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Aims Estimated population sizes of key populations are critical for resource allocation as well as for monitoring program performance to prevent HIV infection. In 2018, using official records we recruited and conducted a brief survey among People Who Use Drugs (PWUD) to estimate the population size of People Who Inject Drugs (PWID) in Son La Province, Vietnam. Methods In Son La, the authorities develop and maintain a master list of individuals who are suspected of using drugs. The list is updated quarterly and is used to monitor the drug use situation in the province. The list, however, does not distinguish injecting from non-injecting drug users. Individual lists from six districts, 24 communes, and 96 hamlets were selected from among those from 12 districts, 204 communes, and 3335 hamlets in the whole province. After reviewing the lists with the hamlet health workers, based on the length of the lists, a minimum of 20% of the total or five drug users in any hamlet with fewer than 25 PWUD were randomly selected for a brief assessment. In addition to basic demographics, the assessment included any drug use in the last 3 months, injection drug use in the last 3 months, and the last time injected. Results A total of 250 PWUD were interviewed by hamlet health workers. The total number of PWID across all 12 districts in Son La was estimated at 4475 [95% Confidence Interval (CI): 3379-5570] and the corresponding proportion of PWID among PWUD was 48.4% (95% CI: 36.6-60.4). Conclusion The exercise provided an estimated number of PWID in Son La. For planning HIV prevention and care service needs among PWID, additional studies using different methods are needed to validate and improve the population size of PWID in the province of Son La.

Background: HIV/AIDS program managers in Ho Chi Minh City (HCMC), Vietnam have always relied on the police reports and the UNAIDS Estimation and Projection Package for population size estimation of People Who Inject Drugs (PWID). Methods: We used Respondent-driven Sampling (RDS) to implement a two-source capture–recapture study to estimate the population size of PWID in HCMC in 2017. The study was implemented in seven out of 24 districts and included men and women ages 18 years and older who reported injecting illicit drugs in the last 90 days, and who had lived in the city for the past six months. Estimates of the PWID population size for each of the seven districts were calculated accounting for the RDS sampling design. These were then adjusted to account for the district sampling probabilities to give an estimate for HCMC. Chapman two-source capture–recapture estimates of population size, based on simple random sampling assumptions, were also calculated for comparison. Results: The estimates resulted in a population size for HCMC of 19,155 [95% Confidence Interval (CI): 17,006–25,039] using the RDS approach and 17,947 (95% CI: 15,968–19,928), using the Chapman approach. Conclusion: The two-survey capture–recapture exercise provided estimates of PWID in HCMC – based on Chapman estimator and RDS approach – are similar. For planning HIV prevention and care service needs among PWID in HCMC, both estimates may need to be taken into consideration together with size estimates from other sources.

BACKGROUND There is limited population size estimation of female sex workers (FSWs) in Ho Chi Minh City (HCMC)—the largest city in Vietnam. Only 1 population size estimation among venue-based female sex workers (VFSWs) was conducted in 2012 in HCMC. Appropriate estimates of the sizes of key populations are critical for resource allocation to prevent HIV infection. OBJECTIVE The aim of this study was to estimate the population size of the VFSWs from December 2016 to January 2017 in HCMC, Vietnam. METHODS A multistage capture-recapture study was conducted in HCMC. The capture procedures included selection of districts using stratified probability proportion to size, mapping to identify venues, approaching all VFSWs to screen their eligibility, and then distribution of a unique object (a small pink makeup bag) to all eligible VFSWs in all identified venues. The recapture exercise included equal probability random selection of a sample of venues from the initial mapping and then approaching FSWs in those venues to determine the number and proportion of women who received the unique object. The proportion and associated confidence bounds, calculated using sampling weights and accounting for study design, were then divided by the number of objects distributed to calculate the number of VFSWs in the selected districts. This was then multiplied by the inverse of the proportion of districts selected to calculate the number of VFSWs in HCMC as a whole. RESULTS Out of 24 districts, 6 were selected for the study. Mapping identified 573 venues across which 2317 unique objects were distributed in the first capture. During the recapture round, 103 venues were selected and 645 VFSWs were approached and interviewed. Of those, 570 VFSWs reported receiving the unique object during the capture round. Total estimated VFSWs in the 6 selected districts were 2616 (95% CI 2445-3014), accounting for the fact that only 25% (6/24) of total districts were selected gives an overall estimate of 10,465 (95% CI 9782-12,055) VFSWs in HCMC. CONCLUSIONS The capture-recapture exercise provided an estimated number of VFSWs in HCMC. However, for planning HIV prevention and care service needs among all FSWs, studies are needed to assess the number of sex workers who are not venue-based, including those who use social media platforms to sell services.

