Sierra Leone has released its last two known Ebola patients and begun a new 42-day countdown to a declaration that it is officially free of the virus, officials said on Monday.

More than 11,300 people have died of Ebola in Guinea, Sierra Leone and Liberia in an outbreak that was declared in March 2014 and is rated the world's worst.

The two women just released from hospital were treated in Kambia district on the Guinea border after being infected by a 67-year-old woman who posthumously tested positive for Ebola.

Her death lead to almost 1,000 people being put in isolation and stopped the previous countdown a week after it began in August. One community is still under quarantine but that should be lifted this week barring any new cases, officials said.

There were only two new cases in Guinea in the week of Sept. 20 and none in either Sierra Leone or Liberia, according to figures from the U.N.'s World Health Organisation (WHO).

A country must record zero new infections in 42 days to be declared Ebola-free, said WHO, which began the count in Sierra Leone on Sunday.

"It's good news for Sierra Leone and the sub-region. I am hoping that this time we won't fall off," Pallo Conteh, who heads the national Ebola Response Centre, said.

"It is difficult to say you are confident because you know with Ebola one mistake might just lead to a spike again," he said.