After 20-plus months, 28,000 cases and more than 11,000 deaths, the worst Ebola outbreak in history may have finally met its match.

Scientists and health officials are expressing new optimism after an experimental vaccine successfully prevented the spread of the virus as never before in a new study published Friday.

The new vaccine, dubbed rVSV-ZEBOV, could be the long-sought last nail in the coffin of the current Ebola outbreak. Better yet, it could ensure that future outbreaks never get this bad.

That said, there’s still reason to be cautious: a single vaccine, no matter how effective, isn’t going to solve all the systemic health issues that let the West African Ebola epidemic spiral out of control in the first place.

Here’s what you need to know about the new vaccine, why it matters, and what questions still need answering.

The study

Unlike another recent Ebola vaccine trial involving monkeys, this one featured human participants, all in Guinea, who had recently come into contact with the virus either directly or via a third party. Starting in March, more than 3,500 vaccines were administered, with around half of the participants receiving the shot immediately after coming into contact with the virus, and the rest after 21 days. After a 10-day waiting period, researchers determined how many patients in each group contracted the virus.

The results were striking. Of those who received the vaccine immediately, not a single person showed signs of the virus. By comparison, 16 new cases developed among the participants whose vaccination was delayed. Overall, the study concludes that the vaccine could prevent 75 to 100 percent of potential new Ebola cases if administered soon enough.

Granted, the number of new Ebola cases has dwindled in recent months, with just 4 new cases reported in Guinea in the last week of July, so it’s fair to ask how the same trial would have fared at the peak of the outbreak when study participants would have been exposed to the virus much more frequently.

Still, the results of the study prompted Jeremy Farrar, director of the Wellcome Trust (one of the organizations supporting the research), to emphatically endorse the new vaccine, writing in the Guardian, “It is in fact incredibly unusual for any vaccine to show such efficacy, and so swiftly, too."

Why this is good news

What’s most promising about the new vaccine is that, if deployed quickly and effectively, it has the potential to prevent individual vaccine recipients from getting sick, and to stop the virus from spreading anywhere else.

Ebola spreads like wildfire, but only through direct contact. So researchers decided to employ what’s known as a “ringed” vaccination strategy against it. By quickly vaccinating everyone in the social or geographic vicinity of infected people, health workers can build a wall of immunity around the virus, limiting its movements.

Since the start of the outbreak, health workers on the ground in the Ebola zone have been meticulously tracking the virus’ movements in order to treat those affected, and predict where it will move next. Armed with this new vaccine, they may finally be able to stop Ebola in its tracks.

And the possibilities don’t stop there.

Looking ahead to future outbreaks, Ebola experts are adamant that it’s not a question of “if”—it’s a matter of when. Bred in the jungle and capable of striking without warning, Ebola is too unpredictable to fully eradicate. But Dr. Sakoba Keita, Guinea’s Ebola response coordinator and an author of the new study, told the New York Times he was hopeful the vaccine would “contribute to the rapid control of the next epidemics caused by this disease.”

A vaccine that ends the current outbreak and better prepares us for the next time Ebola rears its ugly head? That would truly be something to celebrate.

So what's the catch?

Questions remain.

First off, researchers don’t yet know how long immunity lasts. So until further studies have been completed, there’s no telling if the vaccine could be administered to an entire population as a preventative measure against future outbreaks, the same way kids in many countries are vaccinated to prevent the spread of Measles, for example.

Researchers plan to track the long-term effects of the vaccine, and are considering expanding the trial to neighboring Sierra Leone and Liberia. So far, the vaccine has shown no side effects, which is no small thing for a treatment containing live Ebola virus.

Then there’s the question of what happens once the vaccine moves past the trial phase and actually hits the market. Once the vaccine is approved by regulatory agencies like the US Food and Drug Administration, it will be subject to the same market forces that have driven up the cost of so many drugs before it. Working together with the drug manufacturer, Merck, governments and NGOs must ensure that the Ebola vaccine is within reach of the people who need it most.

That brings me to perhaps the biggest limitation dogging this vaccine. In a recent vlog, I talked about how the devastation caused by Ebola is in some sense just a symptom of the larger health disparities affecting West Africa. A vaccine alone can only target the virus. What’s to be done about the bigger issue?

Dr. Margaret Chan, the head of WHO, echoed this sentiment when she cautioned people not to dub the vaccine a “silver bullet.”

“There is no replacement for very strong and good, resilient health systems with the capability for surveillance,” she told the New York Times.

Amen to that, Dr. Chan.

The fact is, right now much of West Africa lacks the infrastructure and resources necessary to use the vaccine to its full potential.

For one thing, many areas in Africa lack the capacity to keep the vaccine chilled well below zero. For another, health officials here have long struggled to build local trust in Western healthcare.

And then there’s the simple fact that countries like Liberia are desperate for more doctors and health facilities to administer the vaccine—and other services—in the first place.

rVSV-ZEBOV may very well be Ebola’s Kryptonite. But until West Africa has an effective health system to match, the world will never be fully immune to another catastrophic epidemic.