Circulating Zika and dengue IgA-NS1 marks early postnatal exposure without significant association with congenital Zika syndrome-related microcephaly

Background Elevated preexisting maternal IgG-NS1 to DENV, rather than to ZIKV, has been associated with a reduced risk of microcephaly in congenital Zika syndrome (CZS), suggesting a protective role. While serological analysis of IgG and IgM has physiological limitations in infants, IgA-NS1 represents a marker of host exposure to orthoflavivirus, as it is not transferred transplacentally and reflects early active immune responses. This study evaluated circulating IgA-NS1 responses to ZIKV and DENV in mothers and infants prenatally exposed to ZIKV in southern Colombia, a dengue hyperendemic region. Methods In this multicentric study, we analyzed 164 mother-infant pairs with confirmed gestational ZIKV exposure during the Colombia 2015-2016 epidemic. Using optimized ELISAs with well-characterized clinical samples, we simultaneously assessed circulating IgA-NS1 responses against ZIKV and DENV1-4. Neurological outcomes were evaluated through pediatric follow-up to determine associations between IgA-NS1 and CZS-related microcephaly. Results IgA-NS1 ELISAs for ZIKV and DENV1-4 achieved sensitivity/specificity of 94%/97% and 86%/85%, respectively. DENV-exposed samples showed 26% cross-reactivity against ZIKV NS1. There was no evidence of placental transfer, and IgA-NS1 levels for ZIKV and DENV were comparable between mothers and infants with or without CZS-related microcephaly. However, circulating ZIKV and DENV IgA-NS1 increased postnatally and were higher in older children. Conclusion While circulating IgA-NS1 serves as a promising postnatal marker of orthoflavivirus exposure in infants due to its stability and lack of placental transfer, it does not appear to contribute to protection against severe CZS outcomes. These findings demonstrate that protective NS1-mediated immunity against microcephaly may be isotype-dependent.