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Undetectable Variants
Jul 27, 1998

Dear Doctor:

In response to a recent question, you mentioned that a new variant of the virus had been identified in India. What was different about this strain? Was it detectable by conventional tests? Also, what, if any, known HIV strains are NOT detectable by conventional tests? Finally, if a strain is "undetectable," how is it identified? Thank you.

Response from Dr. Holodniy

Your first and third question can be answered together. Subtypes of virus are determined by genetic sequencing of the outer envelop and inner gag gene primarily. So in the major group or M type, subtypes A-J have been determined by sequencing. The sequences are then aligned, and a computer program can determine how close or distant the viral strains are from each other. Sometimes these strains can combine together to form a new kind of virus (recombination). Or a strain can evolve from a parent strain. Some of the viruses can be detected by conventional tests, because they do not vary too much in the areas of the virus which are used to make the antibody tests. Others may not be detected because the variance is too great and the conventional test will not detect them. This was the case for the type O (outlier) virus found primarily in Africa. The envelop sequence was sufficiently different, that the antigens in the conventional antibody assay, could not detect antibodies against this type of envelop. A new assay has been developed to detect this new kind of virus. MH



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