We have observed a marked decline in HIV prevalence among ANC attenders in Karnataka state from 2003 to 2007. This decline occurred in all age groups, and is statistically significant, but was particularly marked in women under the age of 25 years, declining from 1.40% in 2003 to 0.77% in 2007. HIV infections in young antenatal women are more likely to represent recent infections, and therefore reflect recent trends in HIV transmission dynamics. We have also seen that the majority of the decline occurred in the IPI districts. The multivariate statistical model showed that the average annual decline in HIV prevalence among ANC attenders in the IPI districts was approximately 12% greater than in the non-IPI districts. It is a tenet of the intensive intervention programme that the prevention of HIV transmission from FSW to their clients, largely through increased condom use, as well as treatment and prevention of other STI, will result in lower rates of HIV infection in male clients, and therefore less HIV transmission to their other sexual contacts, including their spouses . This should then be reflected in lower HIV prevalence rates among ANC attenders, particularly those in younger age groups, who are more likely to have recently become sexually active. Our findings are consistent with this hypothesis, although the magnitude of the decline was somewhat greater, and occurred over a shorter period of time than might have been expected from the intervention alone . As the intensive intervention was not assigned randomly to districts, however, it cannot be ruled out that different epidemic phases in the districts, or unmeasured systematic differences between IPI and non-IPI districts in behavioural or other population-level factors, may have contributed to the differential declines, and thereby biased our findings.
Over the past two decades, HIV surveillance among ANC attenders has been used as a principal method for analysing and monitoring trends in HIV transmission in the general population [22–28]. It has generally been the only source of HIV prevalence data that is readily available, involves a large segment of the population, and is collected on a regular basis. In different countries and time periods, such prevalence trends have been shown generally to increase during the early years of HIV epidemics, followed by stabilization of prevalence levels, and in more recent years, by declining trends in many countries. It is unclear, however, whether stabilizing or declining prevalence trends reflect real stabilizing or declining HIV incidence rates, or rather reflect other factors, such as high levels of mortality or migration. Indeed, cohort studies from Uganda and Tanzania have suggested the latter [29,30]. In other settings, however, declines in HIV prevalence do seem to have been associated with reductions in HIV incidence, largely through reductions in sexual risk behaviour [31–34], and this notion has been supported by mathematical modelling studies .
The extent to which HIV prevalence among ANC attenders reflects HIV prevalence in the general population of adult women, or adult men for that matter, is not always clear. Over 30 countries have now undertaken population-based demographic and health surveys, in which representative samples of the adult population are sampled, and tested for HIV . In countries with concentrated epidemics, such as India, these population-based surveys have generally shown lower HIV prevalence rates than seen in sentinel surveillance data from ANC attenders. The Joint United Nations Joint Programme on HIV/AIDS (UNAIDS) now estimates that HIV prevalence estimates from general population surveys are approximately 80% of those observed among ANC attenders in the same locations . Studies from a number of African countries have, however, shown that HIV prevalence in ANC attenders can also underestimate HIV prevalence in the general adult female population, because of reduced fertility among HIV-infected women, patterns of contraception use, mobility patterns, age of marriage, age-specific parity and the age distribution of ANC attenders [38–41]. In a recent study from South Africa, population-based estimates of HIV prevalence among all women and pregnant women were significantly lower than that those among ANC attenders, but it was unclear which estimates were more reflective of reality in the general population . The authors concluded that in areas where antenatal care coverage and contraceptive use are relatively high, HIV prevalence is underestimated in population-based surveys because of the unrepresentativeness of the population-based sample, according to age, residence and probably HIV status; whereas HIV prevalence in surveys of ANC attenders generally overestimates the true prevalence because of selection bias in terms of age of sexual debut and contraceptive use. Population-based surveys may also undersample high-risk populations, leading to the underestimation of HIV prevalence .
