Table 1 Criteria for ZIP codes as potential hotspots for HIV and HCV transmission and drug overdoses

HIV/HCV injection transmission hotspot

1. The ZIP code had to have a relatively large number of PWID, such that there were at least 10 participants in the study who reported residing in the ZIP code during the 6-month period prior to their interview.

2. At least 2 respondents seropositive for HIV or HCV residing in the ZIP code had to report potential injecting-related risk for transmitting HIV or HCV, defined as passing on needles and syringes that they had used to other PWID (“distributive sharing”).

3. At least 2 respondents not currently infected with HIV or HCV living in the ZIP code had to report potential injecting-related risk for acquiring HIV or HCV, defined as injecting with needles and syringes that had been used by other PWID (“receptive sharing”).

HIV sexual transmission hotspot

1. The ZIP code had to have a relatively large number of PWID, such that there were at least 10 participants in the study who reported residing in the ZIP code during the 6-month period prior to their interview.

2. At least 2 respondents residing in the ZIP code had to report high potential risk for sexual transmission of HIV, that is, they had to be seropositive for HIV and had to report unsafe sex (vaginal or anal intercourse without consistent condom use) and had to be infected with HSV-2 (be HSV-2 seropositive).

3. At least 2 respondents living in the ZIP code had to report potential high risk for sexually acquiring HIV, that is they were not currently infected with HIV (were seronegative) and had to report unsafe sex and had to be infected with HSV-2 (be HSV-2 seropositive).

Overdose hotspot

1. The ZIP code had to have a relatively large number of PWID, such that there were at least 10 participants in the study who reported residing in the ZIP code during the 6-month period prior to their interview.

2. At least two respondents had to report an overdose.