Background There is limited population size estimation of female sex workers (FSWs) in Ho Chi Minh City (HCMC)-the largest city in Vietnam. Only 1 population size estimation among venue-based female sex workers (VFSWs) was conducted in 2012 in HCMC. Appropriate estimates of the sizes of key populations are critical for resource allocation to prevent HIV infection. Objective The aim of this study was to estimate the population size of the VFSWs from December 2016 to January 2017 in HCMC, Vietnam. Methods A multistage capture-recapture study was conducted in HCMC. The capture procedures included selection of districts using stratified probability proportion to size, mapping to identify venues, approaching all VFSWs to screen their eligibility, and then distribution of a unique object (a small pink makeup bag) to all eligible VFSWs in all identified venues. The recapture exercise included equal probability random selection of a sample of venues from the initial mapping and then approaching FSWs in those venues to determine the number and proportion of women who received the unique object. The proportion and associated confidence bounds, calculated using sampling weights and accounting for study design, were then divided by the number of objects distributed to calculate the number of VFSWs in the selected districts. This was then multiplied by the inverse of the proportion of districts selected to calculate the number of VFSWs in HCMC as a whole. Results Out of 24 districts, 6 were selected for the study. Mapping identified 573 venues across which 2317 unique objects were distributed in the first capture. During the recapture round, 103 venues were selected and 645 VFSWs were approached and interviewed. Of those, 570 VFSWs reported receiving the unique object during the capture round. Total estimated VFSWs in the 6 selected districts were 2616 (95% CI 2445-3014), accounting for the fact that only 25% (6/24) of total districts were selected gives an overall estimate of 10,465 (95% CI 9782-12,055) VFSWs in HCMC. Conclusions The capture-recapture exercise provided an estimated number of VFSWs in HCMC. However, for planning HIV prevention and care service needs among all FSWs, studies are needed to assess the number of sex workers who are not venue-based, including those who use social media platforms to sell services.

To assess hospitalizations among HIV-infected women, we obtained a sample of statewide or hospital-specific 1988 medical records listing a diagnosis of HIV infection in women (≥ 13 years of age) from six sites. HIV/AIDS status, demographic information, mode of HIV exposure, and discharge diagnoses were ascertained through medical record reviews and case investigations. Of 1,207 HIV-infected women hospitalized in the six sites in 1988, most were 25–44 years of age (85%) and black, non-Hispanic (70%), and had HIV infection without AIDS (1987 surveillance definition) (65%). Among the 784 HIV-infected women without AIDS, drug dependence/ abuse was the most frequently listed diagnosis (64%, including 18% with both drug and alcohol dependence). Among pregnant HIV-infected women without AIDS, the prevalence of drug dependence was 43%. Infections were the second most frequent diagnosis in HIV-infected women without AIDS (46%), including respiratory infections (14%), endocarditis (7%), pelvic inflammatory...

Kaposi's sarcoma (KS), the most common cancer in patients with acquired immunodeficiency syndrome (AIDS), occurs predominantly in homosexual men. However, the percentage of homosexual AIDS patients with KS has declined during the past 6 years. This and other findings suggest that one or more cofactors associated with the homosexual lifestyle, rather than a special viral strain, probably influence the development of KS in patients infected with the human immunodeficiency virus (HIV). Possible reasons for the decline include changes in homosexual behaviors, leading to the practice of safer sexual techniques, and a decrease in use of nitrite inhalants. Identification of the KS-AIDS cofactor(s) could be invaluable to developing prevention and treatment strategies.

