Unstable housing among people who inject drugs may be responsible for up to one in five new infections with HIV or hepatitis C in the United Kingdom and the United States, a global modelling study has found.
The modelling exercise, published in the journal Lancet Public Health, was carried out by a team from the University of Bristol.
A systematic review and meta-analysis of 37 studies published in 2021 showed that unstable housing increased the risk of hepatitis C infection by 64% and HIV infection by 39% in people who inject drugs.
The study investigators say that unstable housing – homelessness or lack of a fixed home – is likely to increase the risk of acquiring HIV or hepatitis C in several ways. People with unstable housing may prioritise other survival needs over measures to avoid HIV or hepatitis C, or may lack places to store sterile injecting equipment. They may also face barriers in accessing medical services. Studies have also shown that people with unstable housing are more likely to be involved in sex work and to have a greater number of injecting partners.
Following the findings of the systematic review, the University of Bristol team developed a model to assess what proportion of new HIV and hepatitis C infections globally are attributable to unstable housing – the population attributable fraction – and to identify the countries in which unstable housing in people who inject drugs has the greatest impact on new HIV and hepatitis C infections.
The model of transmission was adjusted to take into account the uptake of antiretroviral therapy in various countries as well as the average duration of unstable housing and injecting drug use. The rate of transmission depended on HIV or hepatitis C virus (HCV) prevalence in people who inject drugs by country or region. It was adjusted for the rates of transmission estimated in people with unstable housing, derived from the 2021 meta-analysis.
The model projected estimates for 56 countries with sufficient data on HCV and HIV prevalence as well as the housing status of people who inject drugs.
Estimates for HIV transmission were available for 50 countries. Overall, unstable housing will account for 7.9% of new HIV infections in people who inject drugs between 2020 and 2029.
Estimates for HCV transmission were available for 49 countries. Overall, unstable housing will account for 11.2% of new hepatitis C infections in people who inject drugs between 2020 and 2029.
The impact of unstable housing on HIV and hepatitis C transmission is greatest in high-income countries. Unstable housing will account for 21% of new HIV infections and 26% of new hepatitis C infections between 2020 and 2029 in North America. In comparison, unstable housing will account for 6% of new HIV infections and 8% of new hepatitis C infections in lower- and middle-income countries.
However, a small number of countries contribute disproportionately to the global total. Six countries – Afghanistan, the Czech Republic, England, India, the United States and Wales - will account for 29% of global HIV infections and 44% of global hepatitis C infections attributable to unstable housing in people who inject drugs. In each country, unstable housing will contribute more than 20% of new HIV and hepatitis C infections in people who inject drugs between 2020 and 2029.
"More than 40% of people who inject drugs are estimated to be unstably housed in the Czech Republic, England and the United States."
Those countries contributing most to the global burden of new infections in the unstably housed have a high prevalence of unstable housing in people who inject drugs. More than 40% of people who inject drugs are estimated to be unstably housed in the Czech Republic, England and the United States compared to Russia (4%) or China (8%). The study authors say that social protection may explain this difference and may therefore prove “an important determinant of the contribution of unstable housing to HIV and hepatitis C transmission among people who inject drugs.”
“With the UNAIDS beginning to incorporate social enablers into their target setting, including for key populations, it is essential to strive for improved access to stable housing for people who inject drugs,” they advocate. “HIV and HCV elimination targets will be missed unless the effect of these structural drivers [is] mitigated.”
However, the study authors say it is unlikely stable housing alone will be sufficient to eliminate the excess risk observed in this study. “Interventions must go beyond simply providing stable housing by addressing individuals’ broader health and social needs and providing access to prevention and treatment services.”
Stone J et al. The contribution of unstable housing to HIV and hepatitis C virus transmission among people who inject drugs globally, regionally, and at country level: a modelling study. Lancet Public Health, published online 7 January 2022.