Introduction

In 2015, the United Nations estimated that 36.7 million people worldwide were living with human immunodeficiency virus (HIV) infection and that 2.1 million people had been newly infected [1].

Primary HIV infection (PHI) is defined as the 6 to 12 weeks between HIV exposure and the appearance of anti-HIV-antibodies, while acute HIV infection (AHI) starts with HIV plasma ribonucleic acid (RNA) detection and terminates when anti-HIV-antibodies appear.

During the few weeks following exposure, HIV replicates exponentially, rapidly seeding cell-associated viral reservoirs. As transmission is correlated with viral load [2], subjects with PHI are highly infectious regarding HIV transmission, all the more since they are unaware of their HIV-infected status. Hence, the risk of HIV transmission per sexual act is 10- to 26-fold higher in subjects with PHI than in individuals with chronic infection [3,4,5]. Modeling estimates suggest that acute and early HIV infections account for 38 to 50% of all forward transmission [6, 7]. Therefore, improving diagnosis and promoting early antiretroviral therapy in the primary stage of HIV infection are essential.

The aim of this review is to summarize the current knowledge about primary HIV infection, through its clinical, immune, viral, and therapeutic aspects.

Clinical Presentation

PHI is responsible for a flu-like illness known as acute retroviral syndrome (ARS) in 65 to 95% of patients [8,9,10,11••], which occurs just before or during the peak of viremia [11••].

The most frequent signs and symptoms are fever, headache, malaise, cough, and lymphadenopathy [11••]. Fatigue, myalgia, weight loss, night sweat, skin rash, pharyngitis, gastro-intestinal symptoms, aseptic meningitis, and oral or genital ulcers have also been reported (Supplementary material 1). Leukopenia, thrombocytopenia, and elevation of liver enzymes are present in one third to half of cases [12].

Two clinical points deserve special attention. First, neurological involvement, including aseptic meningitis, encephalitis, and facial nerve paresis, is reported in 14 to 25% of patients with ARS [9, 12]. Clinicians should be aware of the eventuality of a neurological presentation revealing a PHI. Secondly, ARS can rarely (< 10% cases) be associated with category B or C of the CDC classification symptomatic conditions, such as oral or esophageal candidiasis, gastro-intestinal or hepatic cytomegalovirus infection, multi-segmental herpes zoster, or peripheral polyneuropathy [13].

ARS is self-limited, resolves after 1 to 3 weeks (extremes from 5 to 44 days) [8, 10, 12], but appears to be a prognostic marker in HIV progression. Untreated subjects with ARS undergo a faster CD4 T cell decline and progression to AIDS than subjects with asymptomatic PHI [14]. The 12-month disease progression rate has also been shown to be higher in untreated PHI patients with ARS compared to asymptomatic PHI on a combination of both clinical and biological criteria (CD4 T cells count < 350/mm3, or the occurrence of B or C clinical event in the CDC classification, or death) [8].

Biological Diagnosis Principles

After HIV exposure, local infection is established at transmission sites, and the virus spreads to regional lymph nodes and disseminates. Exponential HIV replication follows, leading to a peak in plasma HIV viral load (median 6.7 log copies per milliliter) about 13 days after HIV-RNA detectability (i.e., 2–3 weeks after infection) [11••]. The peak in genital viral load occurs about 30 days after infection [15]. Then, HIV-specific immune response leads to a decrease of viremia, and viral load reaches a set point (median 4.3 log copies per milliliter) about 5 weeks after infection [11••].

Natural History

Approximately 10–11 days after infection (range 8–17 days), HIV-1 RNA is detectable by nucleic acid testing (NAT) in plasma. Next, HIV-1 p24 antigen becomes detectable 14–15 days after infection. However, p24 antigen detection is transient, as when anti-HIV-1 antibodies appear, they bind to the p24 antigen and form immune complexes that interfere with p24 assay detection. This might lead to a “second window period”, when p24 antigen is not detectable due to the formation of immune complexes, and HIV-antibodies are below the detection threshold. Immunoglobulin (Ig) M antibodies are expressed and can be detected by 21 to 42 days after infection. Finally, IgG antibodies develop and persist throughout the course of HIV infection [16•, 17••, 18].