Background Mean duration of recent infection (MDRI) and misclassification of long-term HIV-1 infections, as proportion false recent (PFR), are critical parameters for laboratory-based assays for estimating HIV-1 incidence. Recent review of the data by us and others indicated that MDRI of LAg-Avidity EIA estimated previously required recalibration. We present here results of recalibration efforts using >250 seroconversion panels and multiple statistical methods to ensure accuracy and consensus. Methods A total of 2737 longitudinal specimens collected from 259 seroconverting individuals infected with diverse HIV-1 subtypes were tested with the LAg-Avidity EIA as previously described. Data were analyzed for determination of MDRI at ODn cutoffs of 1.0 to 2.0 using 7 statistical approaches and sub-analyzed by HIV-1 subtypes. In addition, 3740 specimens from individuals with infection >1 year, including 488 from patients with AIDS, were tested for PFR at varying cutoffs. Results Using different statistical methods, MDRI values ranged from 88–94 days at cutoff ODn = 1.0 to 177–183 days at ODn = 2.0. The MDRI values were similar by different methods suggesting coherence of different approaches. Testing for misclassification among long-term infections indicated that overall PFRs were 0.6% to 2.5% at increasing cutoffs of 1.0 to 2.0, respectively. Balancing the need for a longer MDRI and smaller PFR (

The current challenges to public health in collecting, analyzing, and sharing data necessary to promote the health of the population is exemplified by the current inefficient systems used for public health surveillance. Current data collection systems lack interoperability, speed, and comprehensiveness, among other deficiencies. In part, these inadequacies are products of distinct funding streams and a compartmentalized approach. Yet, many forces now at work are motivating public health and the healthcare system in general to move toward an integrated, efficient, and comprehensive approach to the collection of data important to public health. These forces include recognition of the deficiencies of current data collection and data sharing systems, interest in acquiring new data, the continuing proliferation of systems at all levels, concerns about security and confidentiality, and the opportunity to transform the practice of public health. Such enablers of change as public policy and the multiple developments in information technology are also driving the effort. A new public health initiative, the National Electronic Disease Surveillance System (NEDSS), is a primary example of the long-term vision of data collection and data sharing in public health. NEDSS’s long-term vision is of complementary, interoperable electronic information systems that permit automatic gathering of health data from a variety of sources on a real-time basis, facilitating the monitoring of the health of communities, assisting in ongoing analyses of trends in and detection of emerging health problems, and providing information for setting public health policy. Although that long-term vision faces numerous barriers to and requirements for its realization, the comprehensive, as opposed to disease-based, approach used by NEDSS exemplifies the electronic capture and sharing of information between the healthcare system and public health that will move the United States toward data collection systems that will support public health practice in the 21st century.

To evaluate the accuracy of computerized medical-record coding for human immunodeficiency virus (HIV), medical charts were reviewed in six sites. In 7601 hospital and 867 Medicaid records with a listed diagnosis of HIV, the predictive value for HIV was 91% or higher. HIV was identified in 34% of 1155 Medicaid records listing immune disorder or illness in the acquired immunodeficiency syndrome (AIDS) surveillance definition (without an HIV code). In hospital and Medicaid records, AIDS was identified both in records listing AIDS and records listing HIV without AIDS. HIV codes on hospital and Medicaid records were highly predictive for HIV; undercoding of HIV occurred in Medicaid records.

To evaluate the effect of human immunodeficiency virus (HIV) infection and tuberculosis on hospitalizations and the cost of care.National Hospital Discharge Survey, a nationally representative survey of discharges from U.S. nonfederal short-stay hospitals, and statewide billing information.Patients 15 to 44 years of age with a listed diagnosis of HIV infection (n = 418,200) or active tuberculosis (n = 77,700) during 1985-1990.During 1985-1990, hospitalizations related to HIV infection increased sixfold, from 18 to 102 per 100,000 persons; during 1988-1990, hospitalizations related to tuberculosis increased twofold, from 8 to 16 per 100,000 persons. The prevalence of tuberculosis among HIV-infected patients increased from 2.4% in 1985-1988 to 5.1% in 1989-1990 (P = 0.003). The prevalence of HIV infection among patients with tuberculosis increased from 11% in 1985-1988 to 28% in 1989 to 39% in 1990 (P0.001). Infection with HIV was more prevalent among patients with extrapulmonary tuberculosis (31%) than among those with pulmonary tuberculosis (18%) (P = 0.01). An increase in the duration of hospital stay was associated with both tuberculosis and HIV infection. From 1985 to 1990, inpatient care costs increased 7.7-fold and 3.2-fold for HIV and tuberculosis hospitalizations, respectively. During this period, HIV and tuberculosis hospitalizations resulted in 5,793,000 and 1,107,900 days of care, respectively, with an estimated direct cost of $5.7 to $7.4 billion and $0.89 to $1.07 billion, respectively. Estimated national costs of inpatient care for HIV infection or tuberculosis or both totaled $6.4 to $8.1 billion, 5% of which was for patients with both HIV infection and tuberculosis.This is the first study to use a nationally representative sample of hospitals, combined with cost data, to estimate hospitalizations and their costs for HIV and tuberculosis care. Our findings suggest that the convergence of the HIV and tuberculosis epidemics has had an increasing effect on morbidity and the cost of care among young adults in the United States. The increasing prevalence of comorbidity of HIV infection and tuberculosis in inpatients underscores the need for strict infection control of tuberculosis on the part of hospitals, increased attention to prevention, and early identification and treatment of HIV infection, and tuberculosis to reduce morbidity, hospitalizations, and the cost of care.

