Risk of infection

A number of studies have calculated the risk of HIV infection from any single needlestick injury where HIV-contaminated blood is involved is around 0.32%.¹ Some studies have also calculated the risk of HIV infection on the basis of person-years, and show a similarly low chance of infection.²

This is because the quantity of blood passed on from a needlestick injury is likely to be much smaller than that from an injection of blood when sharing injecting equipment. A 1991 study estimated that the volume of blood likely to be injected as a result of a needlestick injury was approximately one-seventh of the quantity passed on when sharing injecting equipment.³ Although deep injection has been suggested as another factor which increases the likelihood of infection, reports of documented seroconversions fail to show a consistent pattern of type of needlestick injury which leads to HIV infection.

In a CDC study, four factors were associated with increased risk of infection:⁴

• Deep injury.
• Visible blood on the device which caused the injury.
•  Injury with a needle which had been placed in a source patient’s artery or vein.
• Terminal HIV-related illness in a source patient who is not on therapy and has a high viral load.

The Department of Health’s guidance on PEP following needlestick injuries gives an estimate of the average risk for HIV transmission after percutaneous exposure to HIV-infected blood of 3 per 1000 injuries (0.3%), or of 1 per 1000 (0.1%) after mucocutaneous exposure. There is no risk of HIV transmission where intact skin is exposed to HIV-infected blood.⁵

A meta-analysis in 2006⁶ assessed studies of transmission following needlestick injuries. Of 21 studies, 13 gave a transmission risk of 0%. The pooled infectivity estimate was 0.23%, and this was higher when the source of infection had AIDS (0.37%) and lower when studies had explicitly stated that the needlestick injury was the only risk factor (0.13%).

The authors commented that the risk of infection from needlestick injuries depends on factors that influence exposure to a greater volume of blood (e.g. visible blood on the device, procedures involving needle placement directly into a vein or artery, etc.); viral load of the source patient and post-exposure prophylaxis, which affect the 'effective' viral dose received; and the type of injection equipment (syringes with detachable needles retain – and probably transfer – substantially more blood than integral cannula syringes with a permanently attached needle). They also note that laboratory studies have suggested that a greater volume of blood is transferred by deeper injuries and by hollow-bore needles (especially those with larger gauges).