There is evidence the Ebola virus can persist in the bodies of survivors after they have recovered from the disease.
This is not the first time Liberia has been declared free of Ebola transmission - the World Health Organization (WHO) made a similar announcement in May 2015, but the virus returned twice, once in June and then again in November.
What makes this occasion different is that the other two countries most affected by the recent outbreak - Guinea and Sierra Leone - have also been declared free of Ebola. The WHO declared Sierra Leone free of Ebola in early November, and Guinea achieved the same status in late December.
For the first time since the start of the epidemic 2 years ago, say WHO, all three countries have reported zero cases for at least 42 days.
To be declared free of Ebola, a country must achieve 42 days (two 21-day incubation cycles of the virus) since the last confirmed patient tests negative for the disease two times.
The recent outbreak in West Africa is unprecedented in world history. It has infected over 28,500 people and claimed over 11,300 lives. It has devastated families and communities and significantly weakened the health systems and economies of all three countries.
Risk of additional outbreaks continues
While praising the monumental efforts of the governments and people of the countries affected, and the organizations that have partnered with WHO in bringing the epidemic to an end, the UN agency warns that "the job is not over" and "strong surveillance and response systems will be critical in the months to come," as it is likely that flare-ups will occur.
There is a high risk that small outbreaks of Ebola - like the most recent one in Liberia - will occur.
The WHO say 10 of the flare-ups that have occurred were not part of the original outbreak and were likely a result of Ebola persisting in survivors who have recovered.
There is evidence that while the Ebola virus may disappear from the bloodstream of survivors relatively quickly, it can survive in "niches" in the body. For example, it can survive in men's semen for 7-9 months and in the eye for 2 months after recovery.
Dr. Bruce Aylward, special representative of the director-general for the Ebola Response at the WHO, says the epidemic is in a critical period as countries move from managing infected patients to managing the risk of new infections.
He notes that the risk of new infections is gradually reducing as the virus clears from the survivor population, but "we still anticipate more flare-ups and must be prepared for them." He adds:
"A massive effort is under way to ensure robust prevention, surveillance and response capacity across all three countries by the end of March."
Looking after survivors is an important part of this phase. As well as screening for persistent virus, they will need medical and psychological care, and support to help them return to normal life in their families and communities, who will also need education and help to reduce stigma and minimize risk of Ebola virus transmission.
One of the factors that appears to have hampered control of the recent Ebola epidemic in West Africa, and made it difficult to apply aggressive quarantine programs, is the lack of maps or census data. This was the conclusion of a review on the role of mapping in preventing epidemics that Medical News Today reported recently.