We used public health HIV surveillance data to create novel summary measures that track HIV VL dynamics at the population level, and applied these longitudinal measures to the known population of HIV-infected New Yorkers receiving HIV-related medical care in NYC in 2006 and 2007. Findings show that analytic population had an average of six VL test results, suggesting frequent engagement in HIV-related medical care. Almost 40% had virologic suppression for two years, indicating sustained treatment success.
Previous studies have shown that IDUs and persons of color were more likely to delay initiation and have inconsistent utilization of HIV-related care compared to non IDUs and non-Hispanic whites in NYC.
Findings that show IDUs, blacks, and Hispanics at increased risk for SHVL support our hypothesis that SHVL is more common among groups known to have suboptimal engagement in HIV-related medical care. IDUs face challenges in achieving timely linkage to and retention in care.
Delayed initiation of care is associated with poorer immunologic recovery and increased risk of death.
Our analysis suggests that IDUs are an appropriate focus for public health programs aimed at increasing engagement in care among groups most at-risk for HIV-related morbidity.
Blacks, accounting for more than half of PLWHA in the US and with HIV diagnosis rates 8 times that of whites, are also more likely to delay initiation of care after HIV diagnosis. 
Hispanics, accounting for 17% of all new HIV infections in 2006 and with HIV diagnosis rates 3 times that of whites, have similar barriers to engagement in care. 
This trend is consistent with the NYC epidemic where blacks and Hispanics accounted for 50% and 33% of all newly diagnosed HIV cases that were concurrently diagnosed with AIDS in 2009, a marker of late-stage disease and missed opportunities for care.
MSM, disproportionately accounting for prevalent and new diagnoses in NYC, have been shown to have timely initiation and high retention in HIV-related medical care, often serving as the referent group for studies on linkage to care.
Because engagement in care dramatically improves the probability of achieving virologic suppression, it is not surprising that MSM were over-represented among those with DSVL and were not at increased risk for SHVL in this analysis. While blacks and Hispanics comprise less than 60% of MSM overall, they made up almost 90% of MSM with SHVL. Future analyses on SHVL exploring the interaction of transmission risk and race/ethnicity and other markers of SES such as proportion living below poverty, are needed to adequately describe the black and Hispanic MSM population at increased risk for SHVL.
Bronx residents were at increased risk for SHVL, a finding consistent with other HIV indicators showing increased HIV-related morbidity and mortality in the Bronx compared to the other boroughs. 
Bronx residence is likely a surrogate marker for other group membership like race/ethnicity and transmission risk. Known health disparities in the Bronx were part of the impetus for the selection of the Bronx for the DOHMH's community-wide HIV testing scale-up effort, “The Bronx Knows”. 
One of the most concerning findings was that PLWHA aged 13–19 years had 3 times greater risk of SHVL than PLWHA 50 years or older. The relative youth of the 13–19 years age group belies the likelihood that most are highly ART-experienced, given that more than 80% of them were infected with HIV through perinatal transmission, and were diagnosed during the era of mono and dual therapy before highly active antiretroviral regimens were available for HIV treatment. The SHVL among these perinatally-infected adolescents may stem from the development of ART resistant viral strains, or from the adherence gaps often observed among adolescents with chronic illnesses. 
Regardless of its causes, the presence of SHVL among PLWHA who have entered their sexually-active and childbearing years has serious implications for HIV transmission. HIV-infected youth often engage in unprotected sex and do not disclose their HIV status to sex partners. 
HIV-infected youth are also more likely to have a diagnosis of an acute sexually transmitted infection than HIV-infected adults and, among females, to become pregnant. 
SHVL concomitant with risky sexual behaviors places youth at higher risk for HIV-related morbidity and transmitting to others.
Our analysis is subject to limitations. Findings are based on persons presumed to be in care because they had VL results reported to the NYC DOHMH. More than one third of nearly 100,000 PLWHA reported by the end of 2005 were excluded because they had only one or no VL result reported in 2006–2007. This group may represent persons who: 1) are receiving HIV care outside of NYC; 2) died out of NYC and their death was not yet reported to the NYC DOHMH; 3) are out of care; or 4) received care but not VL testing. Because the HIV-infected population in the US is highly mobile and PLWH relocation to jurisdictions outside of NYC are not routinely reported to DOHMH, it is difficult to estimate the proportion of NYC PLWHA without a recent viral load who are engaged in care in another reporting jurisdiction. Despite this distinction, no differences by sociodemographic characteristics between PLWHA with 2 or more VLs and all PLWHA were observed (). 
Characterization of the ‘other’ group where peak VL was detectable but never ≥100,000 copies/mL was limited. Given the heterogeneity in infection times and treatment experiences, some PLWHA may have a more difficult-to-treat virus where achieving DSVL is unlikely with any level of adherence, and thus cannot be considered the goal of therapy. Refinement of the characterization of HIV viremia in this group by exploring measures like area under the curve (AUC), a commonly used measure to quantify rate of change in biomarkers over time, may provide a more comprehensive measure. 
Our ability to infer the level of engagement in care from the number of cumulative VL tests may be limited. We assumed that 1) those more engaged in care had a higher cumulative number of VL tests and; 2) those with the same cumulative number of tests were similar in their access and utilization of care. However, those with the same number of VL tests may be quite different. Among persons with fewer VL tests, for example, some may be non-adherent, experiencing virologic failure and missing the visits at which their VLs would be drawn, while others may be durably suppressed on a stable ART regimen, and therefore require less frequent clinical monitoring. Lastly, VL tests do not necessarily mean that meaningful HIV care was provided to the person tested by a qualified HIV clinician, or that results were received and informed treatment.
Our analyses show that DSVL and SHVL detected HIV disparities, similar to more established HIV population indicators like HIV incidence, diagnosis, and death rates. 
Because undetectable VL is associated with reduced transmission and improved health outcomes, achieving suppression, a specific target of the NHAS, necessitates use of longitudinal measures. 
Our markers not only complement CVL as basic measures of treatment coverage and effectiveness, but improve measurement by characterizing longitudinal patterns of VL dynamics within a population. Moreover, identifying SHVL provides a specific opportunity for outreach to those HIV-infected individuals in a community who are at greatest risk of near-term HIV-related morbidity and mortality. In 2011, DOHMH began active outreach among persons with SHVL who appeared to lack medical follow-up in the months that followed their very high viral load, with the goal of interviewing these clients about partner services and assisting them with returning to medical care for HIV. Prior to this analysis, such outreach was not specifically directed or prioritized based on the magnitude of HIV VL prior to interruption of care. Future analyses will 1) characterize longer-term trajectories of persons with SHVL and DSVL, using AUC to help distinguish rates of virologic change particularly among those with the same number of tests, 2) characterize genotype profiles among SHVLs and measure the proportion who have resistant strains and 3) investigate the relationship between the number of newly reported HIV diagnoses and number of PLWHA with SHVL and DSVL to evaluate its potential as a surrogate marker of incidence. Altogether, findings support the use of these surveillance markers to evaluate treatment effectiveness and HIV transmission risk, especially in high risk groups.