Some needle sticks worse than others.

On average, only about 3 people in 1,000 who are exposed to HIV by being stuck with a contaminated needle or similar sharp object become infected with the virus, research has shown.

Now, a study of health care workers in four countries reveals that deep punctures or cuts, combined with infected blood containing high concentrations of the virus, increase the likelihood that HIV will spread in such accidents, researchers report in the Nov. 20 New England Journal of Medicine.

The nature of the contaminant proved significant. Risk of infection was highest when a worker was jabbed with an instrument carrying blood in visible quantities or received undiluted blood from a vein or artery of an infected person.

Researchers in France, Italy, the United Kingdom, and the United States collected information from 33 people who became HIV-positive through such accidents--30 of which involved needles--and 679 people who didn't. All but six people in the infected group contracted HIV from the highly virulent blood of AIDS patients who died less than 2 months later.

The data were collected between 1988 and 1994 as part of the Needlestick Study of the federal Centers for Disease Control and Prevention (CDC) in Atlanta. Although taking an immediate course of AZT, or zidovudine, seems to lower AIDS risk, only 9 of the 33 infected workers received the drug. Today, CDC recommends an immediate course of AZT and two other drugs for anyone stuck by a needle that may carry HIV, says CDC medical epidemiologist and study coauthor Denise M. Cardo.

In the United States, of a total of 52 health care workers known to have contracted HIV from patients, 46 were infected via cuts or punctures, Cardo says. Although fewer than 1 percent of such accidents result In HIV infection, Cardo says, the new findings show that specific characteristics of the transmission can intensify the risk.