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Epidemiology And Social Science

Determinants of Enrollment in a Preventive HIV Vaccine Trial: Hypothetical Versus Actual Willingness and Barriers to Participation

Buchbinder, Susan P. MD*†; Metch, Barbara MA, MS; Holte, Sarah E. PhD; Scheer, Susan PhD*; Coletti, Anne MS§; Vittinghoff, Eric PhD

Author Information
JAIDS Journal of Acquired Immune Deficiency Syndromes: May 1st, 2004 - Volume 36 - Issue 1 - p 604-612


A safe highly efficacious HIV vaccine could substantially slow the HIV epidemic, 1 currently estimated at 5 million new infections annually. Development and licensure of such a vaccine will require the participation of thousands of HIV-uninfected persons at high risk of acquiring HIV infection in clinical trials. One vaccine manufacturer, VaxGen (Brisbane, CA), initiated 2 efficacy trials of related envelope subunit products, 1 in North America and Europe and the other in Thailand. It took 17 months to accrue more than 5000 HIV-uninfected men who have sex with men (MSM) and 300 high-risk heterosexual women in the North American trial. Preliminary analyses released by the company report no vaccine efficacy overall in the trial but a controversial suggestion of partial efficacy in black participants and other people of color other than Latinos. 2 Regardless of the ultimate interpretation of these data, these results reinforce the limitations in interpreting subset analyses on small samples of participants. Future trials will need to enroll large and diverse cohorts of participants that reflect the demographics of high-risk individuals to ensure that summary measures of efficacy are most applicable to populations in greatest need of HIV vaccines. Understanding of barriers to participation may improve the efficiency and diversity of enrollment in future trials.

Although there is a growing literature on hypothetical willingness to participate in future vaccine trials among high-risk HIV-negative individuals, 3–9 there are few data on enrollment efficiency or barriers to enrollment into vaccine trials among high-risk volunteers. This study extends findings of a previously published study examining the relationship between hypothetical and actual willingness to enroll in an HIV vaccine trial. 10 That study focused on factors associated with hypothetical willingness to enroll and was limited in power to evaluate predictors of actual enrollment in the trial. Another study of enrollment of a preparedness cohort into the VaxGen phase 3 trial focused on factors associated with hypothetical willingness rather than on actual enrollment. 11 To systematically evaluate factors affecting enrollment and refusal to enroll in HIV vaccine trials, we analyzed data from HIV-uninfected high-risk MSM, injection drug users (IDUs), and women at heterosexual risk participating in a vaccine preparedness study (VPS) who were later offered enrollment in a phase 2 HIV vaccine trial.


Vaccine Preparedness Study

The HIV Network for Prevention Trials (HIVNET) VPS was an 18-month prospective cohort study conducted in 1995 through 1997 among 4892 high-risk HIV-uninfected MSM, IDUs, and high-risk women. 7,12 Interest in HIV vaccine trial participation was not a prerequisite for enrollment in the VPS. The VPS consisted of semiannual HIV antibody counseling and testing, assessments of sexual and drug use behaviors, knowledge of vaccine trial concepts, and attitudes toward participation in HIV vaccine and other prevention trials. Participants were recruited from a variety of venues, including sexually transmitted disease clinics, advertising, bars, clubs, street outreach, and rollover from previous preparedness studies.

Vaccine Trial

From May 1997 through January 1998, 435 participants were enrolled in an NIH-sponsored phase 2 randomized controlled trial of live recombinant canarypox ALVAC-HIV vCP205 (Aventis Pasteur, France) with or without HIV-1 SF-2 rgp120 (Chiron, Emeryville, CA). This trial was jointly conducted by 8 HIVNET sites (255 participants) and 6 AIDS Vaccine Evaluation Group sites (180 participants). The HIVNET sites recruited 233 participants from former HIVNET VPS participants, 12 and this report is limited to an analysis of those participants.