The pattern of emergence of laboratory markers is highly consistent and allows classification of HIV infection into distinct laboratory stages [16•, 17••, 18] (Fig. 1; Table 1).

  • The eclipse period is the initial interval after infection with HIV when no laboratory markers are detectable (from day 0 to day 11)

  • The seroconversion window period is the interval between infection with HIV and the first detection of antibodies (from day 0 to day 21, maximum day 42)

  • Acute HIV infection (AHI) is the interval between the appearance of detectable HIV RNA and the first detection of antibodies (from day 11 to day 21, maximum day 42). The evolution of virological laboratory markers was precisely described by Fiebig et al. [17••] (Table 1).

  • Established HIV infection is the stage characterized by a fully developed IgG antibody response sufficient to meet the interpretive criteria for a positive Western blot (from 1-month post-infection).

Fig. 1
figure 1

Schematic display of the progression of HIV markers during primary HIV infection classified by Fiebig et al. [17]

Full size image
Table 1 Fiebig stages: laboratory stages of primary HIV-1 infection based on the emergence of viral markers
Full size table

According to the Center for Diseases Control [16•] and the European guidelines [19••], laboratories should conduct initial testing with a fourth-generation IA (which detects HIV-1 and HIV-2 antibodies and HIV-1 p24 antigen) to screen for established infection with HIV-1 or HIV-2 and for AHI. The performances required for this screening test are a sensitivity of 100% and a specificity of 99.5%. No further testing is required for nonreactive specimens if there is no suspicion of AHI. All reactive specimens must undergo supplemental testing with a confirmation test.

Screening Diagnostic Tests

The sensitivity of fourth-generation IAs for established HIV-1 and HIV-2 infection ranges from 99.7 to 100% and their specificity from 99 to 100% [16•, 20]. However, they perform less in AHI than in established HIV-1 infection. Their sensitivity was 54–83% in patients with AHI who were negative by HIV-1 Western blot but reactive by HIV-1 NAT [16•, 21]. In a comparative study of FDA-labeled HIV-screening tests on sequential sera of patients with PHI, fourth-generation IAs were positive 5.3 to 7.4 days after HIV-RNA and 13 to 23.6 days (median 17.8) after infection. The authors estimated by a simulation model that 99% of HIV-infected persons would be detectable by fourth-generation IAs within 45 days of exposure [22].

Therefore, to take in account the inter-individual variable delay before development of antibodies, the lower sensitivity of screening tests for AHI than for established infection and the possible negativity of p24 antigen when antibodies develop, a fourth-generation IA formally excludes HIV infection if performed at least 6 weeks after exposure. In case of negative fourth-generation IA with suspicion of symptomatic PHI, an HIV viral load should be performed.

Confirmation Tests

A positive screening test requires a confirmation test: a HIV-1/HIV-2 antibody differentiation assay (ADA) in the USA, a Western blot (WB) or an immunoblot in Europe. In addition, according to the CDC, specimens which are reactive on the initial fourth-generation IA and nonreactive or indeterminate on the ADA proceed to HIV-1 NAT to distinguish acute HIV-1 infection from false-positive initial IAs results.

Interpretation of Western Blot

The WB is interpreted as positive, negative, or indeterminate, depending on the number and specificity of the positive bands, revealing the presence of antibodies to HIV-1 antigens (Supplementary material 2).

However, WB has limitations: its results are negative or indeterminate early in the course of HIV infection, and HIV-2 infections may be misclassified as HIV-1 by the HIV-1 WB [23]. Three alternative confirmation tests might overcome these limitations: the antibody differentiation assay (ADA), a specific HIV-2 WB, and the HIV1 NAT.

