1 RTI International, Berkeley, CA, United States; University of California San Francisco, San Francisco, CA, United States; University of Washington, Seattle, WA, United States. Electronic address: blambdin@rti.org.
2 University of Southern California, Los Angeles, CA, United States.
3 RTI International, Berkeley, CA, United States.
4 North American Syringe Exchange Network, Tacoma, WA, United States.