Higher educational attainment (secondary/higher) tended to be positively associated with the risk of infection among men (OR 3.7; 95% CI 0.8–17.8), especially among highly educated urban men (Table 2), and tended to be negatively correlated in women. However, among the youth, HIV prevalence was less than 0.1% in the highly educated compared with about 0.8% in noneducated men and women. Men without jobs and those working in agricultural sectors were found to have a very low HIV prevalence (<0.25%) compared with men in professional jobs (1.32%) or labour/service jobs (1.7%). No such differences were observed in women across job categories. The wealth index was found to be significantly correlated with HIV. Men who scored high on the wealth index had the highest HIV prevalence (OR 5.7; 95% CI 2.0–16.4). The pattern was similar among women (OR 2.0; 95% CI 1.0–4.2). Among sexually active people, 9.3% of men reported multiple sex partners, about 65% of whom were sex workers. Urban men reported multiple sex partners at about twice the rate (16.6%; 95% CI 13.0–20.9) of rural men (7.9%; 95% CI 6.6–9.4). In contrast, fewer than 1% of women reported multiple sex partners. Men who reported multiple sex partners were four times more likely to be infected than those with a single partner (95% CI 1.3–12.5) (Table 2).
This survey revealed the HIV prevalence to be relatively low and substantially below previous estimates based on the surveillance system among ANC attendees, that is, 0.6 vs. 1.9%. The prevalence did not differ by sex but was substantially higher in urban than in rural settings and among married individuals than among singles. Furthermore, a high age difference between spouses substantially increased the risk among women of being HIV infected. The two indicators used for SES revealed somewhat different association between sex and infection. The prevalence among men increased in correlation with both household wealth and education level, whereas among women, infection increased with household wealth but tended to decrease with education level. Women did not report nonmarital sex, whereas 10% of the men reported more than one sex partner in the previous year. The majority of these men reported paid sex. This finding, together with the observed sociodemographic pattern of HIV, suggests that most women were infected by their spouses.
Our study indicated a positive relationship between the household wealth index and HIV infection. A number of studies [18,19] showed that wealth, rather than poverty, has been positively related to HIV prevalence in the past. However, a possible interpretation is that the rates may differ by sex. In our setting, in which the HIV epidemic is mainly driven by sex work and male clients, paid and extramarital sex were likely to be strongly associated with higher SES among men. In contrast, the higher HIV prevalence among women in the wealthier quintiles is likely to be linked to their husbands' behaviours rather than the women themselves. Hargreaves and Glynn  have previously suggested this precise relationship. This seems to be supported by our finding of higher education to increase the risk of HIV among men but not among women. Studies [20–23] in the earlier stages of HIV epidemics in Africa reported a positive association between educational attainment and HIV. However, this pattern later reversed to a situation in which HIV became negatively associated with education level [23–26]. In Cambodia, where the magnitude of the epidemic is still relatively low, educated men may feel freer to engage in risky behaviours associated with different types of sex work. On the contrary, educated women may be more independent in terms of economic and negotiation power with their partners regarding safer sex behaviour. There is a need for intensive interventions aimed at curbing the transmission between spouses while sustaining the preventive programme efforts focused on commercial sex [8,27].
We found the likelihood of HIV infection among women to increase with the age difference between spouses. Furthermore, the age-specific HIV prevalence among women peaked about 5 years earlier than among men, and among spouses, the women were on average 4 years younger than men. This has also been reported in African settings [28,29]. As older men were more likely to be infected with HIV, the age mixing pattern seems to play a major role in the transmission from the older partners to the younger women [2,29]. In Africa, this type of sexual mixing pattern (young women forming sexual relationships with older partners) is observed in nonmarital sex, and there have been suggestions that efforts to reduce it could be an important HIV preventive strategy .
Surveys are faced with a number of potential biases. In our survey, the nonresponse was low and is not likely to have substantially affected the results. The second concern is that survival among HIV-infected individuals may be associated with SES , that is, longer survival among the better off than among the poor due to better access to care and treatment. However, this type of bias is not likely to be substantial as the scaling up of antiretroviral treatment began in 2005. Social desirability bias is well known in surveys and is especially true when asking women about past sexual practices . However, a study  among female factory workers in Cambodia found that fewer than 4% of women reporting never having had sex tested positive for sexually acquired herpes simplex virus. This study  confirmed a high reliability of behavioural data that were consistent with biological data. A final concern is the possible bias that may be due to the undersampling of some unreachable groups (i.e., high-risk groups, migrant workers). However, the Cambodia's Consensus Group found little impact on HIV estimates when taking into account these nonhousehold populations . In the result update of HIV estimates for Cambodia, both the trends of surveillance data and CDHS 2005 data were taken into account to get a better HIV prevalence estimate. The previous estimate that was purely based on ANC surveillance data is likely to overestimate the true prevalence .
In summary, this first national population survey revealed the HIV prevalence to be relatively low and substantially below previous estimates. The 100% CUP has been shown to be highly effective in Cambodia and should therefore remain as the core preventive strategy. Furthermore, there is an obvious need to develop interventions to reduce the HIV transmission from men to women. In this regard, efforts to empower women for better access to information, education and care seem critically important.
H.S., V.S. and C.C. conceived and designed the study. H.S. conducted the statistical analyses, interpreted the data and wrote the first draft of the paper. V.S. and C.C. were involved in the data interpretation. K.F. assisted and guided H.S. in the statistical analysis, data interpretation and provided conceptual ideas in drafting the paper. H.S. and all authors contributed to the final draft. All authors read and approved the final manuscript. We would like to thank the Cambodia's National Institute of Public Health for its permission for the authors to use the CDHS data.
H.S. has received A scholarship from the Quota Programme funded by the Norwegian government for his PhD programme at the University of Bergen. There are no conflicts of interest.
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