Why is injecting drug use a risk for HIV transmission?

At the start of every injection, blood is introduced into the needle and syringe. Therefore, a needle and syringe that an HIV-positive person has used can contain blood with the virus in it. Transmission occurs when another person then uses the same syringe without cleaning it. The reuse of a blood-contaminated needle or syringe by another person can be an effective means of transmission because a large quantity of blood can be injected directly into the bloodstream.

Although HIV does not generally survive well outside the body, it can survive for long periods of time (over 28 days) if hermetically sealed in syringe.^{1 2}

There is a risk of HIV infection through intravenous injecting, subcutaneous injecting (injecting into the fat under the skin) and intramuscular injection. Some injecting drug users may believe, wrongly, that they are not at risk of HIV transmission if they simply avoid intravenous injecting.

But HIV infection will not automatically occur from a single incident of shared needle/syringe use. In fact, estimates of the infection risk from an injection range from 0.63 to 2.4%.³

Two factors are likely to determine the chances of HIV infection from any single incident of shared needle/syringe use:

• The level of HIV present in the blood injected. Very low levels of circulating virus in the blood may make HIV infection less likely. But assuming a fairly high viral load, it is thought that amounts of blood invisible to the naked eye may be sufficient to permit HIV infection.

• The quantity of blood injected. Evidence from the follow-up of needlestick injuries and other occupational injuries involving blood shows that the likelihood of HIV infection is dose-related – the more blood injected, the more likely it is that seroconversion will take place.

But HIV infection from blood can occur in other ways as a result of injecting drug use:

• Through sharing water used to flush blood out of a needle and syringe.
• Through sharing syringes or 'works' that have been cleaned in a way that does not eliminate all the blood they contain.
• Through reusing bottle caps, spoons or other containers used to dissolve drugs in water and to heat drug solutions.
• Through reusing filters – often small pieces of cotton or cigarette filters – used to filter out particles that could block the needle.
• Theoretically, through unsafe disposal of needles or syringes used for injecting drugs, leading to accidents in which blood gets into the body of another person.