Results from the Thai Phase III HIV vaccine study (RV144), the largest HIV vaccine trial conducted to date, have demonstrated for the first time that a vaccine can prevent HIV infection . The modest protective effect of the experimental vaccine, while not sufficient for licensure, represents a significant step in ongoing HIV vaccine development efforts [1–3].
A vaccine would obviate many of the social and behavioral challenges of existing HIV preventive interventions, e.g., the need to correctly and consistently implement and negotiate condom use at each sexual encounter. Nevertheless, HIV vaccine acceptability, that is, potential users’ judgments of the satisfactoriness of future HIV vaccines and willingness to use them, is not guaranteed. Challenges related to HIV vaccine acceptability and access portend a broad gap between projected need and actual uptake of future HIV vaccines [4–6]. The extent to which we can close this gap will be key to the success of HIV vaccines in controlling the epidemic.
The objective of this systematic review is to synthesize results from over a decade of quantitative and qualitative investigations of HIV vaccine acceptability. Acceptability is based on hypothetical HIV vaccines that may be available in the future; at present there is no HIV vaccine approved for public licensure. Specifically we aim to understand: (1) rates of HIV vaccine acceptability; and (2) factors that may impact HIV vaccine acceptability.
Criteria for considering studies for this review
Types of studies included in this review include original research studies using qualitative, quantitative, or mixed methods design. Studies must have examined rates of HIV vaccine acceptability and/or barriers, facilitators, attitudes, socio-demographic characteristics, or other factors that impact acceptability of future HIV vaccines. Only studies published in peer-reviewed journals were included, with no language, geographical, or time restriction. Studies that did not use quantitative or qualitative methods, report original data, or focus on HIV vaccine acceptability were not included in this review. Types of participants included youth and adults reporting on HIV vaccine acceptability for themselves, others, or their children.
The primary outcome is HIV vaccine acceptability rates. The secondary outcomes are factors that impact HIV vaccine acceptability, including demographic characteristics (i.e., ethnicity), vaccine characteristics (i.e., efficacy), HIV vaccine attitudes (i.e., perceived benefits), HIV risk perceptions (i.e., perceived susceptibility), social factors (i.e., HIV stigma), and structural factors (i.e., HIV vaccine cost).
Search methods for identification of studies
We used a comprehensive search strategy to locate articles meeting the inclusion criteria across multiple electronic databases: ScholarsPortal, *MedLine, *Campbell Collaboration, *Cochrane Library, PsychInfo, ERIC, Social Work Abstracts, Dissertations Abstracts, and *OVID. The search strategy had no date/time restriction and included the terms HIV, AIDS, vaccine, accept*, attitude, uptake, barriers, and facilitators. We identified studies reviewed in this article by searching the databases using the designated terms and by searching references cited in peer-reviewed articles.
Data collection and analysis
Selection of studies
All titles and abstracts from the reference lists of articles were screened for inclusion. The full article was obtained when the first reviewer determined the article might meet inclusion criteria based on the objectives of the study. Once the shortlisted studies were compiled, two reviewers (P.A.N., C.L.) assessed each article for inclusion based on study type and outcome measures. Each article was independently reviewed and reviewers provided a summary statement describing reasons for exclusion. In case of disagreement, a third reviewer was available to arbitrate.
Data extraction and management
We developed a data extraction form using Microsoft Excel. The following data were extracted independently by two reviewers: article information (i.e., year of publication, author, journal); descriptive data (i.e., sample size, country, participant demographics); methods and study design; and outcomes/key findings.
Analysis of outcome measures
For studies that quantified HIV vaccine acceptability, we linearly transformed acceptability ratings onto a 0–100 scale. We further identified studies that provided acceptability ratings for both highly (80% – 95%) and partially (50%) efficacious hypothetical HIV vaccines, and conducted a paired sample t-test to assess statistically significant differences.
Based on a review of the articles, we developed a list of themes and subthemes related to vaccine acceptability. We reviewed each article several times to ensure findings were catalogued under relevant themes. To reduce bias and ensure that studies reported in several articles were not overrepresented, we included themes and acceptability scores from only one article per study.
We conducted meta-analysis on studies that examined similar correlates or predictors of HIV vaccine acceptability. Version 2 of Comprehensive Meta-Analysis Software  was used to calculate effect sizes for each variable with a random effects model to compensate for clinical and methodological diversity between studies.
We combined coefficients across studies for each variable that was examined to derive a global estimate of its correlation with HIV vaccine acceptability . We examined factors associated with HIV vaccine acceptability categorized according to the themes identified in the literature review. We calculated the Q statistic to assess homogeneity of correlations across studies  and the I2 index to assess the degree of heterogeneity .