The details of the phase 2 study are published elsewhere. 13 In brief, healthy HIV-seronegative men and women 18 to 60 years of age were eligible to enroll if they had normal medical histories, physical examinations, and laboratory evaluations (purified protein derivative, rapid plasma reagin, complete blood cell count, alanine aminotransferase, and urinalysis); were willing to provide written informed consent for 48 months of follow-up; and met HIVNET behavioral criteria at entry into the vaccine trial. Behavioral criteria were based on those used in the HIVNET VPS: (1) MSM: any anal sex in the 12 months before enrollment; (2) IDUs: any injection drug use in the 6 months before enrollment; and (3) high-risk heterosexual women: current relationship with high-risk man (HIV-positive, MSM, IDU, or sexually transmitted infection [STI] in past year) or history of any of the following in the past year: smoked crack cocaine, exchanged sex for drugs or money, had 5 or more sex partners, or had a diagnosed STI.

Vaccine Trial Screening Procedures

The HIVNET Statistical Center generated randomly ordered lists of potentially eligible participants to be contacted at each HIVNET site from a list of former VPS participants who were alive, age-eligible, HIV-seronegative, and in active follow-up as of their last VPS follow-up visit. Participants were included without regard to their previously stated interest in HIV vaccine trials. To address barriers and incentives to participation among persons traditionally underrepresented in clinical trials, random ordering of the selection list was weighted to give younger persons and persons of color a higher probability of being assigned near the top of the list. Participants without current locating information were classified as being unable to contact.

After obtaining written informed consent for screening procedures, participants underwent screening during 3 or more separate visits. Willing participants underwent behavioral risk assessment, medical history, physical examination, and phlebotomy. Participants received a detailed informational booklet about trial participation and staff review of the vaccine trial consent form. At a follow-up screening visit, participants received HIV antibody posttest counseling and took a 15-item questionnaire assessing knowledge of vaccine trial concepts, with review of incorrectly answered items. Knowledge questionnaires were readministered to participants with 2 or more incorrect answers. Separate informed consent was obtained for vaccine trial enrollment, and participants were asked which of 8 selections best described their primary motivation for vaccine trial participation.

The screening process was terminated at any point at which an individual was found ineligible or refused further participation. Because of this procedure, not all screened individuals had a complete eligibility assessment, and participants who refused screening at any point, regardless of eligibility, were categorized as refusers. Individuals who refused participation were asked their main reason for refusal. After the trial began, a more extensive refusal questionnaire was administered to those participants who refused during a screening visit. This questionnaire, developed from investigators’ prior experience and results of prior focus groups, measured participants’ level of concern about specific items measured on a 3-point scale of “very,” “somewhat,” or “not at all” concerned. Participants could reply that they were very or somewhat concerned to multiple items.

Data Analysis

Data were included from all HIVNET sites except a single subsite of female participants whose documentation of recruitment status could not be verified. To compare risk behaviors between those enrolling and those refusing enrollment in the vaccine trial, reported risk behaviors at the last (18-month) VPS visit were used. To compare hypothetical with actual willingness to enroll in the vaccine trial, data on hypothetical willingness were taken from the 12-month VPS visit, when participants were specifically queried about their willingness to enroll in a future hypothetical phase 2 trial.

Unadjusted associations between categorical variables were assessed using the χ2 or Fisher exact test. Multivariable logistic models were used to identify the independent predictors of refusal to participate among those who refused compared with those who enrolled in the vaccine trial. The final multivariable model was adjusted for site and included demographic variables, risk group, and any behavioral variables that were associated with refusal on univariate analysis at P < 0.1. Similar results were found in larger models, including the other potential predictors of refusal in Table 1.

Participants Screened for HIVNET 014 Vaccine Trial


Disposition of Screenees

The 8 HIVNET sites attempted to contact 2531 former VPS participants before fully enrolling the trial. Of these, sites were unable to contact 469 (19%) participants, 241 (10%) were in the process of screening when enrollment for the vaccine trial closed, and screening disposition data were incomplete for 26 (1%) participants. Of the remaining 1795 participants, 13% enrolled in the trial, 34% were ineligible for enrollment, and 53% refused enrollment. Staff were able to determine refusals or ineligibility by a single telephone pre-screening call or a single screening visit for 79% of those ultimately refusing or found to be ineligible.