The HIV-1/HIV-2 Antibody Differentiation Assay

ADA is the confirmation test recommended in the USA. It is a rapid, qualitative immunoconcentrating assay that detects and differentiates antibodies against HIV-1 and HIV-2. Sensitivity of the ADA for established HIV-1 ranges from 98.5 to 100%, and specificity is 99%. In acute HIV-1 infection, the ADA was positive 7 days earlier than the WB and reduced indeterminate results [24].

The HIV-2 WB

A negative or indeterminate HIV-1 WB after a positive fourth-generation IA might indicate either an early infection, or an HIV-2 infection, or a false-positive. A control of HIV-1 WB 14 days later associated with a specific HIV-2 WB concludes on the definite result (Supplementary material 2).

HIV-1 NAT

HIV-1 NAT is the earliest positive test during PHI (11 days after infection), with a sensitivity of 100%. However, it is less specific (2–5% false-positive), more time-consuming, and more expensive than other diagnostic tests [25]. Its indications are limited to specimens that are reactive on the initial IA and nonreactive or indeterminate on confirmation tests.

Point-of-Care Tests

Disadvantages of the reference diagnostic method include the need for venipuncture and the requirement of a follow up visit, while up to half of the tested persons do not return to receive their HIV test results [26]. Point-of-care tests (POCTs), also known as rapid disposable tests, have been developed to increase access to HIV-screening and reduce barriers to early diagnosis of HIV infection (Supplementary material 3). To be performed outside laboratories, they use direct, unprocessed specimens (finger-stick capillary whole blood or oral fluid) but some are also available for plasma or serum. Results are available within 30 min of testing. These POCTs are based on immunofiltration or immunochromatography: HIV-1 and HIV-2 synthetic antigens are affixed to the test strip or membrane, and if the specimen contains HIV-antibodies, the binding of HIV-antibodies to the antigen results in a colorimetric reaction.

POCTs detect HIV-antibodies but not p24 antigen (except the Determine® HIV-1/2 Ag/Ab Combo®), and become positive between 21 days and up to 42 days following HIV infection.

Sensitivity of POCTs for established HIV infection is ≥ 98% with specificity around 99%. In AHI, POCTs on whole blood and even more on oral fluid, lack sensitivity [27, 28]. In a prospective study in men who have sex with men in the USA, the POCT OraQuick® identified 91% of HIV-antibody-positive patients. Most of the Ora-Quick® negative, EIA-positive patients had early infection [29]. In another study, the sensitivity in 53 patients with recent HIV infection was 88.5% for EIA, 79.2% for the POCT on oral fluid, and 81.1% to 88% for the POCTs on whole blood [30].

There are a number of reasons for this. First, all but one POCTs detect antibodies and not p24 antigen; so their detection window is at least 21 days—longer than the 14–15 days detection window with a fourth-generation IA. Secondly, the dilution of whole blood compared to serum, and the low concentration of antibodies in oral fluid might contribute to the decreasing sensitivity of diagnostic tests on whole blood and oral fluid compared to serum [28]. At last, the short migration or filtration time for antigen-antibody binding and a weaker antibody affinity due to hemolysis and reaction at room temperature instead of 37 °C have also been suggested [28].

In pre-exposure prophylaxis (PrEP) settings which involve frequent HIV testing, POCTs on whole blood reveal less sensitivity compared to antigen/antibody tests [31]. PrEP with tenofovir in injecting drugs users has also been associated with a higher risk of false-negative results of oral POCT compared to patients receiving placebo in Thailand [32]. However, this was not confirmed on two other PrEP trials analyzed by the same authors [32].

The Determine® HIV-1/2 Ag/Ab Combo® test was an attempt to improve the diagnosis of early HIV infection. Unfortunately, its sensitivity for the detection of p24 antigen is much lower than for the detection of antibodies [28, 33, 34], and it did not perform better than other POCTs. Overall, in a recent meta-analysis, POCTs were less sensitive than fourth-generation IAs for the diagnosis of HIV infection in clinical settings, and this difference was stronger in high-income than in low-income countries, maybe as a result of a higher incidence of AHI [35]. The authors estimated that in high-income countries about one in seven HIV infections were missed by POCTs when they were used alone. Newer and more sensitive RNA/DNA-based POCTs are being developed and validated but are not available for now.

