EFEMexico City

A bacteria caused two young children to die and dozens of others to become sick last weekend in the southern Mexican state of Chiapas, according to a senior health official, who said the vaccines administered to the infants did not trigger their illness.

"We can offer a clear and unequivocal conclusion: that the isolated problem that occurred in Chiapas was not due to a faulty vaccine, to anything being wrong with the vaccine," the director of the Mexican Social Security Institute, or IMSS, Jose Antonio Gonzalez, said at a press conference, adding that the investigation is ongoing.

While other vaccines were administered to the children, the only one that all of the affected infants received was the hepatitis B vaccine, and therefore that has been the focus of the investigation, Gonzalez said.

The Federal Commission for Protection against Sanitary Risk, or Cofepris, analyzed and released the lots the vaccines came from in October of last year, he said, noting that those batches are the source of "tens of thousands of doses that have been administered (since then) in other parts of the country, without any instance of a reaction like this one."

Although only 72 hours have elapsed since the infants were hospitalized, "we have results from the cultures that are beginning to show the presence of local external contamination, unrelated to the vaccine," Gonzalez said Tuesday.

"The contamination came from outside," he said, adding that the type of bacteria and the source of the contamination still must be determined.

State health officials said Saturday that two babies died and 31 other young children became ill in the rural community of La Pimienta, Chiapas, due to suspected adverse reactions to IMSS vaccines.

Twenty-four of the children remain hospitalized, six of whom are listed in serious condition.