Guinea and Sierra Leone only registered one new Ebola case each last week, the UN's health agency said, hailing dramatic improvements in monitoring but warning the danger was far from over.
During the previous seven-day period, the two countries had together recorded just seven new cases, already marking the fewest weekly infections in over a year in the west African Ebola epidemic.
But the World Health Organization (WHO) warned against "unrealistic expectations" that the epidemic that has claimed around 11,300 lives since late 2013 would just peter out.
The single case reported in Guinea, in the capital Conakry, during the week ending August 2 was a 28-year-old woman who had been sought since she was a known contact of an Ebola patient, said WHO.
While she was missing, the woman "is likely to have generated a substantial number of further high-risk contacts," it said, pointing out that she had travelled from Conakry to nearby Forecariah and onto Kambia, Sierra Leone, where she reportedly visited a traditional healer, before returning home again.
Health professionals use the term "contact" to refer to someone they know has been in contact with someone with Ebola.
The case reported in Sierra Leone last week was also a known contact who had provided care to a family member with Ebola, said WHO.
In both countries nearly 2,000 contacts remain under surveillance, it said, but cautioned that a small number of contacts in both countries had not been traced or had been lost to follow-up.
"The response is definitely getting better, the epidemiology is reflecting that but there is a huge risk of unrealistic expectations that this will go from here to zero," told Bruce Aylward, who heads WHO's Ebola response.
"We will have additional flares, and this could still go on for additional months before it gets stopped," he said, adding that finding the missing contacts is vital to ending the epidemic.
Aylward said it was a "realistic goal" to halt Ebola transmission by the end of the year, but warned that even once that was done risks would remain and insisted tight surveillance would be required well into 2016.
He pointed to the example of Liberia, which was declared free of Ebola transmission in May after reporting no new cases for 42 consecutive days, only to see the virus reemerge a month later.