JUDY WOODRUFF: Now the last in our series on Ebola in West Africa — tonight, a look at new research to help stop or slow the next outbreak. The best hope may ultimately come from a new vaccine.

Science correspondent Miles O'Brien reports, part of his series on Cracking Ebola's Code.

MILES O'BRIEN: It's dark and early in Freetown, Sierra Leone. A team of pharmacists is in a nondescript government building preparing the day's supply of an experimental vaccine against Ebola.

The clock starts running when they take the vaccine out of a very deep freeze. This is likely the coldest spot in the whole country. The vaccine can only be thawed out right before it is injected, or it will lose its potency, and all of this will be a waste of time, money and hope.

So, right now, timing and temperature are absolutely critical. And then it happens.

WOMAN: The power went out.

MILES O'BRIEN: Another reminder of how hard it is to conduct a high-tech vaccine trial in one of the poorest countries on the planet. But they are ready. They have got two backup generators for the building, solar-charged batteries, and, if all else fails, a special container that maintains about 100 degrees below zero Fahrenheit for five days without power.

Pharmacist Morrison Jusu is delivering the vaccine three-and-a-half miles across Freetown. After a seemingly endless national nightmare, he carries a cooler full of expectations. He knows much is riding with him.

MORRISON JUSU, STRIVE Trial Research Team: Some people lost family members. And some families were essentially wiped out as a result of this thing. And if this vaccine proves out to be something that prevents such in the future, then it's — it's — words cannot describe how much relief that would be to this community.

MILES O'BRIEN: While Jusu and the vaccine are wending their way, a line is growing outside their destination, Freetown's Connaught Hospital.

The volunteers start showing up before dawn. They are health care workers. This trial is limited to them because they are, by far, the most at risk of contracting Ebola virus disease. Even though there is no evidence the vaccine poses any real danger, they must weigh the rumors and the uncertainties.

MOHAMED SAMBOLA, Vaccine Trial Volunteer (through interpreter): Life is all about risks. But I believe it will be of help in the job that I do.

RUGIATU CONTEH, Vaccine Trial Volunteer (through interpreter): It is a high risk for me. I believe this can protect me from Ebola, and that's why I came here for this vaccine.

MILES O'BRIEN: It wasn't always this way. The trial got off to a slow start when it began in April. People were too afraid.

Dr. Mohamed Samai is one of the principal investigators.

DR. MOHAMED SAMAI, College of Medicine and Allied Health Sciences: People said the vaccine was the Ebola virus, so, once you get it, you become infected. So, a lot of people were not willing to come forward there in the first week to take the vaccine, because they thought they should wait and see what happens.

MILES O'BRIEN: The vaccine does contain a piece of the Ebola virus, a protein. It's enough to trick the body into triggering its natural defenses, but won't give the recipient Ebola virus disease.

On the wall in the lobby of the hospital, a spontaneous memorial to some of the doctors and nurses who died here during the epidemic, a grim reminder of what motivates volunteers like Richard Kanu.

RICHARD KANU, Vaccine Trial Volunteer (through interpreter): I became aware of it through my friends who got the shot three days ago. Since they're not having any side effects, I decided to come and have a go at it myself, because I feel it will protect me.

MILES O'BRIEN: Kanu works on a team that buries the highly contagious dead. He has been shunned by friends, even forced out of his own home.

RICHARD KANU (through interpreter): I will go back and tell them that I have had the vaccine and they should rest assured that I won't have the virus. I will probably encourage them to step forward as well.

MILES O'BRIEN: Bad traffic delays Jusu's ride to the hospital, but, when he arrives, the vaccine isn't spoiled, and the volunteers are ready. The nursing team doesn't waste any time prepping for the jabs.

Aruna Thorlie is a chlorine sprayer who disinfects Ebola treatment units.

Have you felt anything different? Did it hurt, anything?

ARUNA THORLIE, Vaccine Trial Volunteer (through interpreter): I feel the same as I did before. I hope and pray that it works.

MILES O'BRIEN: The vaccine is made by Merck. The trial is a partnership between the Sierra Leone Ministry of Health, the U.S. Centers for Disease Control and Prevention and the medical college in Freetown.

DR. MOHAMED SAMAI: Once we can document the effectiveness of the vaccine, and we are now sure that it can really protect them, we can move to another stage, where we will look at the community and the population at large.

MILES O'BRIEN: Across the border in Liberia, a separate trial is testing the same vaccine, along with another one made by GlaxoSmithKline. It is open to all and based at this hospital in Monrovia.

In the darkest days of the crisis here in Liberia, a senior doctor and nurse here at Redemption Hospital contracted Ebola virus disease, and they subsequently died. Many of the health care workers here became afraid to come to work and the hospital had to close for a time. In all, 13 members of the hospital staff died here.

DR. MARK KIEH, Redemption Hospital: It was scary. It was confusing

MILES O'BRIEN: DR. Mark Kieh is the site physician for the trial.

So far, there are 1, 500 volunteers. He is carefully watching them for side effects. So far, so good.

DR. MARK KIEH: Those we have seen are expected side effects of fever, muscle pain, pain at the injection sites, some joint pain, some — few people with rashes that resolve over time.

MILES O'BRIEN: The trial is run by Liberia's Ministry of Health and the U.S. National Institute of Allergy and Infectious Disease.

Its director, Dr. Anthony Fauci, has intense, intimate understanding of the ravages of Ebola.

DR. ANTHONY FAUCI, Director, National Institute of Allergy and Infectious Diseases: At least in the most recent patient that we took care of, that that's about as sick as you can get without dying.

MILES O'BRIEN: On March 14, 2015, he suited up to treat a U.S. health care worker who became infected in Sierra Leone, and was airlifted to the NIH hospital outside Washington, where he received the highest level of intensive care possible. It kept him alive while his body mounted its own defense.

It was touch and go for a week, but he survived.

DR. ANTHONY FAUCI: All of the people who have recovered from Ebola, even those who have been very ill, it was their immune system that ultimately suppressed and eliminated the virus.

MILES O'BRIEN: This is why Fauci and others are optimistic they have a found a way to stop Ebola its tracks. The human body can create the antibodies to fight off Ebola, but usually not fast enough.

An effective vaccine creates an army of Ebola antibodies that can stop the virus before it stampedes through the body. But here is the ironic rub.

DR. THOMAS FRIEDEN, Director, Centers for Disease Control and Prevention: If we are successful in controlling Ebola, we won't be successful in determining whether the vaccines are effective at preventing Ebola.

MILES O'BRIEN: Dr. Tom Frieden is director of the CDC. Ebola cases in Liberia are now at zero, and Sierra Leone is close behind.

If no one is getting Ebola, you can't tell for sure if the vaccine is protective, so they have to rely on other evidence.

DR. THOMAS FRIEDEN: So, the trials under way now will at least tell us whether they're safe, and they will tell us whether they lead to an immune reaction.

We wish we were further along with the vaccine, but it's very difficult to do research in the middle of an epidemic.

MILES O'BRIEN: In Freetown, pharmacist Morrison Jusu is among those who scrambled to get this trial under way at all. He is anxious to know if the hard work will pay off.

MORRISON JUSU: We are really looking forward to it being successful, so that someday in the future, we will be able to say, yes, I was a part of that team and I contributed. So, it will be a great feeling then.

MILES O'BRIEN: The answer will have to wait, but no one doubts the vaccine will meet its viral foe someday.

Miles O'Brien, the PBS NewsHour, Freetown, Sierra Leone.