Participants who were successfully contacted for screening were statistically significantly more likely than those unable to be contacted to be white (52% vs. 45%; P < 0.01) and to be older than 40 years of age on enrollment in the VPS (38% vs. 32%; P < 0.01). Differences by risk group and education were not statistically significant. Compared with participants who were not contacted, contacted participants were statistically significantly more likely at their 18-month VPS visit to have reported injecting drugs (23% vs. 18%; P = 0.04) and significantly less likely to have reported exposure to HIV (17% vs. 22%; P = 0.04) or more than 1 sex partner (47% vs. 58%; P < 0.001) in the prior 6 months.

The demographic and risk characteristics of the screened participants and their disposition in the HIVNET 014 trial are shown in Table 1. As a result of oversampling young persons and persons of color for trial screening, 27% of the screened participants were 30 years of age or younger and approximately half were people of color, providing substantial data on the age range and racial diversity of participants who might be approached for future vaccine efficacy trials. Participants also appeared to have substantial risk: 52% of those screened had reported having unprotected anal or vaginal sex in the 6 months before their 18-month VPS visit, 47% having had multiple sex partners, 22% having injecting drugs, and 17% having had a possible recent exposure to HIV.

Participants enrolling in the trial were substantially different from those refusing or ineligible to enroll. Rates of enrollment were higher among MSM, white participants, persons with higher educational levels, and persons with high-risk sex and drug-using practices. Overall, 29% of male IDUs, 14% of women, and 4% of MSM were ineligible because they no longer met high-risk criteria at screening. Only 28 of the 275 participants found to be medically ineligible on screening had conditions likely to cause them to be excluded from future vaccine efficacy trials (e.g., pregnancy or desire to have children, history of idiopathic anaphylaxis, prior HIV vaccine trial participation, HIV infection). Other reasons for ineligibility included age restrictions, lack of availability for the duration of the trial, and other assorted reasons.

Willingness and Enrollment

Although interest in phase 2 trials at the 12-month VPS visit was significantly correlated with ultimate enrollment in this phase 2 trial, 28.9% of those who had previously stated they would be definitely willing to participate in a future phase 2 trial refused enrollment in this phase 2 trial, as did nearly half of those stating they would probably be willing to participate (see Table 1).

Knowledge of Vaccine Trial Concepts

Although knowledge about vaccine trial concepts was a prerequisite of vaccine trial enrollment, no participants were excluded from enrollment because of lack of sufficient knowledge as assessed by a 15-item assessment tool. Of 343 participants who took the knowledge test, 83% passed on the first try (fewer than 2 errors), 11% had 2 errors, and 7% had 3 or more errors. There was a statistically significant difference between risk groups in the proportion passing the test on the first try: 89% of MSM, 74% of male IDUs, and 67% of women (P = 0.001). A subgroup of participants failed to understand that “the placebos used in this vaccine study could cause side effects,” including 15% of MSM, 17% of male IDUs, and 24% of women. Only 4 screened participants agreed with the statement, “Participants in this vaccine study will definitely be protected against HIV,” and only 2 agreed with the statement, “You could get HIV infection from the vaccines being tested in this study.”

Correlates of Refusal

Univariate and multivariate analyses of factors associated with refusal to enroll are shown in Table 2. Older participants, those who had been to college, and those reporting 6 or more sex partners in the 6 months before their last VPS visit were less likely to refuse enrollment on multivariate analysis. African-American participants were more likely to refuse enrollment. Although women and Latino participants appeared more likely to refuse on univariate analysis, these factors were no longer significant after controlling for other demographic and risk variables.

