(CNN) The Ministry of Health in the Democratic Republic of Congo officially declared on Tuesday that the country's battle against a ravaging Ebola outbreak has come to an end, according to the World Health Organization .

The government of Congo declared an outbreak of Ebola hemorrhagic fever in May, the country's ninth recorded outbreak.

According to the Ministry of Health , 54 cases of Ebola virus were recorded during the outbreak, including 33 deaths.

The organization provided guidance to the ministry to declare the outbreak over if no other cases were confirmed and two full incubation periods of 21 days each have passed, starting the day after the last Ebola patient was released from care, WHO spokesman Tarik Jašarević wrote in an email last week.

The last patient was released after blood-testing negative for the Ebola virus on June 12.

"DRC beat this outbreak of Ebola through traditional methods such as case investigation, contact tracing and providing care to the sick, and with new tools such as vaccinating people who were at risk of being infected by the virus," Jašarević said.

"The government was speedy, transparent, and welcomed support where needed. These were key to an effective response," he said.

A fast-spreading outbreak

WHO officials originally thought the recent outbreak would remain within Congo's isolated rural communities and not spread to more densely populated towns, which was the case in previous outbreaks, said Dr. Peter Salama, a medical epidemiologist and deputy director-general of emergency preparedness and response at WHO.

"Then, in the first few days, cases were confirmed in the urban center of Mbandaka with more than a million people. That meant that we quickly had to change strategy," he said.

He added that, as the outbreak grew, the risk was massive to Kinshasa, the capital of Democratic Republic of Congo, as well as major cities in surrounding countries.

The fast spread of the outbreak to a big city was a move that Salama pointed out has become more common in recent years among various infectious diseases.

"Perhaps because of climate change, because of changes in reservoir animals or vectors, and because of human encroachment on animal populations, we're seeing more and more urban outbreaks of these kinds of high-threat pathogens -- and that poses a very different set of risks," Salama said.

As for Ebola, scientists think people are initially infected with Ebola virus through contact with an infected animal, such as a bat, and then the virus spreads from person to person. Ebola can only spread between humans by direct contact with an infected person's bodily fluids, such as urine, saliva, sweat, feces, vomit, breast milk and semen.

"In urban areas, the chances of what we call a super-spreading event -- an event where one case can trigger literally tens or hundreds of cases -- is higher just because of the density of population," Salama said.

"Therefore, these diseases become even more deadly and even more risky in urban circumstances, and that's something we've seen now with Ebola this time around," he said.

Within hours of the Ebola outbreak being declared in Congo, WHO released $2 million from its contingency fund for emergencies to support response efforts, deployed a team and activated an emergency incident management system.

To fight the outbreak, health workers turned to not only traditional approaches but also new tools, such as an experimental vaccine and investigational drugs.

'One thing that's different ... is the availability of the vaccine'

"We had a vaccine -- and that I think is going to be extremely important for the future of Ebola control," Salama said.

"In addition, although we didn't get to use them this time, we had under investigational protocol the potential to use of four drugs. So for people already infected with Ebola, we had drugs on site by the end of the outbreak and able to be used," he said. "It's not enough to do traditional public health containment measures. We really feel that the same kinds of vaccines and drugs that we would offer to patients in the West should be offered to people in developing countries as well."

"One thing that's different this time that I think really made a big difference is the availability of the vaccine from Merck," said Don Sodora, a virologist at the Center for Infectious Disease Research , which recently announced joining Seattle Children's Research Institute . The combined scientific team will form the largest pediatric infectious disease research program in the United States.

"It's still being tested," he said of the vaccine. "Merck is still trying to figure out how efficacious it is, but it can really provide an opportunity where you can start protecting people."

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Among those eligible to receive the vaccine were health care and front-line workers, as well as the contacts -- and even the contacts of contacts -- of confirmed Ebola virus disease patients, dead or alive.

Health care workers administered the vaccine using the ring strategy : The vaccine was administered to groups of people who were in close contact with each Ebola case, such as family members or caregivers, as well as the contacts of those contacts.