Objective. —To investigate potential transmission ofMycobacterium tuberculosisin aircraft from a crew member with tuberculosis. Design. —Retrospective cohort study and survey. Setting. —A large US airline carrier. Participants. —A total of 212 crew members and 59 passengers who were exposed to a crew member with tuberculosis during a potentially infectious period (May through October 1992). Comparison volunteer sample of 247 unexposed crew members. Main Outcome Measures. —Positive tuberculin skin test (TST) result or tuberculosis. Results. —Rates of positive TST results were higher among foreign-born persons in all study groups. Among US-born comparisons and contacts, rates of positive TST results did not differ between comparisons and contacts exposed from May through July (5.3% vs 5.9%, respectively). However, contacts exposed from August through October had significantly higher rates of positive TST results than did contacts exposed from May through July (30% vs 5.8%, respectively;P Conclusions. —Data support the conclusion thatM tuberculosiswas transmitted from an infectious crew member to other crew members on an aircraft. Because of the clustering of TST-positive frequent flyers in October when the index patient was most infectious, transmission ofM tuberculosisto passengers cannot be excluded. (JAMA. 1994;272:1031-1035)

During 1983-1988, hospitalizations of patients with a diagnosis of human immunodeficiency virus (HIV) infection increased from 1.3 to 33.7 per 100,000 persons. We used the National Hospital Discharge Survey, which is based on a representative sample of discharges from nonfederal short-stay hospitals, to describe illnesses among hospitalized patients with HIV infection. Of 222,200 such hospitalizations during 1983-1988, most occurred among persons who were 25-44 years of age (79%), white (66%), and male (90%). Among men 25-44 years of age, HIV admissions increased from 8.5 to 148.6 per 100,000 persons during 1983-1988; among black men 25-44 years of age, HIV hospitalizations increased from 43.1 to 387.4 per 100,000 persons. Among women, hospitalizations increased 3.4-fold. Frequently listed illnesses in the Centers for Disease Control (CDC) AIDS case definition were Pneumocystis carinii pneumonia (30%), candidiasis (20%), and Kaposi's sarcoma (13%). Other frequently listed illnesses included infections (39%) such as pneumonia, sepsis, and urinary tract infections; blood dyscrasias (30%) such as anemia, thrombocytopenia, and agranulocytosis; metabolic (17%), gastrointestinal (16%), and respiratory disorders (12%); and drug abuse (9%). These data provide a minimum estimate of HIV hospitalizations because for some patients HIV infection may not be specified on the discharge record. HIV hospitalizations are increasing markedly and are associated with a broad spectrum of severe morbidity.

In recent years, many individuals have developed models to predict the future course of the human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) epidemic at the local, state, and national levels. The number of forecasters and resulting projections reflects not only the complexity and uncertainties in predicting the epidemic, but also the critical importance of such estimates. 1 Many health care policies are based on these forecasts, which influence the scope and direction of both public and private initiatives related to HIV. Overall, the number of reported cases of AIDS in the United States continues to increase. During 1989, 35 238 cases were reported, for a yearly increase of 9.4% over the 32 196 cases reported during 1988. This increase is lower than in past years, partly because of the large number of previously diagnosed cases reported in 1988 that met only the expanded AIDS case definition. 2 However, even after

