I recently heard a report that a woman from Africa in the United States developed Aids, but tested negative on the usual antibody test. Subsequently, other tests revealedthat she had a variant strain of HIV known as Type O. If many individuals can go several yearswithout symptoms, what are the chances that the Type O variant is spreading without detection in this country? Do blood banks screen for the Type O strain?

There are multiple subtypes of HIV-1, which are designated A through H, M, and O, based on genetic differences. The subtypes differ geographically, and these differences may explain some of the differences in the efficiency of transmission (and thus, epidemiological differences) worldwide. In North America and Europe, most infection is with subtype B. In South America and Asia it is B and F; in Africa A-H. The routine HIV blood test will detect A-H, but does not detect subtype O, which is found primarily in West Africa, especially Cameroon. Until recently there had been no reported cases of subtype O infection in the United States, but as you point out, the first such case (from an African woman residing here) was recently reported. To date, blood banks have not been screening for this subtype. I'm sure that this recent case will cause them to reexamine this policy, but I cannot tell you what will ultimately be decided.