AIDS TREATMENT UPDATE, Issue 28 - April 1995
Keith Alcorn

At January's Second National Conference on Human Retroviruses in Washington, Dr Patricia Fultz of the University of Alabama revealed that she had been able to infect chimps first with one sub-type of HIV and then later with another. Within two weeks, the second sub-type had become the dominant (most common) sub-type in the animals' bodies. The revelation was met with stunned silence by researchers, who had been discussing whether a vaccine could provide protection against all sub-types of HIV. If this revelation is borne out by future study, it suggests two things:

- that re-infection does happen - that some forms of vaccine may be useless.

The conference also heard that so-called 'recombinant' strains of HIV have been identified in Brazil, Africa and India, countries where several different sub-types of HIV can be found in the population. Sub-types are genetically distinct families of HIV which diverged from a common ancestor many years ago. Recombination occurs when two different viruses combine their genetic material to produce a new hybrid. If recombinant strains are emerging, says Dr Fultz, "it means that re-infection is probably taking place, although at the moment it seems very rare".

Dr Fultz warned that the same problem could also emerge in countries where only one sub-type of HIV dominates, such as the US and Europe. This is because even within a single sub-type there are different virus strains, which become increasingly divergent the longer a virus is present in a population. If people are repeatedly infected with these differing HIV strains, the viruses could recombine and result in entirely new strains.

Virtually all HIV vaccines currently in development are based on sub-type B virus, which is the dominant strain in North America, Europe and amongst drug users in Thailand. If vaccines only work against a single sub-type, new ones would be required for each different sub-type. This problem already occurs with influenza, a highly variable virus which requires the concoction of new 'flu vaccines every year. But Professor Jonathan Weber of St Mary's Hospital in London disputes this view, saying that whilst the sub-typing of HIV is fascinating in evolutionary terms "we don't see any biological significance in these variations. There is no evidence that these genetic sub-types are relevant to vaccine development".

IS RE-INFECTION DANGEROUS?

If re-infection can take place, the question remains whether or not it can speed up disease progression. The answer partly depends on whether the strain of HIV with which an individual is infected plays a significant role in determining the rate of disease progression. There was no consensus on this issue at the conference. However, it's known that on average people infected with HIV-2 experience much slower disease progression than people with HIV-1, suggesting that viral variation can have an important impact on disease progression. It may be that some variation in the virus makes HIV-2 less harmful than HIV-1, or that the immune system can control HIV-2 better.

Two years ago AIDS Treatment Update reported that a survey of the US MACS cohort, a group of gay men with HIV who have been tracked for the past ten years, found that men who practised unprotected receptive anal sex after they became infected with HIV were at increased risk of developing AIDS within five years. At the Washington conference, the same researchers presented findings which show that the men with the most rapid rate of fall in their CD4 cell count tended to be those who had unprotected receptive anal sex to ejaculation with a high number of partners _ ten or more in the previous year. In other words, getting fucked without a condom by many different partners was associated with the most rapid disease progression, so safer sex may be advisable even between HIV-positive people.

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