Coronavirus was first detected at the hospital in Tlanepantla de Baz, in Mexico State just outside the capital, on March 10, IMSS Director Zoe Robledo said at the president's regular news conference.
He said the transmission did not originate inside the facility, and that it had been tracked to three separate cases - one patient and two doctors who did not have contact with each other.
He said the cluster was not similar to an outbreak in the city of Monclova in the northern border state of Coahuila, where 45 health workers have tested positive for the coronavirus, according to the state's health ministry.
"Compared with Monclova, where there is epidemiological proof that there was contagion inside the hospital, it's not the same case," he said. "The epidemiological study, which took several weeks, is showing that it came from outside."
Two weeks ago, workers at the Tlanepantla hospital protested outside the building to demand more medical equipment to handle coronavirus cases.The outbreak of the novel coronavirus, which began in China at the end of last year, has led to 2,785 cases of coronavirus in Mexico so far, and 141 deaths.