Monday to receive a trial vaccine for Ebola in Congo as world health officials play catch-up in getting promising preventive measures to market for emerging health threats.

The World Health Organization is speeding the vaccine to Mbandaka, a city of more than 1 million people where four Ebola cases have been confirmed, hoping to prevent the infection from spreading down the Congo River “highway” to other major cities.

Officials say 26 deaths nationwide have been linked to the outbreak, and they have identified 600 people who may have come into contact with affected people. They are now looking to vaccinate those people, along with health care workers and those responsible for safe burial of victims.

The technique is known as a “ring” vaccination. It was tested in Guinea during the 2014-2015 outbreak, which totaled more than 28,000 cases in West Africa, led to 11,325 deaths and sparked global dread.

Congo has received more than 7,500 doses of a vaccine candidate, known as rVSV-ZEBOV, which is offered by Merck. The vaccine is not fully licensed but is available for “compassionate use” in emergencies.

“It’s concerning that we now have cases of Ebola in an urban center, but we are much better placed to deal with this outbreak than we were in 2014,” WHO Director General Tedros Adhanom Ghebreyesus told the World Health Assembly in Geneva on Monday. “I’m pleased to say that vaccination is starting as we speak.”

Though the trial vaccine is better than nothing, researchers say, they would rather have full approval to deal with scares like Ebola because it would help cut through a lot of red tape to deliver lifesaving protections to those trying to combat the spread.

Regulatory authorities and ethics committees wouldn’t have to issue special declarations to use the vaccine, for instance, nor would each recipient have to go through a special consent process.

“It’s much, much, much easier to distribute a licensed vaccine than an experimental vaccine,” Dr. Anthony Fauci, director for infectious disease research at the National Institutes of Health, told The Washington Times on Monday.

For now, WHO is rallying against a third global health scare in five years without a fully vetted antidote, after the initial West African Ebola outbreak and then the mosquito-borne Zika virus that spread to the Americas in 2016.

Pursuing a vaccine against Ebola, Zika or any other emerging infectious disease is a long and costly undertaking for drug companies, said Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security.

Earning a sufficient return on investment is doubtful because the market size and purchasing power of affected countries are often limited and interest tends to wane as outbreaks disappear from the headlines.

Dr. Fauci said the federal government can take steps to “de-risk” the endeavor by supporting some of the initial work.

For instance, the NIH developed a DNA-based Zika vaccine candidate that is undergoing phase two trials around the Americas. The head start should make it more attractive to drug companies interested in manufacturing and marketing it.

“One of the issues is if they don’t see this as a reasonable market where they can market their product,” Dr. Fauci said. “They don’t want to get involved and make a major investment.”

Researchers say that was what held back efforts to vaccinate against Ebola, which was discovered in 1976 and popped up intermittently in tropical areas of sub-Saharan Africa before the massive outbreak in Guinea, Liberia and Sierra Leone.

“I think it’s definitely the [lack of] interest,” said Brittany Kmush, an assistant professor at Syracuse University who specializes in global health. “On Ebola, it’s not on the scientific end because we were able to find an antigen very quickly.”

The West African outbreak served as a wake-up call for the medical community as infection ripped through major population centers, spawning a global response, travel bans and multipronged efforts to pin down a vaccine.

Merck entered into an exclusive worldwide licensing agreement with NewLink Genetics in November 2014 to research, develop, manufacture and distribute the rVSV-ZEBOV vaccine, which was kick-started by the Public Health Agency of Canada.

The vaccine was given to more than 5,800 at-risk people in Guinea during the outbreak, and none of the people who received the shots developed Ebola nine days or more after vaccination, leading researchers to conclude it was effective.

WHO said the vaccine has been studied in trials involving more than 16,000 volunteers in Europe, the Americas and Africa and proved to be safe for people 6 and older.

Merck said it is compiling evidence from those studies and plans to file for full licensure by next year from major regulatory authorities such as the Food and Drug Administration and the European Medicines Agency.

“We have been in the process of gathering, validating and compiling the data from the multiple partners involved in the clinical program to support planned regulatory submissions in addition to transferring the manufacturing process to a site in Germany that will make the vaccine,” Merck spokeswoman Pamela Eisele said.

She said Merck did encounter “unforeseen facility and engineering issues” at the German facility, though the company is making progress in correcting those problems.

“The development of vaccines are complex, and this complexity can often result in confronting the unexpected,” she said.

Merck does appear to have moved relatively swiftly since the West African outbreak.

The threshold is high for safety and efficacy of vaccines that are given to healthy people.

“If you’re already healthy, you don’t want to give somebody something that potentially has side effects and doesn’t work very well,” Ms. Kmush said. “A couple years is a short time to get a candidate and thoroughly test it.”

WHO said the 7,500 doses available in Congo are enough for 50 “rings” of 150 people. An additional 8,000 doses will be arriving in the coming days.

Many of the recent cases are in a remote region of the country, Bikoro. Transportation and storage there is a major undertaking because the vaccines must be kept at minus-75 to minus-110 degrees Fahrenheit.

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