Sierra Leone has officially been declared free of Ebola by the World Health Organization(WHO). Thousands of people took to the streets of the capital, Freetown, at the stroke of midnight - marking 42 days without a single declared case of the disease. There were further cheers when the WHO local representative made the official announcement later on Saturday. The outbreak killed almost 4,000 people in Sierra Leone over the past 18 months. Many gathered around a giant cotton tree in the center of the city. Some lit candles in memory of the victims, while others danced with joy. Oliver Johnson, from the King's Sierra Leone partnership, worked at an Ebola clinic in Freetown, and has strong links with medical professionals there. "[For] everyone I've spoken to, there's a sense of relief that this might finally be over and maybe a bit of disbelief that after so many false starts, or false ends, we might finally be there," he said. A country is considered free of human-to-human transmission once two 21-day incubation periods have passed since the last known case tested negative for a second time.