A review commissioned by the World Health Organization estimates that 2.3 million people worldwide living with HIV are co-infected with hepatitis C virus.

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The study shows people infected with HIV are at much higher risk of also becoming infected with the hepatitis C virus, particularly if they use injected drugs.

More than half of this co-infected population includes 1.3 million people who inject drugs (PWID), say researchers from the University of Bristol and the London School of Hygiene & Tropical Medicine - both in the UK - who report their work in The Lancet Infectious Diseases.

The study - commissioned by the World Health Organization (WHO) - systematically reviews and analyzes nearly 800 medical studies from around the world.

The analysis reveals that people living with HIV are on average six times more likely to be co-infected with hepatitis C (HCV), compared with people who are not HIV (human immunodeficiency virus) infected.

The authors say this shows there is a need to improve integrated HIV/HCV health services. Coauthor Dr. Philippa Easterbrook, of WHO's Global Hepatitis Programme, says:

"The study shows that not only are people with HIV at much higher risk of HCV infection, groups such as people who inject drugs have extremely high prevalence of HCV infection - over 80%."

For the review, the team searched several databases for studies published during 2002-2015, measuring prevalence of HCV and HIV. They searched for studies estimating HCV co-infection in the main HIV population, as well as sub-groups of PWID, heterosexually exposed and pregnant women, men who have sex with men, other high-risk groups and the general population.

To qualify for the analysis, each study had to have at least 50 participants and meet other inclusion criteria - for instance, self-reported HIV infection status did not count, and studies where infected individuals also had other diseases, or were receiving treatments that put them at risk of infection, were excluded.

The authors also included studies that measured exposure to HCV by presence of antibodies, as testing for active infection costs a lot more and not many studies used it. Around 20-30% of people exposed to HCV and found positive with antibody will clear the virus, they note.

Greatest burden is in eastern Europe and central Asia

Of nearly 32,000 studies the researchers identified, 783 studies met their inclusion criteria, resulting in 902 estimates of the prevalence of HIV/HCV co-infection.

From these, they estimated that 2.4% of the general global population is infected with both HIV and HCV.

The researchers also estimated HIV/HCV co-infection rates in the other populations: 4.0% in the pregnant or heterosexually exposed; 6.4% in men who have sex with men; and 82.4% in PWID.

The analysis shows that the greatest burden of HIV/HCV co-infection (27% of global burden, or over 600,000 cases) is in eastern Europe and central Asia, particularly among PWID.

Sub-Saharan Africa accounts for 19% (nearly 430,000 cases) of the global burden of HIV/HCV co-infection, due to high numbers of people living with HIV.

WHO commissioned the study to inform their guidelines on screening for HIV/HCV co-infection and on starting antiretroviral therapy (ART). The findings will also inform regional and national strategies for HCV screening and management, says the United Nations (UN) health agency.

There are 37 million people around the world living with HIV, and around 115 million people living with chronic HCV infection. But before this study - the first to look at this area on a global scale - we knew little about the extent of global HIV/HCV co-infection.

The authors also note there is a need to improve surveillance of HCV and HIV, especially in countries with growing numbers of PWID and also in sub-Saharan Africa, where rates of co-infection are high due to high burden of HIV.

Senior author Peter Vickerman, a professor in Bristol's School of Social and Community Medicine, says their figures show how injecting drug use is driving up HIV/HCV co-infection rates, especially in eastern Europe and central Asia, and adds:

"It also shows the need to scale up prevention interventions, such as needle and syringe programs and opioid substitution therapy, as well as access to HIV and HCV treatment, to reduce morbidity and new infections."

Nine countries in sub-Saharan Africa account for over three quarters of their continent's burden of HIV/AIDS. From a recently published modeling study, Medical News Today learned there is a significant shortfall in the funding these countries will need to control the disease in the years to come.