As I suspected when I first heard that the accident was due to “external contamination”, the tragedy in Mexico appears to have been due to Staph. species bacterial contamination on the outside of the vial, or on the hands of someone involved in the process of administering the vaccines. I was mocked for making the statement “sounds like someone didn’t wash his hands” but that’s exactly what it sounds like– perhaps a little oversimplified. No, you do not have to handle the needle to transfer bacteria, simply handling the baby and not following hygiene protocol would be enough to give the bacteria entry into the skin.
According to the Mexican government agency IMSS, the species found was Staphylococcus hominis, a very common bacteria that colonizes human skin. It can cause severe sepsis in newborns. The antibiotic resistant strain is transmitted by healthcare workers, but it is not clear which strain was found in the infants in Chiapas. The bacteria was also isolated from the children, and their clinical symptoms were consistent with an infection from this bacteria.
They concluded that the infections were a result of handling and administration, since the vaccines were apparently properly stored and the vials themselves were not contaminated.