A British nurse who suffered a relapse after contracting Ebola in Sierra Leone has been taken off the critical list while remaining "serious but stable", the hospital treating her said Monday.

Despite recovering from the virus in December after returning to the Scottish city of Glasgow from Sierra Leone, Pauline Cafferkey, 39, was readmitted to hospital on October 6.

Her condition deteriorated and doctors at London's Royal Free Hospital issued a statement on Wednesday saying she was "critically ill".

In Monday's update, the hospital said that "Cafferkey's condition has improved to serious but stable."

Cafferkey's case is believed to be only the second recorded incident of "reactivated" Ebola after American doctor Ian Crozier.

Just three weeks ago, she was at Downing Street meeting Prime Minister David Cameron's wife Samantha and receiving a bravery award.

She reportedly visited a primary school the day before being readmitted to hospital.

During her initial bout, the nurse spent almost a month in the isolation unit at the Royal Free Hospital and was treated with an experimental anti-viral drug and blood from Ebola survivors before being discharged.

After relapsing, she was treated in Glasgow before being flown by military aircraft to the Royal Free Hospital.

Her family accused local medical staff of failing to recognise quickly enough that the infection had returned.

Health officials said they were monitoring 58 people who have been in close contact with Cafferkey. Forty have had direct contact with her bodily fluids -- the main mode of Ebola transmission.

Of the 40, 25 are receiving a vaccine and a further 15 have either declined it or were unable to receive it due to existing medical conditions.

The deadliest-ever Ebola outbreak since the virus was identified in central Africa in 1976 has killed over 11,000 people and infected over 28,000, according to the latest WHO figures.