About 100 Montreal health-care workers had to get another H1N1 flu shot because the previously administered ones were improperly mixed.

One nurse at Centre Le Cardinal Inc. in the city's east end told CBC News that on Wednesday, she and others had to be vaccinated a second time this week after a colleague administering the shots realized the dosage was wrong.

French-language television network TVA reported that the vaccine given to workers at the long-term-care facility last Sunday and Monday contained too much of the adjuvant, the substance added to the vaccine to stimulate a stronger immune response.

The centre's director, Sylvie St-Hilaire, said the vaccine contained double the amount of adjuvant normally found in the vaccine.

"These kind of errors do happen. They are not common but they do happen every year, usually with errors in doses," she said.

Dr. Terry-Nan Tannenbaum, chief of health protection at Montreal's Public Health Department, said the H1N1 vaccine requires more steps to prepare it.

"It's a little bit different than what we usually need to do when we prepare vaccines," she said. "They sometimes come completely prepared. Other times the vaccine is in a powder form and you mix it with a liquid called a dilutant. So this is different and more complex."

Public health officials said the people responsible for preparing the vaccine did not understand the guidelines issued by the government, but they added that those receiving the shots were not in danger.