Zika in Infants and Pregnancy (ZIP) study: results from a prospective international cohort study of prenatal Zika virus infection and adverse fetal and infant outcomes

Background: Before Zika virus (ZIKV) infections were observed in the Americas, an association between ZIKV and microcephaly or other congenital malformations was not well documented. Initial reports suggested strong associations between ZIKV and congenital malformations, but plausible estimates of causal effects from prospective studies with adequate sample size and covariate data were few. Methods: From 2016–2018, the Zika in Infants and Pregnancy (ZIP) study enrolled pregnant people before 18 weeks gestation or with confirmed symptomatic ZIKV in a prospective cohort across 10 sites in South and Central America, and in Puerto Rico. Pregnancies were followed monthly through delivery and 6 weeks postpartum. Infants were followed quarterly to age 12 months. Prespecified co-primary analyses evaluated the associations between a composite endpoint of adverse fetal, neonatal, and infant outcomes with intrauterine ZIKV exposure overall and with symptomatic intrauterine ZIKV exposure. Secondary analyses separately evaluated the association of intrauterine ZIKV exposure with individual components of the primary endpoint. Results: Six thousand one hundred pregnant participants were included in the primary analysis, including 61 with ZIKV infection during pregnancy confirmed by a ZIKV-specific RNA test. For the primary analyses, the relative risk (RR) for the composite endpoint associated with any ZIKV exposure was 1.64 (95% CI: 0.65, 4.13) and with symptomatic ZIKV exposure 1.08 (95% CI: 0.15, 7.64). Sensitivity analyses provided similar results. Secondary analyses showed significant adjusted RRs [95% CI] for stillbirth (4.28 [1.39, 13.21]), infant death within six weeks (6.20 [1.08, 35.60], and fetal loss before 20 weeks (3.72 [1.82, 7.59]). Conclusions: The ZIP study identified an elevated but not statistically significant risk of the primary composite outcome with intrauterine ZIKV exposure, and a significantly increased risk of some adverse fetal and infant outcomes with intrauterine ZIKV exposure in secondary analyses. Fewer than expected infections observed during pregnancy, coincident with a waning epidemic, limited study power to evaluate risk. Combining data from multiple cohorts for future meta-analysis may better define the risks of intrauterine ZIKV exposure. Trial Registration: NCT02856984. Registered August 5, 2016. Retrospectively registered.