Health workers also followed up with and tracked the health of those in close contact with people who either had Ebola virus disease or died from the disease. The process was called contact tracing, which helps prevent further spread of the virus.

"In the typical ways to contain these types of outbreaks, you want to identify the patients very quickly. You want to make sure that people who are feverish and have the disease do not leave an area," Sodora said.

"The hemorrhagic fever viruses have a number of issues that make them particularly problematic with regard to transmission. They have this initial period when people are infected but don't show symptoms, and that's a problem," he said, adding that once people are infected, they are extremely contagious.

"So the best strategy is to identify infected people, get there rapidly with trained health care workers and get quarantine established quickly," he said. "That is the major lesson we have learned."

The risk of Ebola 'flare-ups'

There is still much left to learn about the Ebola virus and how how long the risk might continue.

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A study published Monday in the journal The Lancet Infectious Diseases found that about a year after a woman in Liberia survived an Ebola infection, she passed the virus to one or more of her family members, leading to the death of her 15-year-old son.

This cluster of cases suggests that, on rare occasions, Ebola virus can persist in a person long after that person recovered from acute infection. The study also is the first, to the authors' knowledge, to provide suggestive evidence of Ebola virus transmission from a female survivor after such a long period of time.

Such transmissions "are rare but underscore the need for continued prevention and disease surveillance," said Dr. Emily Kainne Dokubo, a researcher at the US Centers for Disease Control and Prevention and lead author of the study.

"Risk factors for Ebola virus persistence in body fluids are unknown, and the length of time that the virus persists in some survivors is varied," she said.

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The woman's teen son tested positive for the virus in November 2015 after showing symptoms. The researchers collected specimens from the family members and tested blood samples.

The woman's husband and their 8-year-old son also tested positive. Their two youngest sons, 5 years old and 2 months old, tested negative. The woman herself also tested negative but had Ebola antibodies, indicating prior infection.

The researchers discovered that the woman must have been infected with the virus after she cared for her sick brother, who died of Ebola virus disease in 2014.

After caring for him, she reportedly became very ill with symptoms consistent with Ebola virus disease, but she did not seek care. In September 2015, she gave birth to her youngest son and fell ill after delivery.

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The researchers noted that the disease could have re-emerged because pregnancy and birth could have altered the woman's immune defenses.

"The mother's infection may have flared up after her pregnancy, and the virus was then transmitted to other family members," Dokubo said.

The woman also could have transferred protective antibodies to her newborn son, which is why he didn't test positive for the virus. Meanwhile, she probably transmitted the virus to her husband, who in turn transmitted it to their oldest sons.

"The findings from this and recent Ebola virus disease clusters highlight the risk of Ebola virus disease flare-ups even after an outbreak is declared over," the researchers wrote.

"We already know that this persistence has led to occasional transmissions ," Kraft said. "Women who become pregnant after having been exposed or recovered from Ebola virus disease will need to be closely monitored clinically for relapse."

Ebola's legacy and lessons learned

In responding to this year's outbreak in Congo, the world seemed to be better prepared -- especially in comparison with the previous outbreak in West Africa.

"One of the many painful lessons from the devastating West African Ebola epidemic of 2014 was that the world expected much more from the World Health Organization than it was then able to deliver," WHO's Jašarević said.

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During that outbreak, 28,616 cases of Ebola virus disease and 11,310 deaths were reported in Guinea, Liberia and Sierra Leone. An additional 36 cases and 15 deaths occurred when the outbreak spread outside those three countries, according to the CDC

"Since then, we have dedicated ourselves to ensuring that the world is better prepared, not only for Ebola but for the many high threat pathogens, including pandemic influenza, that can cross the species barrier, from animals to humans, at any moment," Jašarević said.

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As of now, WHO and other global health organizations are ensuring the end of the Ebola response in Congo and maintaining an increased vigilance to identify lessons learned and good practices in responding to such outbreaks moving forward, Jašarević said.

"Already, we can say we have contributed to improving surveillance systems, and for next time, will have in place protocols for vaccines and therapeutics," he said. "Another legacy is capacity building, such as training vaccinators who will now not only be able to respond domestically but can also help neighboring countries, too."