The last-known Ebola patient in Sierra Leone has been released from hospital, and the African country has reported no new infections for two weeks, health officials say.

It represents a major milestone in the battle against the Ebola outbreak that began in the country 15 months ago and has claimed almost 4,000 lives, say health officials and the World Health Organization.

With the release of 34-year-old Adama Sankoh from a hospital in Madeni, Sierra Leone's third-largest city, the country is now in the standard 42-day WHO countdown, after which it could be declared Ebola free if no new cases are reported.

Forty-two days is twice the incubation period of the Ebola virus.

Neighboring Liberia is in the same kind of countdown, leaving Guinea as the only country in West Africa still reporting active and new cases.

The release of Sankoh was accompanied by ceremonies and celebrations, with television and radio stations carrying the news.

Her discharge "represents a significant milestone in the fight against Ebola and the countdown towards a resilient zero," says Steven Gaoja of the country's National Ebola Response Center.

The outbreak that began in December 2013 is considered history's worst ever, infecting almost 28,000 people — mostly in Liberia, Sierra Leone and Guinea — and killing some 11,300 people.

The first case confirmed in Sierra Leone was in May 2015, in a woman who tested positive for the virus after returning from attending a funeral in Guinea.

Health officials are saying that despite the celebrations, caution is in order.

"We might have hidden cases, so we have to continue to be vigilant, continue our surveillance, maintain our discipline of hand-washing and temperature checks, screening and avoid over-crowding," says NERC situation room Director O.B. Sisay.

Malaria, a disease common in Sierra Leone, has many symptoms similar to those of Ebola, so medical personnel are performing ongoing monitoring of suspected cases, says Vanessa Wolfman from the International Medical Corps, which runs a British-funded treatment center that includes a diagnostic laboratory.

"As we see the tail end of this we still have safe places for sick people to be referred," she says. "We can get people in, isolate them from other community members, do rapid testing and get them back home as soon as possible."

Symptoms of Ebola can include fever, diarrhea, vomiting, liver and kidney failure and internal and external bleeding.

WHO officials said the outbreak could be completely over by the end of the year but cautioned against a false sense of security since even a single undetected case could cause a major flare-up of the disease.