Table 1.
Model parameters, data sources and values used in model
| Parameter | Source | Values used |
|---|---|---|
| Base case data | ||
| HIV infection rates | Department of Health and Population Development 199619 | 15% |
| Infant mortality | Harrison and Nielson 199520 | 35/1000 live births |
| Discount rate | World Bank 199321 | 5% |
| Natural rates of breast feeding | Wagstaff et al 199322; Ross et al 198323 | Proportion who breast feed from birth=95%; likelihood of stopping breast feeding/month=14% |
| Vertical transmission of HIV | ||
| Proportion of infants born to HIV positive mothers infected at birth: no intervention | Gray et al 1997* | 26% |
| Proportion of infants born to HIV positive mothers infected at birth: antiretroviral interventions | Centers for Disease Control 199818; Mayaux et al 19979; Saba 1999† | CDC regimen=13% (50% reduction); ACTG076 regimen=8% (70% reduction); PETRA regimen=16% (37% reduction) |
| Risk of transmitting the virus by breast feeding (cumulative): age specific rates derived from Weibull decay curve | Gray et al 1997* | Cumulative infection risk: 3 months=10%, 6 months=13%, 12 months=16%, 18 months=17% |
| Survival | ||
| HIV infected children: Weibull survival curve calibrated to survival data from Uganda and South Africa (figures are numbers surviving at end of period‡) | Spira and Msellati 1997§; Gray et al 1995¶ | 1 year=66%, 2 year=46%, 3 year=33%, 4 year=24%, 8 year=9% |
| Uninfected children: life expectancy at birth | Central Statistical Service 199724 | 66 years |
| Relative risk of mortality due to not breast feeding in HIV negative children (relative risks of health care use assumed to be same as for mortality) | Cunningham et al 199125** | 0-3 months=2.5, 3-6 months=2.0, 6-12 months=1.5 |
| Cost data | ||
| Unit costs of counselling, screening, antiretroviral drugs, and formula feed | Kinghorn et al 199826 | Counselling and screening=$7.30 (£4.86) per pregnant woman; antiretroviral drugs=from $400 (£267) (ACTG076) to $89 (£59) (PETRA regimen) per person; formula feed and bottles (6 month period) $60 (£40) |
| Average public healthcare expenditure on HIV positive children | Zwi 199927; Söderlund 199728; Söderlund and Peprah 199829; Brown and van den Heever 199430 | Range of 4 to 18 days of hospitalisation required per year depending on age; cost of hospitalisation=$57 (£38) per day; cost of outpatient attendances=$48–$183 (£32-£122) per year depending on age |
| Average public healthcare expenditure per HIV negative child | Söderlund and Peprah 199829; ReHMIS dataset 199531; Brown and van den Heever 199430 | 1-12 months old =$72 (£48) per year; ⩾1 year olds=$8.30 (£5.5) per year |
Gray GE, McIntyre JA, Lyons SF, XIth international conference on AIDS, Vancouver, 1997. †Saba J, 6th conference on retroviruses and opportunistic infections, Chicago, 1999.
Use of Weibull distribution to model hazards implies that hazards were different for every month since birth. These were calibrated to cumulative survival data derived from sources mentioned; calibration source figures are shown in right hand column. §Spira R, Msellati P. Care of HIV infected children in developing countries: a workshop for clinical research, Paris, 1997.
Gray GE, McIntyre JA, Pettifor JM, IXth international conference on AIDS and STD in Africa, Kampala, 1995. **It was not possible to extract exact figure from this review article because of differences in nature of adverse events measured by different studies. Figures used represent reasonable estimate of overall risk of mortality on basis of review data.