Despite a national program to test all donated blood for the presence of the AIDS virus, there is still a small risk -- estimated at about one in 40,000 -- of becoming infected from transfusions, according to a new study.

Centers for Disease Control researchers who conducted the study said that current tests for the AIDS virus often do not detect its presence if the infection is very recent and called for greater efforts to dissuade persons at risk of infection from donating blood. The study was published in today's New England Journal of Medicine.

The report estimated that, nationally, as many as 460 people may have become infected each year from contaminated units that tested negative since routine testing of donated blood began in March 1985. About 60 percent of these infected individuals probably would have died from the illnesses that necessitated their transfusions.

In contrast, the study said that 7,200 people are estimated to have become infected through transfusions in 1984, before routine testing began.

The risk estimate, which researchers called a "worst case" figure, was based on a Centers for Disease Control study of 13 people who contracted the virus, known as human immunodeficiency virus or HIV, from blood that tested negative in routine screening.

The infected units were traced to seven donors, all of whom later tested positive for infection with the AIDS virus. Six of the seven had behavioral risk factors for infection, such as a history of homosexual activity or sexual contact with a drug addict.

The contaminated units of blood apparently tested negative because each donor had become infected only a few weeks earlier, and had not yet developed antibodies to the virus, according to the study.

Antibodies are proteins manufactured by cells of the immune system in response to infection. It usually takes between four and 14 weeks for an infected person to develop antibodies, which must be present in blood for the tests routinely used for screening to show a positive result, according to Dr. John E. Ward, a medical epidemiologist with the CDC's AIDS program and the study's principal author.

Transfusion-related cases of AIDS make up 1,272 -- or 2 percent -- of the total of 52,975 adult cases of the disease, according to figures reported to the CDC as of Feb. 15. In children with the disease, transfusion-related cases make up 117 -- or 14 percent -- of the total of 839 cases.

The American Red Cross has been aware of the study's findings for months and has taken steps to improve donor education and reduce the likelihood of a contaminated unit slipping through the screening, according to Dr. S. Gerald Sandler, associate vice president of blood services for the organization.

Sandler said he thinks the risk estimate of one in 40,000 is too high, because the accuracy of tests used to screen blood has improved since the study was completed and because rates of HIV infection in the population are much lower in some areas of the country than in others.

In an editorial accompanying the study, Dr. Thomas F. Zuck of the University of Cincinnati Medical Center noted that a risk of one in 40,000 is less than that of dying in an automobile accident (one in 5,000 annually).

Potential blood donors are given a pamphlet listing behavioral risk factors for HIV infection and are asked not to donate if they have any risk factors, Sandler said. Besides those with AIDS or who have tested positive for HIV, risk categories include:Any man who has had sex with another man since 1977.

Anyone who has taken illegal drugs by needle.

Natives of Haiti or certain central African countries who arrived in the United States since 1977.

Hemophiliacs.

The sex partner since 1977 of anyone in the above groups.

A woman or man who has been a prostitute since 1977, or the heterosexual partner of a prostitute within the last six months.