Nine health workers went on trial in northern Poland Monday accused of having tested a vaccine against bird flu on nearly 200 patients without their knowledge, court officials said.

The accused -- three doctors and six nurses -- are charged with "fraud, creating false documents and delivering health care without authorisation" to 196 patients, judge Piotr Szadkowski of the Torun region told AFP.

If found guilty, they risk up to 10 years in jail.

All nine accused, some reportedly clad in wigs and sun glasses to avoid being identified, pleaded not guilty.

The medical personnel are charged with administering a vaccine banned in Poland against the deadly H5N1 strain of bird flu that can be transmitted to humans.

The patients were paid for the vaccines, Polish news agency PAP reported.

They allegedly led their patients, many of them poor and homeless, to believe they were being vaccinated against ordinary flu.

Police discovered the scam by chance when they were called to break up a fight at a homeless shelter, PAP said.