In India, HIV prevalence estimates in the general population have recently been revised downwards, largely as a result of data from the population-based National Family Health Survey, round 3 (NFHS-3) carried out in 2006 . NFHS-3 found infection rates among adults to be considerably lower than those observed in ANC attender surveillance, which had been one of the major inputs used to derive previous estimates. The figures from NFHS-3 for HIV prevalence in Karnataka state were 0.69% overall (0.85% among men and 0.54% among women) . This compares with the ANC attender sentinel surveillance figure of 1.13% for 2006, i.e. 39% lower overall, and 52% lower compared with all women. In a population-based survey in the Guntur district of neighbouring Andhra Pradesh state conducted in 2004–2005, HIV prevalence overall was 1.72% (1.74% among men and 1.72% among women), compared with 3% observed in ANC attender sentinel surveillance . This differential was attributed to the preferential use of public hospitals by people in lower socioeconomic strata (who are also at higher risk of HIV infection), as well as the selective referral of HIV-positive individuals to public hospitals. In a general population-based survey undertaken in Bagalkot district in northern Karnataka in 2003, HIV prevalence was 2.9% overall (2.5% among women and 3.3% among men), which was similar to that seen in ANC attender sentinel surveillance for the district (3.1%) . Similarly, a general population-based survey in Mysore district in southern Karnataka in 2005–2006 found an overall HIV prevalence in the general population of 0.8%, just less than the prevalence of 1.0% found among ANC attenders in Mysore in 2006 of 1% .
Declining trends in HIV prevalence in India, as determined from sentinel surveillance among ANC attenders under the age of 25 years, have been reported from four states in south India, using sentinel surveillance data from 2000–2004 [18,47]. Karnataka state was included in this analysis, although the decline in Karnataka was small, and as indicated above, sentinel surveillance coverage before 2003 was incomplete and sampling sites changed each year. A declining trend in HIV prevalence has also been reported recently from Pune, in neighbouring Maharashtra state . The Pune study was conducted among young, recently married, monogamous primigravid women attending ANC, so HIV infections in this group are likely to be recent infections, and HIV incidence was approximated through estimates of person-time exposure. Between 2002 and 2006, HIV incidence declined from 2.2 to 0.73 per 100 person-years, paralleling declines observed in HIV prevalence among ANC attenders during that period. The authors attributed this decline to decreased high-risk sexual behaviour among young, recently married men, contributing to a decreasing risk of HIV transmission to their wives.
There are limitations to analysing and comparing data obtained from sentinel surveillance, and attributing trend differentials on an ecological basis to specific activities or events. It is difficult to measure district-level variability precisely in programme coverage or quality, and changes in intervention intensity over time, although we have shown that condom distribution per FSW was much greater in the IPI districts. The intervention districts were also not chosen randomly, but districts that were felt a priori to be associated with a higher risk of HIV tended to be selected for the intensive intervention programme, although this was not always the case. We tried to control for variability in HIV risk among districts by including HIV prevalence per district at baseline (2003) in the statistical model, but there may be other, unknown factors that are important in determining HIV prevalence, which we could not capture. For example, it may be that the higher prevalence districts that were overrepresented among the IPI districts were in a later epidemic phase than the other districts, and that the decline in ANC attender prevalence had already begun before the intervention programme was put in place.
Whatever the reasons for the decline in HIV prevalence observed in sentinel surveillance among ANC attenders in Karnataka, there seems little doubt now that the decline has been real, and it seems likely that at least one of the major reasons for the decline has been the intensive HIV preventive programmes that have been undertaken over the past 5 years, within both the governmental and non-governmental sectors. An added benefit of the decline in HIV prevalence among pregnant women, whatever the cause, is the prevention of maternal to child transmission. There are approximately 1.2 million births each year in Karnataka, so a reduction in HIV prevalence among pregnant women of 0.6%, as has occurred between 2003 and 2007, translates into 7200 fewer HIV-infected pregnant women in 2007. Approximately 30% of infants born to these women would have been HIV infected, unless covered by programmes for the prevention of maternal to child transmission of HIV. The reduced HIV prevalence in the population of pregnant women would thus result in over 2000 HIV infections among newborns having been prevented in 2007. This compares to less than 200 such infections having been prevented in Karnataka in 2007 through screening for HIV in pregnancy and providing HIV-positive pregnant women and their newborns with antiretroviral therapy to prevent HIV transmission, as estimated from prevention of parent to child transmission programme data (Karnataka State AIDS Prevention Society, personal communication). In 2007, just over 202 000 pregnant women were screened for HIV in Karnataka, of which 2599 tested positive, and 858 were treated with nevirapine to prevent maternal to child HIV transmission. If 30% of the 858 women (257) would have had positive newborns without treatment, and if two-thirds of those were prevented by nevirapine, that would amount to approximately 172 cases of HIV prevented in newborns.
In conclusion, this study supports the notion that scaled-up, intensive, targeted HIV preventive interventions among high-risk groups can have a measurable and relatively rapid impact on HIV transmission in the general population, as represented by young ANC attenders. Such focused intervention programmes should be rapidly taken to scale in all HIV epidemics, and especially in concentrated epidemics such as is being experienced in India.
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