All studies examining correlates of HIV vaccine acceptability that provided sufficient data (i.e., correlations, odds ratios, chi squared statistics, or t-values) were included in meta-analysis. We contacted corresponding authors, as needed, to provide missing and unreported data (e.g., group means, standard deviations, confidence intervals, sample sizes, demographics) to facilitate inclusion in meta-analysis. As no studies evaluated interventions to enhance HIV vaccine acceptability, we did not conduct meta-analysis on dichotomous (intervention vs. control group) data.
We then quantified and ranked each theme and subtheme based on its prevalence across studies. Fisher's exact test was used to assess significant differences between subthemes found in qualitative and quantitative articles, and independent sample t-tests to assess statistically significant differences between overall themes explored in qualitative versus quantitative articles.
Results of the search
The systematic literature search yielded 186 studies (see Fig. 1), with 100% agreement between reviewers in selecting relevant articles (n = 52). Of the relevant studies, 13 were excluded because they did not include original research results; did not examine barriers, facilitators, or attitudes regarding acceptance of future HIV vaccines; or did not focus on HIV vaccine acceptability. Thirty-nine remaining studies, 21 quantitative and 18 qualitative, were included in this analysis.
Both authors reviewed articles to determine whether the same sample and study were used more than once. We identified 30 original studies (n = 11 711) that are included in the final analysis. The 21 quantitative articles [5,11–30] reflect 18 original studies [5,11–13,15,16,18–20,22–30] (n = 11 155). The 18 qualitative articles [31–48] reflect 12 original studies [31,33–37,39,41,42,45,48] (n = 556). All studies included in this review (n = 30) were published in English. Studies were conducted in seven countries: the United States (n = 21), Canada (n = 2), South Africa (n = 2), Uganda (n = 2), the Dominican Republic (n = 1), Thailand (n = 1), and Vietnam (n = 1).
We identified seven major themes across the 30 studies: (1) demographic factors; (2) vaccine characteristics; (3) HIV risk perceptions; (4) HIV vaccine attitudes; (5) behavioral factors; (6) social factors; and (7) structural factors.
Quantitative methods included structured questionnaires that were self-administered (n = 10), interviewer-administered (n = 6), and administered by telephone (n = 2). Two self-administered and two interviewer-administered surveys were computer assisted. Sampling methods included: random sampling (n = 3); stratified random sampling (n = 3); venue-based sampling (n = 2); and convenience sampling (n = 10). Quantitative study samples included: adults at elevated risk for HIV infection [i.e., men who have sex with men (MSM), injection drug users (IDUs), low socioeconomic African Americans and Latinos, prison inmates] (n = 6); college students (n = 4); adolescents (n = 3); adults from the general population (n = 2); parents and their adolescent child (n = 1); healthcare practitioners (n = 1); and military personnel (n = 1).
Qualitative methods included semi-structured interviews (n = 7) and focus groups (n = 5). Sampling methods included: venue-based (n = 5), convenience (n = 4), snowball (n = 2), and online recruitment (n = 1). The sample composition for the qualitative studies included: high-risk adults (i.e., STD clinic attendees, MSM, IDUs, female sex workers) (n = 6), adolescents/young adults (n = 2), the general population (n = 2), parents (n = 1), and U.S. immigrants (n = 1).
HIV vaccine acceptability
Twenty-one studies published between 1996 and 2010, with sample sizes ranging from 14 to 1615, quantified HIV vaccine acceptability (see Table 1). Twenty studies (n = 7576) were included in the calculation of HIV vaccine acceptability scores because two studies [20,21] were based on the same sample. These 20 investigations were conducted among high-risk adults (i.e., STD clinic attendees, U.S. ethnic minorities at elevated risk of infection, MSM, prison inmates) (n = 6), college students (n = 4), adolescents (n = 4), parents (n = 2), adults in the general population (n = 2), military personnel (n = 1), and Asian and Pacific Islanders in the United States (n = 1).
HIV vaccine acceptability ranged from 37.2 to 94.0 on a 100-point scale, with a mean acceptability of 65.3 (SD = 20.0). The weighted mean HIV vaccine acceptability score was 65.6 (SD = 21.1).
Eleven studies [12,18,19,21,23,24,26,29,35,40,47] compared HIV vaccine acceptability at high (80–95%) versus moderate (50%) efficacy. Mean acceptability at high efficacy was 73.8 (SD = 9.2) versus 40.4 (SD = 20.2) at moderate efficacy (t(9) = 4.97, P < 0.001).
