Treatment Information Officer, John Cumming*, looks at the superinfection panic and says, 'relax'
Several cases of hiv superinfection presented at international conferences this year have generated more confusion than answers. Many people remain unclear on the meaning of the term '"superinfection'". Is that the same thing as re-infection? How often does it happen? To begin to address these questions, it's useful to look at the genealogy of hiv, or to put it another way, its evolutionary tree.
Worldwide, there are two types of hiv: HIV-1 and HIV-2. HIV-2 is similar to HIV-1, but is mostly restricted to the countries of West Africa. Worldwide, the predominant virus is HIV-1. Two major groups of HIV-1 exist, '"M'" and '"O'". The M group contains the virus that causes the great majority of HIV-1 infections. Contained in the M group are genetic variations of HIV-1 called subtypes, also known as clades. Each subtype is subject to mutations, which can cause different strains of subtypes. There are at least 9 nine of these subtypes, most of which can be found in Africa. Elsewhere in the world, single M subtypes predominate in particular regions. In Australia, Europe and the US, the predominant subtype is B. The standard viral load tests used in Australia and other developed countries are geared specifically to detect and measure the B subtype.
Superinfection confirmed
The possibility of superinfection (what used to be called re-infection) has long been debated. One school of thought suggested that the immune response produced after hiv infection would prevent repeated hiv infection. However it now seems that the initial immune response to hiv infection is specific only to the subtype of the original infection, and not necessarily to infection from a different subtype. The first confirmed case of superinfection was reported in February 2002. A 38 year old Swiss man diagnosed in 1998 with hiv subtype A, started treatment soon after infection and quickly achieved an undetectable viral load. In January 2001 he stopped treatment and three months later developed mild fatigue, fever and a viral load of more than 400,000. Investigation revealed he had unprotected sex in March 2001 on a holiday to Brazil, and as a result had been infected with a different type of hiv, subtype B.
A similar case was reported in July 2002 at the International AIDS Conference in Barcelona. A man infected with subtype B had participated in a treatment interruption study and, after stopping his third cycle of treatment, was maintaining an undetectable viral load. When his viral load reappeared, researchers investigated and found that an unprotected sexual encounter had exposed him to a different strain of the B subtype. Although this subtype was only 12% genetically different from the original subtype, the existing immune response that was controlling his initial hiv infection was less able to recognise the new virus, leading to uncontrolled viral replication.
No hiv transmission = no superinfection
In the cases reported so far, none were on hiv medication when they were superinfected. Does this mean that hiv medication can prevent superinfection, in the same way that it can prevent mother-to-child transmission? We don't know. We do know that a low or undetectable viral load lowers the likelihood of hiv transmission. A recent study of 415 heterosexual hiv discordant couples in Uganda demonstrated that when the viral load in the hiv positive partner fell below 1,500 copies, no transmission occurred. However it would be wrong to assume from this study that someone with a low or undetectable viral load is never infectious.
For instance, a sexually transmitted infection can cause a dramatic increase in hiv levels in the genital tract, leaving blood viral load unchanged. More information on superinfection will emerge as research continues. Although superinfection appears to be rare, it may be another issue for hiv positive people to consider when thinking about sexual practices and health.
*John Cumming was a Research and Policy Officer at People Living With HIV/AIDS New South Wales. He wrote this article as Treatment Information Officer at ACON.