Tapen Sinha writes:

Living in Mexico, I have been witness to many strange (and beautiful) things. Perhaps the strangest happened during the first outbreak of A(H1N1) in Mexico City. We had our university closed, football (soccer) was played in empty stadiums (or should it be stadia) because the government feared a spread of the virus. The Metro was operating and so were the private/public buses and taxis. Since the university was closed, we took the opportunity to collect data on facemask use in the public transport systems. It was a simple (but potentially deadly!) exercise in first hand statistical data collection that we teach our students (Although I must admit that I did not dare sending my research assistant to collect data – what if she contracted the virus?). I believe it was a unique experiment never to be repeated.

The paper appeared in the journal Health Policy. From the abstract:

At the height of the influenza epidemic in Mexico City in the spring of 2009, the federal government of Mexico recommended that passengers on public transport use facemasks to prevent contagion. The Mexico City government made the use of facemasks mandatory for bus and taxi drivers, but enforcement procedures differed for these two categories. Using an evidence-based approach, we collected data on the use of facemasks over a 2-week period. In the specific context of the Mexico City influenza outbreak, these data showed mask usage rates mimicked the course of the epidemic and gender difference in compliance rates among metro passengers. Moreover, there was not a significant difference in compliance with mandatory and voluntary public health measures where the effect of the mandatory measures was diminished by insufficiently severe penalties,