Although the overall number of new HIV infections has declined worldwide, it is estimated that 5000 new infections occur each day.1 With continuing progress in the implementation of treatment and prevention programs, it is becoming increasingly important to define metrics by which HIV incidence may be considered controlled to gauge success and guide resource allocation.
In a paper published in the Lancet HIV2 in October 2018, researchers from the Yale School of Public Health and Oregon State University examined the 4 potential types of control criteria and their shortcomings, which were discussed in 2017 at a United Nations Programme on AIDS (UNAIDS) meeting. This meeting used a transmission model to simulate trajectories of prevalence, incidence, and mortality through 2030 in 38 sub-Saharan African countries.
- A percentage reduction in incidence over time, although useful for assessing progress, is not an adequate measure of control. However, even in the case of achieving the UNAIDS goal of 90% incidence reduction, the incidence of HIV would remain unacceptably high in some countries compared with others; for example, 21 per 10,000 in Lesotho vs 0.14 per 10,000 in Niger.
- The incidence-to-mortality ratio, in which a higher rate of mortality vs incidence is considered indicative of control, is problematic for several reasons. However, given that more than 70% treatment coverage is required for incidence-to-mortality ratio-based definitions of control, this would be inapplicable to 35 of the 38 countries assessed.
- The value of using an incidence-to-prevalence ratio is also limited, partly because of variations in life expectancy. “For a lifelong infection such as HIV, prevalence rises as life expectancy increases,” as stated in the paper.2 “As the life expectancy for an uninfected person can vary substantially across nations, an [incidence-to-prevalence ratio] indicative of low incidence in one context may translate to an unacceptably high incidence in another setting.”
- The authors supported the use of incidence-based criteria; specifically, fewer than 1 in 10,000 new cases per year, to define HIV control. This metric is unaffected by fluctuating HIV mortality rates, and it reduces empirical uncertainty while more clearly conveying the magnitude of the problem compared with the other metrics.
“Efficiently achieving a low national incidence will undoubtedly involve targeted interventions within high-risk groups who might also be less likely to access services,” the researchers wrote.2 “Outreach to high-risk groups will be instrumental in effective treatment-as-prevention approaches and will ensure that control is steadily achieved at subpopulation levels and for the country overall.”