Austria said Thursday that several COVID-19 patients were cured after receiving transfusions using blood plasma from people who had recovered from the virus.

The treatment has been tested in several countries but there is still little medical data available about its effectiveness.

Three patients who had transfusions at a hospital in the southeastern city of Graz are now cured, Robert Krause, an infectious disease specialist at the hospital told a press conference.

However, it is "an option for hand-picked patients and is not readily transferable" to anyone who falls sick with the virus, he said.

Two of the three patients had other diseases and very weakened immune systems.

A total of 20 people have been treated in Austria with the therapy, which uses part of the blood rich in antibodies, known as convalescent plasma, from COVID-19 patients.

The Red Cross launched an urgent appeal to plasma donors on Thursday to allow this therapy, which is still experimental, to be tested more widely.

Some 200 donations have so far been made in Austria.

Convalescent plasma has proven effective in small studies to treat infectious diseases including Ebola and SARS.

Its use to treat COVID-19 patients has been authorised in other countries such as France, the United States, Switzerland and China.