The Research Committee was notified on the beginning and the design of this trial: 6,000 tablets of chloroquine 150 mg (Aralen) and 6,000 identical tablets of placebo were commercially purchased and sent to the laboratory of Dr. Roberto Medina at the National Polytechnic Institute (selected as monitor). 30 sets of 375 tablets each were separated (15 contained chloroquine and 15 placebo) in identical flasks, the assignment of treatments was randomly allocated; the codified treatments numbered 1-30 were then sent back to our Institute. The code was sealed and kept by the monitor until the end of the study. Patients included in the study were given the corresponding treatment in sequential order.