Diagnosing human immunodeficiency virus 1 (HIV-1) infection is most commonly based on a multi-serological assay algorithm. Usually, enzyme-linked immunosorbent assay (ELISA) such as the HIV-1/2 Combo assay (performed on either the Architect or Alinity, Abbott Diagnostics, Chicago, IL) is initially employed to screen for infection with either HIV-1 or HIV-2 (a less virulent variant of HIV)[1,2]. Reactive samples are then tested by a confirmatory assay such as the Geenius HIV-1/2 supplemental assay (Geenius; Bio-Rad Laboratories, Redmond, WA) that is more specific and distinguishes between HIV-1 and HIV-2[3,4]. A molecular assay further tests all cases that are negative or indeterminate in this assay to detect acute infections diagnosed during the confirmatory seroconversion window[5,6].

Discriminating between recent and long-term, non-recent infections and estimating HIV-1 incidence is currently not part of this diagnostic algorithm. Identifying recently acquired HIV infections is important for several reasons. First, understanding the current transmission of HIV in a country, region, or population subgroup. Second, evaluating the impact of specific prevention interventions. Additionally, focusing on resources for prevention or treatment services for specific risk groups or geographic locations with the greatest potential of reducing risk for onward transmission. HIV assays designed to detect recent infection, also known as “recency assays”, can be used to gain an understanding of these epidemic dynamics[7].

Avidity assays, such as the Sedia limiting antigen avidity enzyme immunoassay (Sedia, LAg EIA, BioSciences, Beaverton, OR), developed specifically for incidence analysis by the CDC, is such an assay that is used to assess recency of infection[8,9]. Sedia defines a normalized optical density value of 1.5 separating recent from non-recent infection[10]. Any result below 1.5 is considered recent[11,12], occurring approximately 180 days before the diagnosis or having a mean duration of recent infection, MDRI, of six months[13].

The Geenius assay can also assess the recency of infection based on the intensity of the bands identified by the Bio-Rad accessory reader[14]. MDRI values of approximately six months (179 days, 95% CI: 155-204) were calculated for a calculated cutoff of 1.5[14]. Architect HIV-1/2 Combo test was also suggested to discriminate between recent and non-recent infections. A value of 400 S/CO units was calculated below which, an MDRI value between 4 and 12 months was estimated[15].

In Israel, the annual incidence rate of HIV-1 is low and was estimated to be approximately 3.9 cases per 100,000 people in 2021[16,17]. HIV-2 infections have rarely been recorded[18], [19], [20].

Here, using the Sedia LAg avidity EIA assay, we determined recency (less than 180 days post-infection) in a randomly selected sample of HIV-1 positive sera and compared the Sedia results to those obtained by the Geenius recency algorithm. In addition, an optimal Combo S/CO threshold for recency was calculated, and used to distinguish recent from non recent infections in the same cohort of samples and compared to the Sedia. Lastly, the prevalence of recent infections among all newly diagnosed HIV-1 infections in Israel in 2017-2021 was assessed (using a combined Geenius and the calculated Combo recency verdicts), and the demographic and virological factors associated with recency were examined.