Monrovia - Liberia said on Thursday it was free of Ebola, meaning there are now no known cases in west Africa of the tropical virus that has left more than 11 300 people dead in the region since late 2013.Liberia was the country worst hit with more than 4 800 Liberians killed by the virus, and was awaiting the all-clear following the discharge of its last known patients in May."Liberia is again free of Ebola. We have just ended the incubation period following the last case," Sorbor George, chief of communication at the ministry, told AFP.The west African nation has now passed the World Health Organisation (WHO) threshold of 42 days - twice the incubation period for the virus - since the last known patient tested negative for the second time."WHO commends Liberia's government and people on their effective response to this recent re-emergence of Ebola," said WHO Representative in Liberia Alex Gasasira in a statement. "WHO will continue to support Liberia in its effort to prevent, detect and respond to suspected cases," Gasasira added.The country now enters a 90-day period of heightened surveillance for any new cases.At its peak in 2014, Ebola sparked anxiety about a possible global pandemic and led some governments to threaten or unilaterally enforce travel bans to and from the worst-affected countries - Liberia, Sierra Leone and Guinea.The World Health Organisation declared an end on June 1 to Ebola cases in Guinea - where it first broke out in December 2013 - and in Sierra Leone on March 17.However, previous declarations announcing the end of Ebola flare-ups in West Africa have been followed by new cases - the virus has re-emerged three times in Liberia.Vigilance Health authorities were monitoring for new cases after a woman died of Ebola in the capital of Monrovia on March 31 after arriving from Guinea.Two of her three children, aged five and two, subsequently tested positive for the virus.In all, the virus affected 10 countries, including the United States and Spain, with more than 28 000 cases reported.The Liberian health ministry called on people to remain vigilant in order to avoid another outbreak in the future. "We have been carrying on a sensitisation campaign. This campaign will continue, and we will still be in readiness to contain any eventual outbreak," George said.The risk of infection lasts beyond the 42-day period because the virus can survive in certain bodily fluids of survivors, particularly sperm, where it can linger up to a year, according to experts.In Paynesville, the Monrovia suburb where the most recent spate of cases were registered, residents were glad to be moving on."It is good to hear that Ebola is gone again, but from what we saw recently we remain resilient in our preventive measures. We don't want our neighbourhood's name to be attached to the outbreak," said Bubakar Sanor, 56."We are happy that our health workers are now up to the task, containing the virus with bravery and professionalism," he told AFP.China's economic attaché to its Sierra Leone embassy announced on Thursday it would help to build a $5bn tropical disease research and prevention centre in the country to strengthen west Africa's readiness to combat Ebola and similar conditions."The recent Ebola outbreak damaged the country's economy and health sectors so we have decided to construct a research centre which will help in any future disease attack in the country and in the sub region, Shen Xiaokai told journalists.