Overall current POCTs must not be used as the sole screening test in suspected PHI settings and should be completed by a fourth-generation IA.

A positive POCT must be confirmed by the reference diagnostic process, including a fourth-generation IA and a confirmation test with a WB or immunoblot in Europe [36] or an ADA in the USA [16].

Antiretroviral Treatment

Early antiretroviral treatment (ART) of PHI has demonstrated virological and immunological benefits and potential clinical benefits. The earlier the stage of PHI, the more important it is to start the treatment early [37].

Virological Benefits

Early ART in PHI lowers HIV-RNA viral load set point [38,39,40]. In 173 patients enrolled during PHI to receive either no treatment or 24 or 60 weeks of ART from baseline, the mean viral set point was measured 36 weeks after randomization in the untreated arm and 36 weeks after treatment interruption in the treatment arms. It was 4.8 log10 copies/mL (standard deviation (SD) 0.6) in the untreated arm, and 4.0 (SD 1.0) and 4.3 (SD 0.9) log10 copies/mL, respectively, in the 24- and 60-week treatment arms (p < 0.001) [39].

HIV establishes reservoir early in infection, rapidly replicating and seeding peripheral blood mononuclear cell (PBMC) and tissue sanctuary (gut and lymph nodes principally) in an integrated form [41]. With HIV plasma kinetics, there is a rapid seeding of the viral reservoir when evaluated by HIV DNA monitoring. HIV DNA set point is established early in PHI, around 4 weeks after infection [42]. Early ART dramatically reduces this set point. Among 90 Thai patients with early acute HIV infection, 71 received immediate ART while 19 were not treated. The total and integrated HIV DNA levels were closely monitored. The total DNA level in untreated patients was 20-fold higher than in treated patients at week 2 following enrollment, and 316-fold by week 144. For integrated DNA, the ratio was 25 by week 2 and 100 by week 144 [42]. The French cohort PRIMO, including patients with PHI, also showed that the earlier ART was initiated, the faster cell-associated HIV-DNA level decreased [43].

In addition, early treatment prevents colonic mucosal mononuclear cells infection. Among 41 patients with AHI who underwent a sigmoidoscopy at diagnosis, before starting ART, 10 had an undetectable HIV RNA in homogenized colon biopsy specimens. The median delay between HIV exposure and colon biopsy was 11 days in these patients, versus 16 days in the 31 participants with detectable HIV RNA in colon (p = 0.02). All but one patient received immediate ART after biopsy and underwent a measure of total HIV DNA in colonic mononuclear cells after 24 weeks of ART. Participants with baseline detectable colonic HIV RNA demonstrated persistent elevation in total HIV DNA in colonic mucosal mononuclear cells, while it was undetectable in the 10 patients with undetectable HIV RNA in biopsy at baseline [44].

Virologic failure is uncommon after treatment initiation during PHI: of 264 Thai patients treated during AHI, only 1.1% did not achieve HIV RNA less than 200 copies/mL at 24 weeks [45]. Participants who initiated ART during Fiebig I stage demonstrated a shorter median time to virological suppression (8 weeks, interquartile range (IQR) 4–12) than did all other stages combined (12 weeks, IQR 8–16, p < 0.001) [45].

Between 5 and 15% of HIV-1-infected patients treated early during PHI, also known as post-treatment controllers, maintain viral suppression after stopping ART [46,47,48,49,50,51]. However interruption of ART is not recommended because in most cases it has a negative impact on CD4 cell recovery [52], inflammation [53], and risk of AIDS and non-AIDS events in chronic HIV-1 infection [54].

