In our analysis, the prevalence of both HBV and HCV infection declined by about half among young IDUs in Seattle during 1994–2004, while HIV prevalence remained low. Two observations support our argument that these declines are not an artifact of differences among the component study populations. First, the declines were significant after controlling for potentially confounding variables, and they remained significant after control for residual confounding associated with the study in which subjects participated. Second, decreases in prevalence were observed over time within each of the studies, although not all of these declines were statistically significant.
Our results are consistent with a report documenting a decrease in HCV prevalence, from 91% in 1990–1991 to 62% in 2000–2001, among persons entering a drug detoxification program in New York City.17
Decreases in HCV24
infection incidence during ongoing follow-up in IDU cohort studies have also been reported in Baltimore, possibly due to retention of lower-risk IDUs over time. International data have shown declines in HCV prevalence in Scotland26
though prevalence patterns in these countries may differ from those in the United States. It will be of interest to determine to what extent the declines in HBV and HCV infection prevalence we observed have occurred among other IDU populations throughout the United States.
We observed no decrease in needle sharing, or sharing of cookers or cottons, or backloading, which are potentially significant components of blood-borne pathogen transmission,15,16,29
even though increasing proportions of Seattle area participants reported needle exchange as their primary source of new needles. According to internal Seattle King County Public Health reports, between 1994 and 2004, the annual volume of needles exchanged increased from 750,000 to almost 2 million and the number of needle-exchange sites increased from five to eight.
The different time periods for reporting of sharing of injection equipment among the four studies included in this analysis could have influenced the pattern of time trends. For instance, even if the proportion of IDUs sharing needles were constant, a higher proportion would be expected to report needle sharing over a longer reference period. Thus, the longer reference period of the DUIT study (3 months vs. 30 days in the other studies) could potentially yield spuriously higher rates of sharing and mask a decline, had it occurred. For indirect sharing of injection equipment, the shorter reference period of the DUIT study (3 months vs. 6 months in the other studies) could have had the opposite effect.
The ever/never sharing variables used in these studies may have been too crude to reflect subtle behavioral changes. However, ever/never variables have recorded significant reductions in sharing in the late 1980s to early 1990s in Baltimore30
and New York.31
It is interesting to note no change in ever sharing of syringes was found among IDUs in a New York drug treatment facility from 1990 to 2001,32
while HCV and HIV prevalence declined markedly.17
The latter finding raises the possibility that the declines we observed were the delayed result of behavioral changes occurring before the beginning of the study period.
Another possibility is that IDUs are decreasing their risk of infection by selective choice of injection equipment-sharing partners. The reported high levels of HIV testing, and, in recent years, of HCV testing, make it possible for IDUs to practice such serosorting. In the DUIT study, 20–30% of participants reported having made a decision not to share a needle based on knowledge of a partner’s HCV or HIV positive status. A finding that HIV-infected IDUs in New York were less likely than HIV-negative persons to engage in distributive needle sharing suggests they may have practiced such serosorting.32
Further studies should evaluate the extent to which IDUs are serosorting.
Hepatitis B vaccination rates increased in the Kiwi study population during the late 1990s, in contrast to low vaccination rates reported among IDUs elsewhere in the United States.11,33–36
It would be of interest to know how vaccinated participants came to be vaccinated. Vaccination was offered at times by the needle exchange. Adolescent hepatitis B vaccination was available in teen clinics at Seattle high schools and through Group Health Cooperative, a large local HMO. However, we have no data on the extent to which participants were vaccinated through these programs.
None of the study participants were young enough to have been vaccinated as infants, first recommended in 1991.37
After a recommendation for universal hepatitis B vaccination of adolescents in 1995,38
it became a school requirement in Washington State in 1997 to vaccinate 11 and 12 year olds; only 24 DUIT participants were subject to this requirement. In 1999, the Advisory Committee on Immunization Practices recommended hepatitis B vaccination for all adolescents up to age 19 years,39
which would have included 13% of study participants. However, this recommendation was not accompanied by any mandate or program. The prevalence of vaccination in our study population does not appear to be primarily a product of programs universally mandating vaccination.
We noted an increase in condom use among young Seattle IDUs enrolled in the Kiwi study between 1998 and 2002. The pattern of the time trend in condom use resembles that for hepatitis B vaccination. This suggests that multiple preventive measures were being adopted concurrently among young Seattle IDUs. Nonetheless, as has been reported in other IDU populations,40–43
absolute levels of condom use remained modest.
As with other studies of IDUs, the clandestine nature of injection drug use complicates efforts to ascertain the extent to which study participants reflect the universe of young Seattle IDUs. Systematic changes over time in the characteristics of recruits within each study could conceivably account for the observed trends within individual studies. However, if progressively different subpopulations were being sampled in the course of the individual studies, the pattern of HBV, HCV, and HIV seroprevalence in the different subpopulations sampled would need to reflect, in register, the overall trends we observed, which seems unlikely. Our measures of risk behavior and prior vaccination and testing are based on self-report and could be influenced by participants’ desire to respond in a socially desirable way. Nonetheless, participants reported high levels of sharing of injection equipment. If socially desirable reporting was a substantial factor, DUIT participants, using the more private reporting methods of ACASI might be expected to report higher levels of risk behaviors than persons interviewed face-to-face,44
which was not observed.
We can only conjecture the effects of the decline we observed on future prevalence rates as the cohort ages. Data from the full Kiwi study population, which is not restricted to persons under 30 years old, indicate that among persons who have injected 6–10 years HCV seroprevalence was 57% and among those injecting 11–20 years the figure was 79%. Thus, even among persons injecting for longer periods of time, HCV infection was not universal. It seems reasonable to predict lower prevalence rates than these as the members of the present study population age. Even with widespread eventual infection, a delay in seroconversion offers a window of opportunity for future developments in treatment and vaccination to reduce morbidity and mortality among IDUs.
Because the three viruses differ in the relative importance of sexual and parenteral modes of transmission, our results suggest that multiple factors influenced the observed trends. The increases reported in preventive measures likely contributed to our findings for the three viruses: needle-exchange use for all three viruses, condom use for HIV and HBV, and vaccination for HBV. Although our data do not allow us to identify with certainty any specific programs that are responsible for the declines, they do suggest that HCV and HBV are amenable to control by public health efforts. Despite declines, the prevalence of HCV and HBV infection remains substantial among Seattle IDUs and sharing of injection equipment is widespread. There is ample opportunity for improvement preventive measures to further reduce risk behaviors and increase hepatitis B vaccine coverage.