Risk of Seroconversion following Occupational Exposure to HIV
The average risk of HIV transmission following accidental percutaneous injury (needlestick) involving an HIV-infected source patient is approximately 0.3%, assuming that no postexposure chemoprophylaxis is given to the health care worker; the risk with a mucous-membrane exposure is approximately 0.09%.[,,,] These risks are significantly lower than the risk of acquiring hepatitis C virus or hepatitis B virus from a similar injury (Figure 1).[,] As of December 2001, the Centers for Disease Control and Prevention (CDC) had reported 57 documented cases in which health care personnel became infected with HIV following an occupational exposure and most of these cases involved a percutaneous injury (Figure 2).[,] In an analysis of the first 56 cases, 49 of these health care workers were exposed to HIV-infected blood, 3 to concentrated HIV in a laboratory, 1 to visibly bloody fluid, and 3 to an unspecified fluid (Figure 3). Following the widespread use of combination antiretroviral therapy for occupational postexposure prophylaxis, reports of occupational HIV transmission have been rare. The CDC estimates that more than 360,000 percutaneous injuries occur each year in United States hospitals, with more than 60% of these involving hollow-bore needles.[,]
Risk Factors Related to the Exposure that Correlate with Seroconversion
In 1997, the CDC Needlestick Surveillance Group published findings from their retrospective, case-controlled investigation of factors that influence the risk of HIV transmission among health care workers who sustain a needlestick injury (Figure 4). In this study, the investigators compared 33 cases (health care workers who contracted HIV following an occupational percutaneous needlestick injury) with 665 controls (health care personnel who failed to contract HIV despite suffering a similar percutaneous injury involving an HIV-infected source patient). Multiple factors related to the percutaneous exposure correlated with the risk of HIV seroconversion, all of which pertained to the amount of virus contained in the inoculum. High-risk injuries included deep injury, visible blood on the needle or device immediately prior to the injury, and needles that had been used in an artery or vein of the source patient. Injuries involving large-diameter hollow-bore needles probably enhance the risk of HIV transmission when compared with solid-bore needlestick injuries, but this association did not reach statistical significance in a multivariate analysis (p = 0.08). Indeed, to the best of our knowledge, there have been no well documented infections as a result of needlesticks with solid-bore needles.[,] Increased risk of HIV transmission occurred among cases where the source patient had end-stage AIDS, which likely reflected high HIV RNA levels (routine testing for HIV RNA was not done at the time the study was performed). Although absence of glove use by the health care worker was not documented as a risk factor by the CDC Needlestick Surveillance Group, a study of simulated needlestick injuries using an animal model has shown that glove use reduces the volume of blood transmitted to the underlying skin by approximately 50%. Given that the risk of transmission appears to increase with higher volumes of blood, any measure that decreases the volume of blood, such as wearing gloves, would likely reduce this risk. Laboratory experiments suggest that HIV infectivity decreases approximately 10-fold every 9 hours when it is exposed to drying conditions. Other experiments, however, suggest that in a cool, humid climate, viable HIV may persist for up to 6 weeks within a syringe. Hence, dried blood at the tip of a discarded needle probably carries a very low risk of HIV transmission, but the liquid contents within a needle could contain infectious HIV for weeks.
Transmission Reduction with Zidovudine Postexposure Prophylaxis
In the CDC Needlestick Surveillance Group study many of the health care workers among both cases and controls had used zidovudine (Retrovir) for postexposure prophylaxis, allowing for an estimate of the efficacy of this intervention. Most of the health care workers who took zidovudine received their first dose within 4 hours and most took at least 1000 mg/day. The investigators found that zidovudine reduced the risk of HIV transmission by 81%. The study has several limitations, including the small sample size of HIV transmission cases, the use of cases and controls drawn from different populations, and the biases that can occur with a retrospective, non-randomized trial. Nevertheless, if zidovudine had provided no protective effect, one might have expected a higher proportion of zidovudine recipients to have become infected compared with those who did not receive zidovudine, since at the time of this study postexposure prophylaxis was not routinely given and zidovudine use might have served as a marker for higher-risk injuries. Following this publication, the CDC did initiated a prospective, randomized, placebo-controlled trial to examine the efficacy of zidovudine postexposure prophylaxis, but investigators discontinued the trial because of insufficient enrollment.
Defining Health Care Personnel and Occupational Exposure
The definitions for health-care personnel and occupational exposures are the same in 2013 U.S. Public Health Service guidelines for the management of occupational exposures to HIV as outlined in the 2001 and 2005 guidelines.[,,] The guidelines define health care personnel as persons who have activities that place them in contact with patients or with blood or other body fluid from patients. Exposures defined as potentially placing a health care workder at risk are percutaneous injury, or contact of either mucous membrane or non-intact skin with blood, tissue, or other potentially infectious body fluids. The guidelines categorize the risk of transmission with different body fluids into one of three categories: (1) potentially infectious fluids with know risk, (2) potentially infectious fluids and unknown risk, and (3) fluids not considered potentially infectious (Figure 5).[,,] Body fluids considered potentially infectous are blood, visibly bloody body fluids, semen, vaginal secretions.