Brucellosis is a disease transmitted by cattle or animal products. The pathology can cause fever, joint pain and headache.
In total, 3,245 people tested positive for the bacteria, according to health officials in Lanzhou, the capital of Gansu province.
According to the local government, there was no transmission from human to human.
A state laboratory used an expired disinfectant in July and August 2019 to produce vaccines against brucellosis for animals, officials said on Tuesday.
As a result, sterilization was incomplete, and the bacteria were still present in the gas emissions of the company, the Lanzhou Biopharmaceutical Unit for Livestock.
The contaminated gas spread through the air to the neighboring Veterinary Research Institute, where it infected almost 200 people in December.
The Lanzhou Department of Health said on Friday that the bacteria usually originated from sheep, cows or pigs.
The lab apologized earlier this year and saw its license to produce revoked brucellosis vaccines.
Patients will receive compensation starting in October, according to Lanzhou authorities.