Meta-analytic results: correlates of HIV vaccine acceptability
Sufficient data were provided to examine the association between HIV vaccine acceptability and factors in five categories: vaccine characteristics, structural factors, HIV risk perceptions, HIV vaccine attitudes, and demographic factors. Table 2 reports weighted mean correlational effect sizes measuring associations of each factor with HIV vaccine acceptability and 95% confidence intervals, as well as the Q test of homogeneity and I2 index of between-study variability. Thirteen studies (n = 5023) were included in the meta-analysis [5,13,15,17,19,20,23–29].
The I2 index estimates the percentage of total variability in effect sizes due to between-study variability rather than sampling error; guidelines provided by Higgins and Thompson  indicate I2 values of 25% represent low, 50% medium, and 75% high heterogeneity. Based on these guidelines, there is high heterogeneity in the reported correlations between HIV vaccine acceptability and cost, efficacy, ethnicity, side effects/safety concerns, perceived vaccine benefits, fear of vaccines, duration of protection and pragmatic obstacles, and moderate heterogeneity among fear of needles/needle-administered vaccine and perceived susceptibility to HIV infection. Non‘risk group’ membership was a consistent correlate across studies.
To account for between-study variability, we used a random effects model in the meta-analysis. As the small number of studies examining each factor precluded sub-analysis of moderator variables, we examined individual results to identify potential reasons for between-study variability. Substantive (i.e., participant characteristics) and methodological (i.e., study design, sample size, sampling strategy) differences may have impacted between-study variability.
Among the four vaccine characteristics, acceptability was positively correlated with efficacy (r = 0.36, P < 0.001) and duration of protection (r = 0.16, P < 0.01) and negatively correlated with side effects/safety concerns (r = −0.21, P < 0.001) and fear of needles/needle-administered vaccine (r = −0.12, P < 0.05).
Structural factors associated with HIV vaccine acceptability were pragmatic obstacles (r = −0.29, P < 0.01) and cost (r = −0.28, P < 0.05). HIV risk perceptions associated with HIV vaccine acceptability were non‘risk group’ membership (r = −0.32, P < 0.001) and perceived susceptibility to HIV infection (r = 0.26, P < 0.001).
HIV vaccine attitudes associated with HIV vaccine acceptability were fear of vaccines (r = −0.20, P < 0.05) and perceived vaccine benefits (r = 0.17, P < 0.05).
Ethnicity was the only demographic variable significantly associated with HIV vaccine acceptability; being African American (in reference to white) was negatively associated with HIV vaccine acceptability (r = −0.13, P < 0.05).
Efficacy and non‘risk group’ membership had medium effect sizes on HIV vaccine acceptability, based on Cohen's  classification. Pragmatic obstacles, cost, perceived susceptibility to HIV infection, side effects/safety concerns, fear of vaccines, perceived vaccine benefits, duration of protection, ethnicity, and fear of needles/needle-administered vaccine had small effect sizes on acceptability.
Comparison between quantitative and qualitative investigations
Themes examined across the 18 quantitative and 12 qualitative original studies are identified in Table 3 in descending order of prevalence.
HIV vaccine characteristics was the most prominent theme across studies, referenced 75 times, with efficacy (n = 19), side effects/safety concerns (n = 17), and fear of vaccine-induced infection (n = 14), the most prevalent subthemes. HIV risk perceptions were evidenced 35 times: perceived vulnerability to HIV infection (n = 14), identification as a member of a risk group (n = 12), and perceived severity of AIDS (n = 9). Additional themes were demographics (n = 33), structural factors (n = 33), HIV vaccine attitudes (n = 31), social factors (n = 30), and behavioral factors (n = 28).
Across the seven thematic categories associated with HIV vaccine acceptability, social factors were examined at a significantly greater frequency in qualitative (n = 17) versus quantitative (n = 8) studies (t(8) = 3.46, P < 0.01). No other statistically significant differences in themes were found between quantitative and qualitative investigations.
Comparisons on the various subthemes using 2×2 contingency tables indicated that risk compensation (n = 9 vs. n = 4, P < 0.01), mistrust and conspiracy (n = 6 vs. n = 1, P < 0.01), perceived behavioral control (n = 5 vs. n = 1, P < 0.05), and the need for vaccine information (n = 8 vs. n = 3, P < 0.05) were significantly more likely to be studied in qualitative vs. quantitative investigations. Three sub-themes assessed in quantitative investigations were not examined in qualitative studies: pragmatic obstacles (n = 6), sexual orientation (n = 2), and healthcare provider attitudes (n = 1).
