Infection with HIV and hepatitis C virus among injecting drug users in a prevention setting: retrospective cohort study

Incidence of HIV-1

During the study period, HIV-1 antibody testing was carried out on 1179 clients who gave a history of injecting drug use. The overall prevalence of infection was 2.5%.

Among 426 injecting drug users initially negative for HIV-1 who underwent repeat testing, two seroconverted, giving an incidence of 0.17 per 100 person years (95% confidence interval 0.09 to 0.44). Median testing interval (time between first negative result and last negative or first positive result) was 12.5 (range 1-44) months. The number of HIV-1 antibody tests per client retested during the study period ranged from 2 to 11 (mean 3.3).

Incidence of hepatitis C virus

During the same period hepatitis C virus antibody testing was carried out on 1078 clients. The overall prevalence of infection was 45%.

Sixteen of the 1078 clients had indeterminate results and were excluded from further analysis. Among the 572 injecting drug users initially negative for hepatitis C virus antibody, 152 (27%) underwent repeat testing. Table 1 compares demographic and behavioural characteristics of those who had repeated tests (n=152) and those who had a single test (n=420). Those who had repeat tests were significantly younger and less likely to have been imprisoned, but there were no other significant differences between these populations. In particular, duration and type of drug use and history of needle sharing were similar between single and repeat testers.

Among the 152 injecting drug users who were initially negative for hepatitis C virus antibody and were retested, 31 were documented as having newly acquired infection during the study period, giving an incidence of 20.9 per 100 person years (13.5 to 28.3). For these 152 subjects the median follow up was 11.7 (range 1 to 42) months. The number of tests per client retested ranged from two to seven (mean 2.6). Median follow up among hepatitis C virus incident cases was 13.3 months compared with 11.2 months for other repeat testers.

The incidence of hepatitis C virus was higher among younger injecting drug users, with a rate of 75.6/100 person years for those under 20 years compared with 14.7/100 person years for those aged 20-29 years and 6.6/100 person years among injecting drug users 30 years or older (table 2). Other factors influencing incidence were type of drug used and needle sharing, with a higher rate among those who used more than one type of drug than those who used a single drug type and among those who reported needle sharing during the study period. Nine injecting drug users who reported that they had not shared needles were found to have acquired hepatitis C virus infection during the study period.

Hepatitis C virus incidence was also substantially higher among injecting drug users who had been imprisoned (60.8/100 person years) than those who had not (12.5/100 person years). Sex, sexual preference, duration of drug use, and a history of methadone treatment did not significantly influence incidence. Hepatitis C virus incidence was similar in the two halves of the study period (18.9/100 person years for February 1992 to December 1993; 22.5/100 person years for January 1994 to October 1995).

There were no new hepatitis C virus infections among the four injecting drug users positive for HIV-1 who were negative for antibody to hepatitis C virus on initial testing and underwent repeat testing.

In the proportional hazards regression analyses, independent predictors of hepatitis C virus seroconversion were age less than 20 years and a history of imprisonment (table 3. A history of needle sharing was associated with an increase in risk that was of borderline significance (P=0.06). Type of drug use was no longer significant in this adjusted model.

Limitations of study

Several methodological issues need to be considered in the interpretation of these results. Although this study is the largest of incident hepatitis C virus cases reported in the literature to date, only 31 incident cases were observed, limiting the power of the analyses and the extent to which confounding can be excluded as an explanation of the observed associations. Despite this, our study had about 70% power (0.05% level) to detect a halving in incidence over the study period.