Immunological Benefits

PHI is associated with an important immune activation consistent with cytokine release which amplifies viral replication in lymphoid organs [55]. Early treatment in PHI reduces immune activation and inflammation and decreases systemic inflammatory biomarkers [56]. Early ART also preserves immune function [57,58,59,60,61] and integrity of lymphoid tissue [62, 63] and enhances CD4 T cell recovery [64, 65]. Whereas CD4/CD8 cell ratio declines rapidly during untreated PHI, initiation of ART within 40 days of the estimated date of infection significantly increases the CD4/CD8 ratio [66].

Treatment of PHI may reduce HIV-induced inflammation in the central nervous system. In the first study, cerebrospinal fluid (CSF) neurofilament light chain (NFL), a measure of axonal injury, was above the normal value in 1 of 32 (3.1%) acutely HIV infected patients and in 10 of 32 (31%) chronically HIV infected patients (p = 0.006). The difference persisted after 6 months of ART [67]. However, in a second study, nine patients who started ART in the 4 months after infection had lower interleukin six levels in CSF, but no difference for Global Deficit Score, NFL, or HIV DNA detectability compared to seven patients who started ART >14 months after infection [68].

Very early after HIV infection (in Fiebig I and II stages), there is substantial depletion of CD4+ T cells in the gastro-intestinal tract lamina propria (LP), with associated epithelial barrier damage, leading to microbial translocation, systemic inflammation and immune activation [69]. In the Thai cohort of acute HIV-infected patients, there was resolution of gastro-intestinal tract inflammation (determined by the numbers of cells that were expressing the proinflammatory cytokine TNF-α in the LP) and immune activation (assessed by the number of Ki-67+ cells in the LP) back to baseline levels after 96 weeks of ART. However, they did not recover gastro-intestinal tract damage with microbial translocation (measured by neutrophil infiltration) nor CD4+ T cells level in the LP after 96 weeks of ART [69].

Early ART in PHI is associated with immune restoration and reduced reservoir size [63, 70, 71, 72••] and may favor post-treatment control [46, 51], thereby enhancing research on strategies to achieve HIV drug-free remission. Therefore, early treatment in PHI is a critical step in any pathway toward an HIV cure [73].

Clinical Benefits

Clinical trial data regarding the clinical benefit treatment of ART in PHI are limited. Many individuals enrolled in studies to assess the role of ART in early HIV-1 infection were identified as trial participants because they presented with signs or symptoms of acute infection. Nevertheless, treatment in PHI appeared to limit the evolution to advanced B and C CDC stages [74] and to reduce both AIDS and non-AIDS morbidity and mortality [66].

In addition, ART started in symptomatic PHI might reduce severity of acute symptoms. Moreover, depression and anxiety, which are common at PHI diagnosis, may be improved by the initiation of ART [75].

As with chronic infection, patients with early HIV-1 infection must be willing and able to commit to treatment. On a case-by-case basis, providers may recommend that patients defer therapy for clinical and/or psychosocial reasons. If treatment during early infection is deferred, patients should be maintained in care and every effort should be made to initiate therapy as soon as they are ready [76••].

Public Health Benefits

Early HIV-1 infection is associated with high viral loads and increased infectiousness [4, 77, 78], and ART-use by HIV-1-infected individuals reduces transmission to uninfected sexual partners, especially during PHI [79, 80]. Early ART is consistent with the principle of Treatment-as-Prevention (TasP) which refers to the use of ART to reduce the amount of virus in patients’ blood (and genital fluids) to decrease the risk of HIV transmission [81].

In order to decrease the overall HIV burden, people living with HIV should be diagnosed as early as possible after acquiring HIV infection, so that they can be linked to prevention and treatment services, and ART can be initiated.

Furthermore, knowledge of HIV-positive status modifies sexual behavior, especially toward more frequent condom use, limiting the risk of transmission [82, 83].

