An estimated 180 million people worldwide are infected with the hepatitis C virus (HCV).
The burden of disease attributable to HCV is huge (it affects over 300,000 Australians and is the major indication for liver transplantation) and will continue to increase for decades.
In Australia and the rest of the developed world, the vast majority of HCV infections occur in people who inject drugs (IDU).
There are an estimated 10,000 new HCV infections annually in Australia, 86% thought to be due to injecting drug use.
Prevention of HCV transmission is vital to halt an ever-increasing burden of HCV-attributable disease: to do so, we must understand the drivers of transmission and the associated behavioural or immunological factors.
Although some individual-level risk factors for HCV are well known (such as sharing used needles), the population-level phenomena driving HCV transmission are not.
We have already shown that HCV risk is dependent on the HCV status of the people in each IDU’s injecting network – more so than on injecting partner numbers or needle-sharing behaviour – and further investigation is underway.
To date, over 400 people have contributed blood samples and detailed behavioural and social network data to the Networks study, and over 240 have had two or more encounters (some up to 15) during the four years of data collection.
This longitudinal design is a major strength, because infections can be observed as they occur and their causes become easier to pinpoint.
A major outcome of the study so far is the finding that people who had cleared an HCV infection were more likely to acquire a second virus than people who had never been infected – suggesting a reduced protective effect from viral clearance, the opposite of what occurs with most viral diseases.
Repeated collection of large blood samples from IDUs has generated a unique dataset for immunological and virological HCV research, and this resource has enabled the development of collaborations with local and international scientists.
Some IDUs are never infected with HCV despite years of risky injecting behaviour with HCV-infected people, suggesting some individual possess innate resistance or develop protective immunity which contributes to their ability to remain HCV free.
Our partners and collaborators are studying these protective cellular responses, with early results showing promise for future vaccine development.