Predictors of Refusal to Enroll in the Phase 2 Vaccine Trial

Reasons for Refusal

Among the 952 refusers, 26% (249) were “passive” refusers who did not return telephone calls after an initial contact or failed to appear for scheduled visits. Another 12% (110) of refusers did not specify their reasons for refusal. The most commonly cited reasons for refusing participation among the 952 refusers included concerns about time commitment (21%), vaccine safety (14%), and potential social harms associated with vaccine-induced antibody positivity (13%). More detailed refusal questionnaires were administered to the subset of participants who refused participation during a screening trial visit; summary data are provided in Table 3 without statistics because of the exploratory nature of these results. Among this group, participants of all races/ethnicities expressed concern about vaccine-induced seropositivity and negative reactions from friends and family members. The safety of the vaccine was a particular concern for African-American and Latino participants. African Americans were also particularly likely to cite mistrust of government and concern about blinding of treatment assignment as important reasons for refusal to participate.

Proportion of Participants Completing the Detailed Refusal Questionnaire Citing They Were “Very” Concerned About the Following Factors

Motivations for Enrollment

Enrolled participants were asked to select their primary motivation for enrollment. Overall, 94% cited altruistic reasons as their primary motivation for participation. There were significant differences in the way members of different risk groups stated their motivations, however. For all 3 risk groups, the highest percentage of participants responded that they wanted to “help find a vaccine that works” (63% of MSM, 61% of male IDUs, and 56% of women). MSM were more likely to respond as wanting to “give to my community” (19% of MSM compared with 7% of male IDUs and 0% of women). Women and male IDUs were more likely to respond that they wanted to participate to “honor people with HIV/AIDS” (33% of women and 24% of male IDUs compared with 14% of MSM). Less than 1% of all participants cited protection from HIV infection as their primary motivation for enrollment.


Many investigators have published studies describing diverse cohorts’ hypothetical willingness to participate in HIV vaccine trials in the United States, 4–12,14–17 Brazil, 18,19 Thailand, 20–22 and Uganda. 23 This is the first multisite study to describe the relationship of hypothetical to actual willingness to enroll in a preventive HIV vaccine trial. Although hypothetical willingness was statistically associated with actual enrollment, 29% of those who had previously said they would definitely be willing to enroll in a future phase 2 trial refused enrollment in this particular trial, as did 49% of those stating they would probably be willing to enroll in a future phase 2 trial. Participants stating future willingness were also significantly more likely than unwilling participants to be ineligible to enroll in this particular phase 2 trial, leading to final enrollment rates of less than 20% for the “definitely” and “probably” willing groups combined. Thus, although measures of attitudes toward vaccine trials can be useful in pointing to potential barriers to trial participation, these data should not be used to project likely enrollment rates for future studies. The initial loss of younger, nonwhite, and sexually risky participants from the original VPS cohort, and demonstrated declines in risk behavior and seroincidence in cohort studies in general 24 point to the hazard of using existing cohorts for recruitment into vaccine efficacy trials.

The major reasons cited for refusal in this phase 2 study were concerns about time commitment, vaccine safety, and potential social harms. Several previous studies of phase 1 and 2 HIV vaccine trial participants have documented relatively low rates of serious social harms (eg, loss of employment or insurance) arising from trial participation, although negative reactions from family, friends, and coworkers can occur 25 and were cited as a barrier to participation by participants in this trial. 26,27 Such harms can be minimized through a variety of strategies, including careful counseling of participants to minimize inadvertent disclosures of study participation and to anticipate needs for outside HIV antibody testing. Other strategies to maximize overall participation include simpler and less frequent visits, development of antibody testing algorithms to minimize misclassification of vaccine-induced responses as HIV infection, 28,29 and creation of mechanisms for coverage of medical expenses for vaccine-related injuries.

