Zika virus serologic responses during pregnancy in a dengue-endemic setting: A prospective cohort study in Thailand

BACKGROUND: Zika virus (ZIKV) infection during pregnancy has been associated with adverse birth outcomes, including severe neurological abnormalities. Confirmation of infection remains challenging due to the disease's predominantly asymptomatic or mild nature. While real-time reverse transcription polymerase chain reaction (RT-qPCR) is the gold standard, its use in clinical settings and surveillance is limited. Serological point-of-care tests offer alternatives but pose interpretation challenges in Flavivirus-endemic regions. METHODS AND FINDINGS: We conducted a prospective cohort of 199 pregnant women attending the antenatal clinic at Rajavithi Hospital, Bangkok, Thailand, in 2021-2022 to describe the incidence of ZIKV serologic responses detected by a rapid test and associated pregnancy outcomes. Participants were monitored using the DPP ZCD IgM/IgG (Zika/Chikungunya/Dengue IgM/IgG) System rapid immunochromatography test (Chembio), with RT-qPCR and plaque reduction neutralization testing performed for further assessment. A ZIKV serologic response was defined as a reactive ZIKV immunoglobulin M (IgM) result at enrollment or seroconversion from non-reactive to reactive ZIKV IgM or IgG between study visits. The DPP ZCD rapid test revealed incident ZIKV-reactive serologic responses at 15.4 per 100 person-years, with all affected participants being asymptomatic. All RT-qPCR results were negative, and participants with ZIKV IgG seroconversion had neutralizing antibodies against ZIKV and all 4 Dengue virus (DENV) serotypes. A prior history of DENV infection was associated with increased odds of ZIKV-reactive serologic response (OR 3.53, P value 0.046). CONCLUSION: This prospective study identified serologic responses detected by a ZIKV-reactive rapid test among pregnant women in Thailand. In a highly dengue-immune setting, these responses could reflect true ZIKV exposure, DENV reinfection, flavivirus immune boosting, or serologic cross-reactivity, and therefore cannot be attributed specifically to ZIKV. Rapid serologic testing may support screening and surveillance but cannot establish ZIKV infection or ZIKV-specific exposure without careful interpretation and additional testing.