The Health Commission of Lanzhou, the capital city of Gansu province, confirmed that 3,245 people had contracted the disease brucellosis, which is often caused by contact with livestock carrying the bacteria brucella.
In preliminary testing, another 1,401 people have been found to be positive, though there have been no fatalities reported, the city's health commission said. In total, authorities have tested 21,847 people out of the city's 2.9 million population.
Human-to-human transmission is extremely rare, according to the CDC. Instead, most people are infected by eating contaminated food or breathing in the bacteria—which seems to be the case in Lanzhou.
This outbreak stemmed from a leak at the Zhongmu Lanzhou biological pharmaceutical factory, which occurred between late July to late August last year, according to the city's health commission. While producing brucella vaccines for animal use, the factory used expired disinfectants and sanitizers—meaning not all bacteria were eradicated in the waste gas.
This contaminated waste gas formed aerosols that contained the bacteria and leaked into the air, carried by wind down to the Lanzhou Veterinary Research Institute, where the outbreak first hit.
People at the institute began reporting infections in November, and it quickly accelerated. By the end of December, at least 181 people at the institute had been infected with brucellosis, according to Chinese state-run news agency Xinhua.
Other infected patients included students and faculty members of Lanzhou University; the outbreak even spread to Heilongjiang province, at the very northeastern tip of the country, where 13 positive cases had worked in the veterinarian institute in August, Xinhua reported at the time.
In the months after the outbreak, provincial and municipal officials launched an investigation into the leak at the factory, according to the Lanzhou Health Commission. By January, authorities had revoked vaccine production licenses for the plant, and withdrew product approval numbers for its two brucellosis vaccines.
A total of seven veterinary drug product approval numbers were also canceled in the factory.
In February, the factory issued a public apology, and said it had "severely punished" eight people who were determined as responsible for the incident. It added that it would cooperate with local authorities in the response and cleanup efforts, and contribute to a compensation program for those affected.
The Lanzhou Health Commission also announced in its report on Tuesday that 11 public hospitals would provide free and regular checkups for the infected patients. The report didn't offer additional details on the compensation for patients, except that it would be launched in batches starting October.