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Sierra Leone - one of the countries worst hit by West Africa's Ebola outbreak - has announced a three-day lockdown to try to tackle the disease.

From 19 to 21 September people will not be allowed to leave their homes, a senior official said.

The aim of the move is to allow health workers to isolate new cases to prevent the disease from spreading further.

The outbreak has killed about 2,100 people in Sierra Leone, Liberia, Guinea and Nigeria in recent months.

The World Health Organization (WHO) announced on Friday that health workers could be given vaccines as from November, when safety tests are completed.

More than 20 health workers have lost their lives to the virus in Sierra Leone since the start of the outbreak in March.

Rights issue

Sierra Leone officials said more than 20,000 people would be deployed to make sure residents stayed indoors.

Sidie Yahya Tunis, the health ministry's communications director, told the BBC: "We don't expect them to refuse. You follow or else you'll be breaking the law. If you disobey then you are disobeying the president."

The BBC's West Africa correspondent, Thomas Fessy in Senegal, says the Sierra Leone population's willingness to obey will be key for the plan to succeed - a forcible implementation is likely to raise human rights issues and could potentially spark violent demonstrations.

Last month, Liberia sealed off a large slum in the capital, Monrovia, for more than a week in an attempt to contain the virus.

The disease infects humans through close contact with infected animals, including chimpanzees, fruit bats and forest antelope.

It then spreads between humans by direct contact with infected blood, bodily fluids or organs, or indirectly through contact with contaminated environments.

Officials in Nigeria have meanwhile decided to reopen schools in the country from 22 September. They were closed as a precaution to prevent the spread of the virus.

Separately, a US doctor infected with Ebola is being treated in a Nebraska hospital after evacuation from Liberia.

Rick Sacra, 51, was described as sick but stable. He is the third American aid worker infected. The other two recovered after treatment in Atlanta.

On Friday, the WHO announced that the blood of patients who recovered from Ebola should be used to treat others.

People produce antibodies in the blood in an attempt to fight off an Ebola infection. The antibodies may be able to help a sick patient's immune system if they are transferred.

However, large scale data on the effectiveness of the therapy is lacking.