Follow-up household serosurvey in Northeast Brazil for Zika virus: sexual contacts of index patients have the highest risk for seropositivity.

Background Zika virus (ZIKV) is a mosquito-borne virus that is also transmitted sexually, however, the epidemiological relevance of ZIKV sexual transmission in endemic regions is unclear. Methods We performed a household-based serosurvey in Northeast Brazil to evaluate the differential exposure to ZIKV and chikungunya virus (CHIKV) among households. Individuals who participated in our previous arboviral disease cohort (indexes) were re-contacted and enrolled, and their household members were newly enrolled. Results The Relative Risk (RR) of sexual partners being ZIKV seropositive when living with a ZIKV-seropositive index participant was significantly higher, while this was not observed among non-sexual partners of the index. For CHIKV, both sexual and non-sexual partner household members living with a CHIKV-seropositive index had a significantly higher risk of being seropositive. In the non-index based dyadic and generalized linear mixed model analyses, the odds of sexual dyads having a concordant ZIKV PRNT result was significantly higher. We have also analyzed retrospective clinical data according to the participants' exposure to ZIKV and CHIKV. Conclusion Our data suggest that ZIKV sexual transmission may be a key factor for the high ZIKV seroprevalence among households in endemic areas and raises important questions about differential disease from the two modes of transmission.