In multivariate modeling, African-American participants were most likely to refuse enrollment in this vaccine trial, whereas older participants, those with higher levels of education, and those reporting greater numbers of sexual partners were least likely to refuse. Previous studies have documented that African Americans are less likely than white patients to volunteer for clinical trials, 30,31 including preventive and therapeutic HIV-related trials. 32–34 In the VaxGen phase 3 trial, only 6% of the enrolled volunteers were African American, making interpretation of the subgroup analysis of this population particularly difficult. One central theme that emerges from multiple studies evaluating racial disparities in clinical trial participation, and confirmed in the subset of participants answering our refusal questionnaire, is the high level of mistrust of scientific research expressed by many African Americans. 30,33,35,36 This mistrust stems from abuses including but not limited to the Tuskegee syphilis study. 33 Indirect evidence suggests that barriers to participation may be partially overcome by educational plans that directly address mistrust of the scientific and research communities 35 and build support with key family members and friends of potential volunteers. 30 Inclusion of African-American staff may also help accrual into clinical trials. 36 More generally, community involvement in research requires ongoing communication, including active involvement of community advisory boards, timely communication of study results to participants and communities, and planning to ensure that communities participating in research have timely access to successful preventive strategies. Our study suggests that educational efforts should also focus on addressing concerns about vaccine safety, a concern particularly cited by our African-American and Latino participants. Future efficacy trials should ensure the active participation of more diverse cohorts of volunteers.

The association of risk practices with willingness to enroll in vaccine trials raises difficult ethical issues. To be efficient, vaccine efficacy trials need to enroll those at highest risk of HIV infection with high seroincidence rates. There is evidence that the highest risk participants may be drawn to participate in such trial 4,7,8,15,18,23 and anticipate 37 or actually engage in increased risk behavior during vaccine trials, 38 leading to the real possibility that HIV vaccine volunteers may suffer undue harm as a result of trial participation. A preliminary analysis of risk behavior change during the VaxGen efficacy trial suggested no substantial increase in risk behavior overall during the first 24 months of follow-up. 39 To ensure adherence to ethical principles, 40,41 HIV vaccine trials must ensure that all participants receive state-of-the-art behavioral counseling and referrals and that ongoing education reinforces knowledge of key trial concepts, including unblinding, the unknown efficacy of vaccine candidates, and the possibility of side effects from placebo. One survey of participants enrolled in any of several phase 1 trials found that 69% thought they knew their treatment assignment based on the presence or absence of symptoms 42; our study found that 15% to 24% of participants incorrectly believed that placebos could not cause side effects.

Our study confirmed others that have found that although baseline levels of knowledge about HIV vaccine trial concepts may be relatively low, 43,44 knowledge increases rapidly in all risk groups with targeted educational strategies. 16,21 A study in IDUs found that participants randomly assigned to receive an informational pamphlet coupled with an informational videotape were significantly more likely to retain knowledge of HIV vaccine concepts 1 month after presentation than those receiving the pamphlet alone. 45 Vaccine trials should consider use of novel strategies such as videotapes or flip charts as part of the informed consent process as well as administration of vaccine knowledge quizzes as a prerequisite for enrollment in vaccine trials. Consideration should also be given to ongoing assessment of knowledge of vaccine trial concepts after enrollment in the trial to ensure retention of essential information.

Most participants enrolled in this trial cited altruistic reasons as their primary motivation. Although few cited protection from HIV infection as their primary motivation, it is possible that this was a secondary motivator for a number of participants. MSM were more likely to cite community benefit, whereas women and IDUs were more likely to mention individuals who had been affected by HIV as a primary motivator for participation. This information could prove useful in targeting recruitment messages for different communities. Consideration should also be given to addressing potential community benefit for all populations.

This study was based on a relatively large and diverse sample of high-risk individuals in the United States. Barriers and motivators to trial participation could be substantially different in international communities or US populations without prior research experience. In study populations in whom HIV vaccine trials are many years in the future, the only methods for assessing barriers to participation are those that ask about hypothetical vaccine trials. Although such data may be of limited usefulness in estimating the proportion of participants enrolling in future trials, they can yield important estimates of HIV seroincidence and information about barriers to enrollment in specific trial populations. More information can be gleaned by systematically gathering data during the recruitment phase of actual trials so as to minimize barriers to the efficient enrollment of large and diverse cohorts of high-risk volunteers.


The authors thank the investigators and staff at the HIV-NET sites who enrolled participants and collected the data and acknowledge the invaluable contributions of the study volunteers. They also acknowledge Goldie Todd and Angela Mc-Clure for administrative assistance with this manuscript.


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HIV vaccine trial; willingness; high-risk; research subject recruitment; enrollment; barriers

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