“Complete findings will be made known to the public by tomorrow [Friday],” he said, adding that further tests were under way.
Earlier Thursday the WHO said a two-year Ebola epidemic that killed more than 11 000 people and triggered a global health alert was over, with Liberia the last country to get the all-clear.
At its peak, it devastated Guinea, Liberia and Sierra Leone, with bodies piling up in the streets and overwhelmed hospitals recording hundreds of new cases a week.
Rick Brennan, the World Health Organisation’s chief of emergency risk management, hailed the milestone but told reporters in Geneva that “the job is still not done”, pointing out that there had already been 10 small flare-ups because of the persistance of the virus in survivors.
UN chief Ban Ki-moon also warned the region can expect sporadic cases in the coming year but added “we also expect the potential and frequency of those flare-ups to decrease over time”.
Liberia, the country worst hit by the outbreak with 4 800 deaths, discharged its last two patients from hospital, the father and younger brother of a 15-year-old victim on December 3, 2015.
Africa’s oldest republic was the last country still afflicted by the outbreak that infected almost 29,000 people and claimed 11 315 lives, according to official data.
The real toll is suspected to be much higher, with many Ebola deaths believed to have gone unreported.
After the last patient is declared in the clear, a 42-day countdown, twice the incubation period of the virus begins before the country is proclaimed Ebola-free.
Ebola causes severe fever and muscle pain, weakness, vomiting and diarrhoea. In many cases it shuts down organs and causes unstoppable internal bleeding. Patients often succumb within days.
From a Guinean infant who was the first victim, the epidemic quickly spread into neighbouring Liberia and Sierra Leone, notching up more deaths than all other Ebola outbreaks combined.
Liberia was first to be declared free of human-to-human Ebola transmission in May, only to see the virus resurface six weeks later.
It was officially credited with beating the epidemic for a second time in September before another small cluster of cases emerged.
Reaction to Thursday’s announcement was muted in the capital Monrovia, where locals have become accustomed to good news on Ebola being followed by setbacks, and there was no official programme of celebration.
The WHO came under fire for its sluggish response to the epidemic, which local healthcare systems were woefully under-equipped to handle. Over 500 healthcare workers died in three west African countries at the height of the outbreak.
Brennan acknowledged the WHO’s initial inertia but said the organisation had “done a lot of soul-searching”, pointing to a “major reform” it is undergoing.
While Cuba sent doctors, Western governments offered little until foreign aid workers started falling ill and returning home for treatment, sparking fears of a global pandemic.
The concerns inched higher when three cases of infections came to light outside Africa — two in the United States and one in Spain.
The US, Britain and other countries eventually rallied to the cause, sending thousands of troops and medics to Africa in 2014 and developing a number of promising potential vaccines and treatments.
But the economic ravages of the epidemic are still being felt.
The World Bank estimates the economic damage of the outbreak, which devastated the mining, agriculture and tourism industries in Liberia, Sierra Leone and Guinea, at $2.2bn over 2014-15.
WHO director Margaret Chan described the next three months as “the most critical”, as foreign medical groups shut down operations in west Africa and national health ministries take over