This systematic review reveals a moderate level of HIV vaccine acceptability (65.6 on a 100-point scale) across 20 studies totaling 7576 participants, with a wide range of acceptability (37.2–94.0) across studies. Overall, the significantly lower acceptability (40.4 vs. 73.8) of moderately (50%) versus highly (80%–95%) efficacious hypothetical HIV vaccines raises cause for concern, given the likelihood that initial HIV vaccines may be of low to moderate efficacy .
Meta-analysis results across 13 studies (n = 5023) indicate the primacy of HIV vaccine efficacy among an array of vaccine-specific, demographic, social, and structural factors on ratings of HIV vaccine acceptability. As public health officials may have little control over the level of efficacy of initial HIV vaccines, it will be important to develop evidence-informed public education and social marketing campaigns to increase the acceptability of partially efficacious HIV vaccines [51,52]. In fact, the lower the efficacy of initial HIV vaccines, the more crucial it becomes to achieve high levels of coverage in order to confer herd immunity . To that end, it is also important to identify other modifiable correlates of acceptability.
The significant impact of self-identification as a member of a risk group and perceived susceptibility to HIV infection on HIV vaccine acceptability in the meta-analysis suggests potentially modifiable factors beyond vaccine characteristics that may influence uptake. Other modifiable factors revealed in the meta-analysis include cost and pragmatic obstacles, among the most significant correlates of HIV vaccine acceptability. Structural interventions to subsidize out-of-pocket vaccine cost and facilitate access by addressing pragmatic obstacles (e.g., providing free transportation to clinics, promoting access through local clinics) may be important measures to facilitate broad HIV vaccine uptake.
A small but significant difference was observed in the lower HIV vaccine acceptability among African Americans versus whites, although based on only 2 studies. In fact, few studies measured or reported associations between HIV vaccine acceptability and age (n = 4), gender (n = 4), race/ethnicity (n = 3), sexual orientation (n = 2), or socioeconomic status (n = 0). Future investigations should explore the impact of sociodemographic characteristics on HIV vaccine acceptability, which may provide evidence to support tailored social marketing interventions to increase HIV vaccine uptake [51,52].
Another notable feature of this review is that over three-quarters (76.7%) of investigations of HIV vaccine acceptability were conducted in North America. Only three of 39 studies were conducted in sub-Saharan Africa, which suffers the highest global burden of HIV and might benefit greatly from even a partially efficacious vaccine . Modifiable structural factors, such as cost and pragmatic obstacles, may be expected to exert a strong influence on HIV vaccine uptake in resource-limited settings . Additionally, risk perceptions  and vaccine attitudes  associated with HIV vaccine uptake may vary as a function of culture and the severity of the epidemic, suggesting the importance of assessing local determinants of HIV vaccine acceptability, particularly in high–HIV prevalence low- and middle-income countries.
Limitations to this review include the absence of intervention studies inherent in evaluating the acceptability of a future (i.e., hypothetical) product and the relatively small number of studies conducted. The lack of intervention studies precludes using RevMan 5 for meta-analyses or application of Cochrane guidelines for assessing risk of bias for experimental and/or observational intervention studies. However, we systematically recorded sampling methods and research design to denote limitations to generalizability, and we calculated weighted mean scores and used a random effects model for the correlational meta-analysis. We also included both qualitative and quantitative studies to increase the breadth of the findings. An additional limitation is that the specific characteristics, cost and time frame for availability of approved HIV vaccines, are at best moving targets. As a result, the current stage of HIV vaccine acceptability research takes place in a dynamic and uncertain field; nevertheless, it constitutes an important contribution to the effectiveness of future HIV vaccines and to further acceptability studies that might be conducted in clinical trial and postmarketing stages  of HIV vaccine development.
The acceptability of future HIV vaccines is fundamental to their effectiveness . Given the sustained, large-scale financial investment required for HIV vaccine development, as well as the tremendous costs of lifetime HIV treatment, it may be prudent to pursue relatively low cost acceptability and other social science research in conjunction with basic and clinical HIV vaccine science [51,52]. With an estimated 2.7 million new HIV infections globally in 2008 alone , each year of delay in dissemination of future HIV vaccines will result in millions of new infections that might otherwise have been averted.
This review was supported by grants from the Social Sciences and Humanities Research Council, the Ontario HIV Treatment Network, and the Canada Research Chairs Program.
A poster based on an earlier version of this manuscript was presented at AIDS Vaccine 2009 in Paris (19–22 October), with an abstract published in Retrovirology 2009; 6(Suppl 3):P212. doi: 10.1186/1742-4690-6-S3-P212.
P.A.N. conceptualized the study, collaborated in reviewing articles and led writing of the manuscript. C.L. reviewed individual articles, executed the meta-analysis and contributed to writing the manuscript.
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