Monitoring the trend of the transmission rate of vertically acquired HIV infection: a simple method applied to Italian data

The objective of this study was to develop and validate a method for estima ting and monitoring over time the transmission rate of vertically acquired HIV infection at the population level. We estimated the annual number of children born to HIV-infected women in It aly in 1991-1994 by multiplying the seroprevalence rates, provided by Anony mous Unlinked HIV Serosurveys among Italian Newborns, by the annual number of births, provided by the Italian National Institute of Statistics. The nu mber of HIV-infected children was estimated by applying a simplified back-c alculation method to the incident cases of vertically acquired AIDS reporte d to the AIDS surveillance registry, using seven different estimates of the distribution of the incubation period identified through a literature sear ch. The annual vertical transmission rates were estimated by dividing the e stimated number of children with vertically acquired HIV infection by the e stimated number of births to an HIV-infected mother. Depending on the chosen distribution of the incubation period, the estimate d transmission rate for the four-year period ranges from 0.10 to 0.30. Five of the seven incubation distributions provided a rate failing within the v ery narrow interval 0.18-0.20. The method provided estimates of vertical tr ansmission rates consistent with those of longitudinal studies performed in European countries. The method presented here could be useful for monitori ng the impact of interventions aimed at reducing HIV vertical transmission rate.