ART Choice

Treatment choice should follow international guidelines and should be based on the local epidemiology of resistance. ART must be started as soon as possible in PHI, especially in case of an symptomatic acute infection, severe or prolonged symptoms, neurological disease, age > 50 years, and CD4 count < 350 cells/μL [19••].

The treatment should include a combination of two nucleoside reverse transcriptase inhibitors (NRTI), namely tenofovir disoproxil fumarate (TDF) or tenofovir alafenamide (TAF), associated with emtricitabine (FTC). Abacavir is not recommended for empiric treatment of acute infection unless the patient is known to be HLA-B*5701 negative.

Because it is important to start the treatment immediately in acute infection, without waiting for the results of antiretroviral drug resistance testing, the third agent should have a high genetic barrier to resistance. The high genetic barrier to resistance is a cornerstone, as the virus may be resistant to at least one antiretroviral drug in up to 16% of patients in the USA [84], 8.2% in recent years in Italy [85] or 4.8% in Thailand during PHI in 2013–2014 [86], and up to 16.3% (95%CI 8.1–30.0%) in 43 acute or early HIV infections in Brazil [87]. The third agent must be chosen among either a pharmacologically boosted protease inhibitor (PI), namely darunavir 800 mg (DRV) boosted with ritonavir or cobicistat, or an integrase inhibitor (INI) namely dolutegravir 50 mg (DTG) [19••, 76].

Some authors reported the use of tenofovir/emtricitabine/elvitegravir/cobicistat as a single tablet regimen during PHI in 23 patients with virological efficacy and safety [88] and in 6 patients, with early and sustained virological response and decrease in HIV DNA [89]. However, the number of patients included in these trials is too limited to propose this treatment for now.

Intensification trials with the addition of maraviroc and raltegravir [71, 90], or only maraviroc [91, 92] to standard treatment of PHI, are not recommended, as they did not significantly improve virological control nor long-term immunological recovery.

Whenever possible, the inclusion of subjects with PHI in clinical trials or studies investigating HIV curative strategies is desirable.

The treatment may be changed if necessary after the results of resistance testing.

The goal of ART is to obtain a viral load < 50 copies/mL after 6 months of treatment, but undetectability may take up to 1 year if the initial viral load is high [36].

The healthcare provider should discuss the diagnosis of PHI with the patient, explain why early treatment is recommended, and provide therapeutic education to ensure adherence to the treatment. ART started during PHI should be continued lifelong, as recommended for any HIV-infected person.

Patients should be also screened for other sexually transmitted diseases such as hepatitis B and C, syphilis, and Neisseria gonorrhoeae and Chlamydia trachomatis infection. Moreover, patients should be encouraged to inform their partners, in order to be tested for HIV and to start post-exposure prophylaxis with ART if appropriate, especially during the first months as patients with PHI have a detectable viral load several months after ART initiation.

Particular Cases

PrEP Users

A previous use of pre-exposure or post-exposure prophylaxis should also be established, as its failure could be predictive of a resistance to the prophylactic ART received. In case of PHI after the use of PrEP (TDF/FTC), the risk of infection with a resistant strain exists [93], but remains exceptional. Therefore, it is still recommended to start initially a TDF/FTC-based treatment [76].

Chronic Kidney Damage

The fixed association of TDF/FTC is not recommended in patients with creatinine clearance lower than 60 mL/min. In subjects with creatinine clearance above 30 mL/min, TAF should be used instead of TDF. The combination of raltegravir and lamivudine (with dose adjustment to creatinine clearance) is recommended as initial treatment, along with darunavir and ritonavir for creatinine clearance below 30 mL/min [36].

Conclusion

Primary HIV infection may be a challenging diagnosis because of its unspecific flu-like symptoms. The diagnosis is based on fourth-generation immunoassay, as POCTs lack sensitivity in early infection. Treatment should be started, as soon as possible, to obtain clinical, virological, and immunological benefits, and in order to prevent further transmission.

Reducing the establishment of HIV reservoir in PHI by early ART may also optimize potential opportunities for viral eradication or functional cure in the future.