Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Research Letters AIDS 2009, 23:533–540 Estimation of HIV incidence in San Francisco Susan Scheer, Chi-Sheng Chin, Anthony Buckman and Willi McFarland The Centers for Disease Control and Prevention recently released the first direct national estimate of HIV incidence. Local jurisdictions have begun to apply this methodology. The national and local estimates have been higher than assumed. When applied to San Francisco, there were 935 new HIV infections [95% confidence interval (CI) 658 – 1212] during 2006. We compared this incidence estimate to an estimate produced in San Francisco in 2006 by a panel of HIV researchers using an iterative Delphi method. Results were similar. Further corrobora- tion of the new method in local areas would strengthen interpretation and identify HIV risk variations. The Centers for Disease Control and Prevention (CDC) recently released the first direct estimate of HIV incidence in the United States [1]. Drawing data from 22 states that participated in a national surveillance system during 2006, HIV incidence was calculated using the BED HIV-1 capture enzyme immunoassay (BED) that distinguishes recent from long-standing infection among new diag- noses and a statistical adjustment for frequency of HIV testing and then extrapolated to the United States [1,2]. The calculation estimated that 56 300 (95% CI 48 200– 64 500) persons were newly infected in the United States in 2006. The figure was similar to that produced by a method of back calculation from AIDS cases, estimating 55 400 (95% CI 50 000 – 60 800). The same methodology was applied to New York City and the resulting estimate was 4762 – three times higher than the national rate [3]. Both the national and the New York City estimates were higher than many public health officials assumed. San Francisco was participating in the CDC-funded HIV incidence surveillance system during 2006. However, the State of California did not implement name-based HIV case reporting until April 2006. As such, data from California was not included in the 22 states from which the national incidence estimate was derived. However, of the 593 newly diagnosed and reported HIV cases in San Francisco, California in 2006, only 17 were originally reported before name-based HIV was implemented in April 2006 and 14 of these were re-reported later in 2006 by name. As a result, 590 cases (99.5%) are included in San Francisco’s 2006 HIV case registry, and all were subject to the same scrutiny regarding checking for duplication and accuracy of date of diagnosis, for example, that cases reported in other name-based reporting systems. After release of the national HIV estimate, CDC released the methodology [2] and accompanying SAS computer programs [4], so that individual incidence surveillance sites could calculate their local estimate. We, therefore, report here the 2006 HIV incidence estimate for San Francisco. Table 1 provides the available data used for the HIV incidence estimation in San Francisco. Ninety cases were classified as recently infected with HIV out of a total of 590 new HIV diagnoses reported in 2006. On the basis of the CDC adjustment and extrapolation methods, we estimate that there were 935 new HIV infections (95% CI 658–1212) in San Francisco during 2006. We used the same 20-fold multiple imputation process that CDC used to calculate the national estimate because HIV testing history and BED results were not available for all cases in 2006. For the population of men who have sex with men (MSM) [including MSM who inject drugs (MSM-IDU)], the estimate was 716 (95% CI 489–944). No other risk population met the criterion of having a minimum of 200 new HIV diagnoses needed for reasonable precision. For comparison, a HIV incidence number was produced for San Francisco in 2006 by the HIV Epidemiology Section of the San Francisco Department of Public. The HIV Epidemiology Section periodically examines all known local data and consults with experts in the HIV/ AIDS field to arrive at a ‘consensus’ estimate of HIV incidence [5]. Data include studies using different methods to estimate HIV incidence in diverse risk popu- lations, including longitudinal cohorts and clinical trials, the less sensitive assay, trends in new HIV diagnoses, trends in sexually transmitted infections and risk behavior and repeat testing histories [6]. An iterative Delphi method is used by an expert panel to arrive at the most plausible estimate and upper and lower bounds. The final estimate is adopted into the Epidemiological Profile of the city’s HIV Prevention Plan and used for programme planning. Using this approach, the estimated number of people acquiring a new HIV infection in San Francisco in 2006 was 975 adults (upper and lower plausible bounds, 801 and 1082, respectively). For MSM (including MSM-IDU), the corresponding figure is 851 (732–1023). Both methods of estimating HIV incidence have their limitations. The CDC methodology extrapolates from a relatively small number of new HIV diagnoses (590), ISSN 0269-9370 Q 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins 533