The high incidence among subjects recruited in this clinical setting may not be representative of hepatitis C virus incidence among injecting drug users more generally. As the centre provides HIV and hepatitis C virus prevention services, its clients may differ from injecting drug users who do not attend the centre. On the other hand, the centre's location in Kings Cross, a focus for the sex industry and drug trade in Australia, may result in the recruitment of injecting drug users at higher risk. Among injecting drug users who attended the centre, those who underwent repeat testing may not have been representative. Decisions by clients to undergo repeat HIV or hepatitis C virus testing may also have been influenced by perceptions of both client and clinician of the client's risk behaviour. Those who underwent single or repeat tests for hepatitis C virus differed significantly in age and history of imprisonment from those who did not. The younger age of repeat testers may have biased upwards the overall estimate of hepatitis C virus incidence but should not affect the relation between age and risk of newly acquired hepatitis C virus infection, an extremely strong association in our study. In contrast, the lower proportion with a history of imprisonment among repeat testers could have biased downwards the overall estimate of hepatitis C virus incidence—but if all those who had an initial test had undergone repeat testing the association between imprisonment and newly acquired hepatitis C virus infection should have persisted.

Another methodological limitation is the dependence on self reports for information on injecting practice, particularly in a clinical context where the client may perceive an incentive to underreport risk behaviour to the attending clinician. Misclassification of variables has the effect of reducing the apparent size of any association, provided the extent of misclassification was not associated with hepatitis C virus status or the true values of the variables. There is no way to assess this issue on the basis of available data.

Incidence of hepatitis C

The extremely high incidence of hepatitis C virus among subjects under 20 years (76 per 100 person years) is a major public health concern. Younger injecting drug users could be at relatively higher risk of seroconversion compared with seronegative older injecting drug users if seronegativity becomes a marker of well established prevention practices with increasing duration of injection and age. Thus, increasingly lower risk cohorts of injecting drug users may be established with increasing age as those at higher risk are removed because of hepatitis C virus seroconversion. The 84% of injecting drug users under 20 years old who were negative for hepatitis C virus on initial testing presumably contains a higher proportion of high risk injecting drug users than the 26% of injecting drug users over 30 years old who were initially negative for hepatitis C virus. While prevalence studies indicate the cumulative impact of hepatitis C virus infection, only incidence studies can identify where infection is currently occurring and allow the direct assessment of the effectiveness of current prevention strategies.

The high incidence of hepatitis C virus coincident with low rates of HIV infection in the population may be due to two factors. Firstly, there is a high risk of hepatitis C virus transmission even for injecting drug users who only rarely share injecting equipment, simply because of the larger pool of hepatitis C virus infection compared with HIV infection among injecting drug users in Australia. 1 3 5 25 A second factor may be the apparent higher infectivity or transmissibility, or both, of hepatitis C virus compared with HIV per episode of blood contact.30

While there was a higher incidence of hepatitis C virus infection among injecting drug users who reported a history of sharing injecting equipment than those who did not, there were nevertheless cases of transmission in the latter group. Underreporting of risk behaviour may explain this finding, but also hepatitis C virus may be transmitted among injecting drug users in ways that do not involve the reuse of injecting equipment—for example, the sharing of items such as razors and toothbrushes, which may draw blood.

An important finding from the study was the strong relation between a history of imprisonment and the incidence of hepatitis C virus. We could not determine on the basis of available data whether the period of imprisonment was between the last negative and first positive test result in subjects who acquired hepatitis C virus infection. The observed association may be due to risk behaviour in prison or a consequence of an association between history of imprisonment and chaotic lifestyle, which may in turn be a surrogate marker of injecting risk behaviour. In either case, the association observed in this study population deserves further investigation, specifically to assess whether preventing the spread of hepatitis C virus should be better dealt with in the prison setting.

A recently published prospective cohort study among injecting drug users in another Australian state revealed a decline in incidence of hepatitis C virus, albeit non-significant, of 16.6 to 8.1/100 person years over the period 1990-5.31 The absence of a similar decline in this study among an inner city population of injecting drug users already attending an HIV prevention service strongly suggests that current efforts aimed at the prevention of bloodborne viral transmission are inadequate to stem hepatitis C virus infection. The vulnerability of this population to hepatitis C virus may ultimately indicate the potential for HIV transmission.