No superinfections among HIV-positive gay men in Amsterdam reporting risky sex

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HIV superinfection appears to be extremely rare, a Dutch study published in the June 1st edition of the Journal of Infectious Diseases suggests.

The study is one of the few attempts to use ongoing virological monitoring and behavioural data to establish the likely incidence of superinfection, albeit in a small cohort, and points to the neglect of a question that remains important for giving advice on HIV prevention strategies that might encourage the practice of serosorting - sex with partners of the same HIV status.

Researchers monitored 15 HIV-positive gay men for evidence of superinfection for an average of almost six years. All the men either reported unprotected anal sex, or had a history of sexually transmitted infections. Despite this risk, no cases of superinfection were detected.



When somebody already infected with HIV is exposed to a different strain of HIV and becomes infected with it in addition to their existing virus.


person years

In a study “100 person years of follow-up” could mean that information was collected on 100 people for one year, or on 50 people for two years each, or on ten people over ten years. In practice, each person’s duration of follow-up is likely to be different.

phylogenetic analysis

The comparison of the genetic sequence of the virus in different individuals in order to determine the likelihood that two or more samples are related. This involves creating a hypothetical diagram (known as a phylogenetic tree) that estimates how closely related the samples of HIV taken from different individuals are. Phylogenetic analysis is not a reliable way to prove that one individual has infected another, but may identify transmission clusters, which can be useful for public health interventions.


A variant characterised by a specific genotype.


longitudinal study

A study in which information is collected on people over several weeks, months or years. People may be followed forward in time (a prospective study), or information may be collected on past events (a retrospective study).

“With no putative case of HIV-1 superinfection detected in 15 individuals over a total of 88.3 PY [person years], we observed a low incidence rate of HIV-1 superinfection (incidence rate: 0 per 100 PY, 95% CI: 0- -4.2),” write the investigators.

However, they do not regard their results as definitive and call for further research into this matter. In particular, they speculate that the level of risk of the men in their study may not have been high enough to lead to superinfection.

Intensive case finding has identified approximately 50 cases of confirmed HIV superinfection (infection with a second strain of the virus).

Some investigators have speculated that the phenomenon may be much more common than this figure suggests.

To try and establish a better understanding of this controversial subject, investigators from the Netherlands studied blood samples obtained from 15 HIV-positive gay men recruited to the Amsterdam Cohort Study between 1986 and 1997.

Blood samples were obtained from these men every three months, and at six-monthly intervals they were interviewed about their sexual behaviour. All the men reported unprotected anal sex with at least two partners in one or more six-month period, and/or infection with syphilis or gonorrhoea.

Phylogenetic analysis was used to see if any of the men had been superinfected with another strain of HIV.

Of note, the study was conducted using samples obtained in the era before effective antiretroviral therapy became available. Superinfection among individuals treated with HIV therapy appears to be extremely rare.

A total of five individuals reported four six-month periods of risk; five patients reported three periods of risk; and the remaining five individuals reported one or two risk periods.

Overall, the men contributed over 88 person years of follow-up, an average of 5.8 years per patient.

Intensive phylogenetic analysis failed to detect a single case of superinfection.

“While we had expected to increase the odds of detecting HIV-1 superinfection in this study by studying longitudinal samples covering time periods of unsafe sexual risk behaviour, no cases of superinfection were detected,” write the authors.

However, they note that they may not have been able to detect minority populations of superinfecting virus, or transient superinfection.

Even though the patients enrolled in their study reported risky sex, the investigators suggest that this may not have been at a sufficient level to lead to superinfection. They note that HIV-positive African sex workers who were superinfected reported “a 10-fold to 30-fold higher number of sexual partners than homosexual men in our cohort.”

The investigators conclude that their analysis “resulted in absent detection of HIV-1 superinfection and hence a low incidence of HIV-1 superinfection in this study may point to external risk and/or host factors involved in acquisition of HIV-1 superinfection.”

They suggest that “additional longitudinal studies are needed to estimate the impact of each factor that may increase the risk for establish of HIV-superinfection.”

However, conducting such studies may be difficult in settings where men could be criminalised either for HIV transmission or for non-disclosure of HIV status. Men may be disinclined to provide frank information about their sexual behaviour in such settings, even where the object of inquiry is sex with partners of the same HIV status.


Rachinger A et al. Low incidence of HIV-1 superinfection even after episodes of unsafe sexual behavior of homosexual men in the Amsterdam Cohort Studies on HIV Infection and AIDS. J Infect Dis 203: 1621-28, 2011 (click here for the free abstract).