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Study objectives: To determine if factors associated with HIV infection vary between subpopulations of women resident in Great Britain.
Design: Case-control analyses on already existing datasets.
Setting: Great Britain.
Participants: 317 cases selected from a MRC Collaborative Study of HIV Infection in Women and 3635 controls selected from a National Survey of Sexual Attitudes and Lifestyles.
Main results: Factors associated with HIV infection varied among subpopulations of women. Among women heterosexually infected, factors associated with HIV infection were residence in London (odds ratio (OR) = 8.3; 95% confidence intervals (CI) 4.6 to 14.9), widowhood (OR = 47.6; CI 20.0 to 113.1), being black (OR = 25.2; CI 15.6 to 40.8) and particularly among white women, having 10 or more sexual partners (OR = 14.5; CI 5.1 to 41.3). Young age was important for black women heterosexually infected. Among women who shared needles, residence in London (OR = 19.0; CI 5.8 to 62.6) or Scotland (OR = 26.9; CI 8.0 to 90.4) and large numbers of sexual partners was important (OR = 19.6; CI 6.4 to 60.0); termination of pregnancy history was also important for those with fewer than 10 sexual partners (OR = 6.7; CI 3.4 to 13.1); and low social class was important for those with 10 or more sexual partners (OR = 4.1; CI 1.7 to 9.6).
Conclusions: Factors vary in importance and significance of association with HIV infection in different subpopulations of women resident in Great Britain. This diversity is also likely to occur in other populations. It is important to identify these differences between subpopulations. Prevention and control policies and activities for HIV/AIDS cannot treat all women as if they were the same.
In the UK, as in the rest of the world, a growing proportion of women is becoming infected with HIV. In the UK, the rate of increase in newly diagnosed and reported HIV infections among women is rising, in 1994 it was 50% higher for women than for men.(1,2) Seroprevalence studies in Europe have shown an increasing HIV incidence among women. (3) Of the three HIV transmission modes for women (heterosexual transmission, injecting drug use and the transfer of infected blood or blood products), heterosexual transmission accounts for the majority of HIV infections, with 78% of all reported HIV infections in women in the UK to March 1998 acquired this way. (1) Also, in 1998, among heterosexually infected people in the UK, the majority (53%) were black African. (4) Heterosexual transmission is also important among injecting drug users (IDUs). Most female IDUs have more than one risk factor for HIV infection, (5-8) as their drug use often puts them at risk of HIV infection directly (for example, via needle sharing) a nd indirectly (for example, via sex with a male IDU or sex for money or drugs). Some argue that these women are at greater risk as a result of heterosexual transmission than via needle sharing. (5,8)
This paper investigates factors associated with HIV infection in women aged 20-59 years resident in Great Britain, by analysing data collected for other purposes. These factors are expected to be of three types, those that lead to greater risk of infection, those that identify high HIV risk groups but do not in themselves lead to greater risk and those that are likely to be effects of infection. These three types of factors are all useful in identifying subpopulations of women at risk of HIV infection.
Case-control analysis of secondary data was carried out on a study population consisting of cases selected from a cohort of HIV infected women, the MRC Collaborative Study of HIV Infection in Women and controls selected from women in a population survey, the National Survey of Sexual Attitudes and Lifestyles. These two surveys are briefly described below, as design and methodology details have been given elsewhere. (10-12)
The MRC Collaborative Study of HIV Infection in Women consists of 505 HIV positive women recruited from 15 genitourinary medical centres in Britain and Ireland between June 1992 and August 1994. The geographical distribution of the centres reflects the spread of HIV infection in women in the UK, with 10 of the centres in London, three in other parts of SE England, one in Edinburgh and one in Dublin. All HIV positive women over the age of 18 years who were available for follow up visits were eligible for the study and written informed consent was obtained from all participants. Questionnaires were completed by investigators with data collected from face to face interviews and laboratory results. The data collected included personal background, sexual activity, obstetric history and probable route of HIV infection. Data on the probable route of HIV infection were derived in a hierarchical manner. Firstly, IDUs who had shared needles, either directly or indirectly, were classified as being infected via injectin g drug use. Then women who could have been infected heterosexually, but had never shared needles were classified as being infected via sex. Injectors who had not shared needles were included in this group. Infected blood products was selected as the transmission mode for women who were definitely infected by needle stick injuries or had had transfusions between 1975 and 1985, had not shared needles and could not have been infected heterosexually. (12) Social class was determined by the woman's longest held occupation, whether she was currently employed or not, according to the register general's classification.
To determine how representative cases in the analyses are of all HIV positive women in the general population, a comparison was done between women from the MRC Collaborative Study of HIV Infection in Women and HIV reports (from voluntary testing) for women in Great Britain, as collected by the Communicable Disease Surveillance Centre (CDSC). (13) A comparison was also done between the cases and CDSC AIDS case reports. (13) AIDS case reports are likely to be more complete records of HIV infection in the population, as all HIV infections would eventually progress to AIDS, whereas HIV reports include only those HIV infected subjects who underwent an HIV test. Women …