From a random sample of patients and hospitals and extrapolation ratios derived from the best available sources of data, the authors estimate that the nationwide nosocomial infection rate among the 6,449 acute-care US hospitals in 1975-1976 was 5.7 nosocomial infections per 100 admissions and that over 2 million nosocomial infections occurred in a 12-month period in these hospitals. Nosocomial urinary tract infections constituted 42% of the infections, surgical wound infections 24%, nosocomial pneumonia 10%, nosocomial bacteremia 5%, and nosocomial infections at all other sites 19%. If adjustments are made for the accuracy of the diagnostic method, the increasing nationwide secular trend, and the number of nosocomial infections in nursing homes, however, as many as 4 million nosocomial infections per year may now be occurring. This greatly exceeds previous estimates and calls for timely and accurate vital statistics on the problem.

To measure the accuracy and consistency of a standardized method--retrospective chart review (RCR)--for estimating nosocomial infection rates (NIRs) in individual hospitals, the authors performed a series of pilot studies in four hospitals of different types. In comparison with a standard based on diagnoses made by physician-epidemiologists supervising intensive prospective data collection teams, the RCR method was found to have an average sensitivity of 0.74 (+/- 0.02 SE; range 0.69-0.78) and an average specificity of 0.964 (+/- 0.002; 0.945-0.991). These values were comparable to those of the physician-epidemiologists' diagnoses and varied less among the hospitals. Two independent teams of chart reviewers were found to have similar levels of sensitivity and specificity, and the reliability of diagnosis at the level of the individual chart reviewer averaged 0.94. In a restudy at one of the pilot hospitals at the midpoint of the actual Medical Records Survey (MRS), there was a substantial increase in sensitivityand a slight increase in specificity as a result of improvements made in the RCR method after the original pilot studies.

From a survey of all U.S. hospitals in 1976 and of a random sample in 1983, we found that the intensity of infection surveillance and control activities greatly increased, and the percentage of hospitals with an infection control nurse per 250 beds increased from 22% to 57%. The percentage with a physician trained in infection control remained low (15%), and there was a drop in the percentages of hospitals doing surgical wound infection surveillance (from 90% down to 79%) and reporting surgeon-specific rates to surgeons (from 19% down to 13%). There was an increase in the percentage of hospitals with programs shown to be effective in preventing urinary tract infections, bacteremias, and pneumonias, but not surgical wound infections. The percentage of nosocomial infections being prevented nationwide appears to have increased from 6% to only 9%, whereas 32% could be preveneted if all hospitals adopted the most effective programs.

In a representative sample of US general hospitals, the authors found that the establishment of intensive infection surveillance and control programs was strongly associated with reductions in rates of nosocomial urinary tract infection, surgical wound infection, pneumonia, and bacteremia between 1970 and 1975–1976, after controlling for other characteristics of the hospitals and their patients. Essential components of effective programs included conducting organized surveillance and control activities and having a trained, effectual infection control physician, an infection control nurse per 250 beds, and a system for reporting infection rates to practicing surgeons. Programs with these components reduced their hospitals' infection rates by 32%. Since relatively few hospitals had very effective programs, however, only 6% of the nation's approximately 2 million nosocomial infections were being prevented in the mid-1970s, leaving another 26% to be prevented by universal adoption of these programs. Among hospitals without effective programs, the overall infection rate increased by 18% from 1970 to 1976.

To predict the likelihood that a patient will develop a surgical wound infection from several risk factors, the authors used information collected on 58,498 patients undergoing operations in 1970 to develop a simple multivariate risk index. Analyzing 10 risk factors with stepwise multiple logistic regression techniques, they developed a model combining information on four of the risk factors to predict a patient's probability of getting a surgical wound infection. Then, with information collected on another sample of 59,352 surgical patients admitted in 1975-1976, the validity of this index as a predictor of surgical wound infection risk was verified. With the simplified index, a subgroup, consisting of half the surgical patients, can be identified in whom 90% of the surgical wound infections will develop. By the inclusion of factors measuring the risk due to the patient's susceptibility as well as that due to the level of wound contamination, the simplified index predicts surgical wound infection risk about twice as well as the traditional classification of wound contamination (Goodman-Kruskal G = 0.67 vs. 0.36, p less than 0.0001). Use of this new index might substantially increase the efficiency of routine surgical wound infection surveillance and control.

