People who have recently become infected with HIV are at increased risk of transmitting HIV to others and may account for up to half of new HIV infections in some populations. Several factors contribute to an increased risk of HIV transmission.
During the first few months of infection (also known as acute HIV infection, or AHI), the amount of HIV (viral load) in the bodily fluids is very high, which can significantly increase the risk of HIV transmission if an exposure occurs. Furthermore, recently infected individuals are more likely to be engaging in transmission-enabling activities as this is what led to their infection with HIV. Finally, since their infection is recent, many are unaware of their HIV-positive status and are consequently less likely to take steps to prevent passing the virus to others.
Early diagnosis of individuals with AHI is important for preventing HIV transmission. Once people living with HIV become aware of their HIV status, the majority take measures to reduce their risk of passing HIV to others. Newly diagnosed individuals can also consider starting treatment, which can improve their health and (by reducing the amount of virus circulating in their body) further reduce their risk of HIV transmission.
Challenges in identifying recent infections
Unfortunately, identifying people with AHI is challenging. Since the timespan of AHI is relatively short, most people pass through this brief stage of infection without getting tested and learning their HIV status. Frequent testing of those at risk is, therefore, important to detect AHI.
Furthermore, all HIV tests have “window periods” (the period of time from when a person first becomes infected with HIV to when a test can detect their infection), which can make it difficult to identify AHI. During the window period, a test may incorrectly find a person with AHI to be HIV-negative.
Newer testing technologies, shorter window periods
Fortunately, newer generations of HIV tests have shorter window periods.
The Western Blot was one of the first tests to be developed and is still used for confirmatory testing in many provincial laboratories. This type of test detects HIV antibodies in the blood and—although very accurate—has a relatively long window period of six to 12 weeks.
Newer types of HIV tests that detect anti-HIV antibodies have shorter window periods. Third-generation HIV tests can identify infection in 95% of people within 34 days of infection. Fourth-generation HIV tests have an even shorter window period, of 15 to 20 days, and detect an HIV protein (known as the p24 antigen) in addition to antibodies.
Another form of testing is nucleic acid amplification testing (NAAT). This type of test, which detects the virus’ genetic material (also known as RNA), has the shortest window period, of seven to 15 days.
However, since NAAT is expensive, a “pooled” strategy is sometimes used to minimize the number of tests performed. With pooled NAAT, blood samples from several different clients are combined and tested as a single sample. If the pooled sample tests positive, the original samples are then tested individually to determine which one was positive.
While third- and fourth-generation HIV tests are used for screening clients in provincial laboratories across Canada, pooled NAAT is not.
British Columbia’s Study of Acute HIV Infection in Gay Men
Researchers in Vancouver recently conducted a study to assess the extent to which a combination of targeted NAAT screening and social marketing campaigns could improve the identification of AHI among men who have sex with men (MSM).
The study took place at five Vancouver-based testing clinics with high rates of HIV infection among MSM clients. Researchers compared the rate of AHI detection at these clinics before and after the implementation of NAAT screening and social marketing campaigns.
Between April 2006 and April 2009 (the pre-implementation period), all clients at these sites were initially screened using a third-generation HIV test. Different actions were taken (also known as a testing algorithm) depending on the result of the test:
- If the initial third-generation test was negative, the client was informed that they were HIV-negative and no additional testing was performed. Therefore, if the person had AHI and was in the third-generation test’s window period, their infection was missed.
- If the initial third-generation test was positive, the result was then confirmed through a Western Blot. If the Western Blot was also positive, the client was informed that they were HIV-positive.
- If the initial third-generation test was positive, but the confirmatory Western Blot was negative or indeterminate, further testing was performed using p24 antigen testing or NAAT. If positive, this indicated the person had AHI and was in the Western Blot’s window period.
Between April 2009 and March 2012 (the post-implementation period), the testing algorithm was changed to determine the extent to which pooled NAAT testing could improve the detection of HIV.
In the new testing algorithm, all male patients (including transgender clients) over 18 who presented for HIV testing at the participating clinics were initially screened using a third-generation HIV test. However, compared to the previous algorithm, new actions were taken if the initial third-generation test was negative:
- If the initial third-generation test was negative, clients were not informed of their status (unlike the previous algorithm). Instead, additional testing was performed using pooled NAAT. If the NAAT was positive, this indicated that the client had AHI and was in the third-generation test’s window period. If the NAAT was negative, the client was informed they were negative.
- If the initial third-generation HIV test was positive, the actions taken were similar to the pre-implementation period.
A sixth clinic adopted this new testing algorithm in September 2009.
In the post-implementation period, two social marketing campaigns were implemented to raise awareness of new testing technologies and AHI among gay men and other MSM. The overall goal of these campaigns was to increase uptake and frequency of HIV testing among those at risk.
Defining acute HIV infection
In the pre-implementation period, AHI cases were defined as men who were in the Western Blot’s window period and whose HIV was detected using third-generation tests.
In the post-implementation period, AHI cases also included men who were in the third-generation test’s window period and would have been missed if pooled NAAT screening was not used.
Between April 2006 and 2009 (pre-implementation period):
- 18,393 men were tested
- 218 were diagnosed with HIV
- 19 of those diagnosed had AHI
- 1.03 AHI diagnoses per 1,000 tests
Between April 2009 and March 2012 (post-implementation period):
- 20,141 men were tested
- 176 were diagnosed with HIV
- 37 of those diagnosed had AHI
- 1.84 AHI diagnoses per 1,000 tests
Implementation of NAAT testing and social marketing campaigns improved the identification of men with AHI. In the post-implementation period, the number of HIV tests, number of AHI diagnoses, and AHI identification rate were all higher than during the pre-implementation period.
If the sixth clinic is included, a total of 54 AHIs were diagnosed in the post-implementation period. Of these 54 infections, 25 were in the window period for the initial third-generation test and would have been missed if not for pooled NAAT screening.
Investigators concluded that the “targeted implementation of pooled NAAT and social marketing has significantly improved detection of AHI among MSM at clinic sites with pre-existing high HIV detection rates.”
In this study, improved identification of AHI was achieved by shortening the window period through NAAT testing and increasing the uptake and frequency of HIV testing through social marketing campaigns. This suggests that social marketing campaigns may play an important role in maximizing the impact of new HIV testing technologies.
Overall, if pooled NAAT had not been implemented, 46% of men with AHI would have been given a negative result. Instead, these men were given a positive diagnosis and were able to learn of their HIV infection early. This has important implications for improving their health and reducing their risk of onward HIV transmission.
While promising, further research is needed to explore:
- the extent to which early detection may help improve health and prevent HIV transmission
- the cost-effectiveness of implementing NAAT screening
- the benefit of adding NAAT screening to a testing algorithm that uses fourth-generation HIV tests
Hopefully this study will create interest in similar pilot programs in other parts of Canada.
Recently infected individuals: a priority for HIV prevention from Prevention in Focus, CATIE’s online prevention resource
Detecting HIV earlier: advances in HIV testing from Prevention in Focus, CATIE’s online prevention resource
HIV: It’s Hottest at the Start – Health Initiative for Men campaign
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- Public Health Agency of Canada. HIV Screening and Testing Guide. 2012. www.catie.ca/sites/default/files/EN_HIV-Screening-Guide-2013.pdf
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