Introduction In Brazil, 92,210 HIV-infected pregnant women were notified from 2000 through June 2015, most of whom living in the Southeast (40.5%) and South (30.8%). Detection rates of pregnant women living with HIV in Brazil have increased in the last ten years. In 2005, rates of seropositivity for newborns were as high as 2.0 cases per 1000 live births, which increased to 2.6 in 2014, indicating a 30.0% rise. The aim of this study was to analyse the frequency of seroconversion among newborns to HIV-positive mothers living in southern Santa Catarina, Brazil, from 2005 through 2015.

Methods A cross-sectional study was conducted to collect secondary data. All the newborns that were exposed to HIV by vertical transmission, and attended the municipal healthcare centre between 2005–2015 participated in the study. The study included all infants between 0 and 18 months of age, exposed to HIV vertically, who attended the healthcare centre that serves 18 municipalities in southern Santa Catarina, Brazil, over the 2005–2015 period.

Results During the study period, there were 93 exposures to HIV, of which 3 (3.2%) seroconversions were confirmed and 2 (2.1%) died of AIDS during the follow-up period. Seroconversion was associated with breastfeeding (PR=29.3; 95% CI=9.6–89.2; p=0.002) and the lack of antiretroviral therapy during pregnancy (PR=21.0; 95% CI=2.4–184.5; p=0.006).

Conclusion The results from this study allowed us to conclude that seroconversion among newborns was 5.4%, resulting in a rate of 3.4 cases per 1000 live births, which was higher than the national average. Seroconversion was associated with breastfeeding and the lack of antiretroviral therapy during pregnancy.