To assess the influence of physicians' diagnostic practices on the recognition of nosocomial infections, the authors analyzed data collected on 339,044 patients selected randomly from admissions in 1970 and 1975-1976 to 338 randomly selected hospitals representative of all acute-care US hospitals. Eight rates representing the frequency of cultures or chest x-rays among patients with or without signs of infection were calculated. These varied widely among hospitals, were highest in teaching hospitals and in the Northeast, but increased more among small hospitals and in the South and West. Rates of performing urine cultures and reporting colony counts were highly correlated with observed rates of nosocomial urinary tract infection. Analogous measures were moderately correlated with observed bacteremia rates and pneumonia rates but were only weakly associated with surgical wound infection rates. These data indicate that the nationwide increase in the use of these diagnostic tests increased the recognition of infectious diseases in US hospitals.

During the period of 1979 to 1983, 38 patients with cystic fibrosis (CF) at the CF center of St. Christopher's Hospital for Children in Pennsylvania developed respiratory tract colonization with Pseudomonas cepacia . Seventeen (45%) of the patients with colonization died. Yearly incidence rates of P. cepacia colonization fluctuated between 1.3% and 6.1%, suggesting an endemic phenomenon. Case-control studies showed that severe underlying CF, use of aminoglycosides, and having a sibling with CF and P. cepacia colonization were significant risk factors for P. cepacia colonization. Once colonized with P. cepacia , patients with CF were likely to be hospitalized longer (P=0.008) and to die sooner (P=0.0001) than control patients with CF. Environmental and microbiologic studies did not identify a common source or mode of transmission of P. cepacia among patients. The results of this investigation suggest that P. cepacia colonization of patients with CF was endemic in the hospital, occurred more frequently in those with severe disease, and was associated with adverse clinical outcome.

Population figures were obtained, and incidence rates of acquired immunodeficiency syndrome (AIDS) for the 12 months from June 1, 1983, to May 31, 1984, were estimated for single (never-married) men aged 15 years or older, intravenous (IV) drug users, Haitians living in the United States, persons with hemophilia A and B, female sexual contacts of male IV drug users, and blood transfusion recipients. Single men in San Francisco and Manhattan, IV drug users in New York City and New Jersey, hemophilia A patients, and recent Haitian entrants had the highest rates of disease (82.0 to 268.9 per 100,000). Male IV drug users and male Haitians were two to four times as likely to experience development of AIDS as were females in each group. Persons with hemophilia A had six times the incidence rate of AIDS as did those with hemophilia B. Persons with severe hemophilia A had three times the rate of those with moderate and seven times the rate of those with mild clotting factor deficiency. Although blood transfusion recipients and female sexual contacts of male IV drug users had much lower average yearly rates than did persons in the four other groups (0.4 to 9.4 per 100,000), they still had a higher incidence rate of AIDS than did persons not belonging to any of these groups (0.1 per 100,000). ( JAMA 1985;253:215-220)

In Reply.— Dr Caceres raises several concerns about AIDS patient data reported to the CDC and the ways in which these data are presented. The CDC has long recognized that some AIDS patients have multiple risk factors. 1-3 The patient classification system most commonly used is hierarchical 2-5 because it is both simple and straightforward. It will not always accurately reflect how each individual acquired HTLV-III/lymphadenopathy-associated virus (LAV) infection and AIDS, but it does reflect the most likely route of transmission. 6 For example, a homosexual/bisexual man who had received a blood transfusion may have acquired HTLV-III/LAV infection in a nonsexual way, but for surveillance purposes, would be classified in the homosexual/bisexual group. However, if 5% of the adult population are gay men and if they have the same risk of transfusionassociated AIDS as the rest of the US adult population, 6 then the number of transfusion-associated cases may actually

In Reply.— As stated in our article, the number of single males aged 15 years or older from the 1980 census is not a true estimate of the homosexual or bisexual male population but was used as the best available source of denominator data to estimate the incidence of AIDS in homosexual or bisexual men. The 1980 census data provided the most recent population figures categorized by sex, marital status, age, and city of residence. Although there are inaccuracies in census figures, those inaccuracies are not likely to vary by geographic areas. Thus, comparisons of incidence rates in single men from different locations would be valid. Within a given geographic area, more accurate denominator estimates for homosexual or bisexual men or other groups with an increased incidence of AIDS may be available, and HMOs may be one source of such information. Since different HMOs collect statistics about participants in different