Zika virus infection was declared a public health emergency of international concern in February 2016 in response to the outbreak in Brazil and its suspected link with congenital anomalies. In this study, we use notification data and disease natural history parameters to estimate the basic reproduction number (R 0) of Zika in Rio de Janeiro, Brazil. We also obtain estimates of R 0 of dengue from time series of dengue cases in the outbreaks registered in 2002 and 2012 in the city, when DENV-3 and DENV-4 serotypes, respectively, had just emerged. Our estimates of the basic reproduction number for Zika in Rio de Janeiro based on surveillance notifications (R 0 = 2·33, 95% CI: 1·97-2·97) were higher than those obtained for dengue in the city (year 2002: R 0 = 1·70 [1·50-2·02]; year 2012: R 0 = 1·25 [1·18-1·36]). Given the role of Aedes aegypti as vector of both the Zika and dengue viruses, we also derive R 0 of Zika as a function of both dengue reproduction number and entomological and epidemiological parameters for dengue and Zika. Using the dengue outbreaks from previous years allowed us to estimate the potential R 0 of Zika. Our estimates were closely in agreement with our first Zika's R 0 estimation from notification data. Hence, these results validate deriving the potential risk of Zika transmission in areas with recurring dengue outbreaks. Whether transmission routes other than vector-based can sustain a Zika epidemic still deserves attention, but our results suggest that the Zika outbreak in Rio de Janeiro emerged due to population susceptibility and ubiquitous presence of Ae. aegypti.

We report Zika virus (ZIKV) vertical transmission in 130 infants born to PCR+ mothers at the time of the Rio de Janeiro epidemic of 2015-2016. Serum and urine collected from birth through the first year of life were tested by quantitative reverse transcriptase polymerase chain reaction (PCR) and/or IgM Zika MAC-ELISA. Four hundred and seven specimens are evaluated; 161 sera tested by PCR and IgM assays, 85 urines by PCR. Sixty-five percent of children (N = 84) are positive in at least one assay. Of 94 children tested within 3 months of age, 70% are positive. Positivity declines to 33% after 3 months. Five children are PCR+ beyond 200 days of life. Concordance between IgM and PCR results is 52%, sensitivity 65%, specificity 40% (positive PCR results as gold standard). IgM and serum PCR are 61% concordant; serum and urine PCR 55%. Most children (65%) are clinically normal. Equal numbers of children with abnormal findings (29 of 45, 64%) and normal findings (55 of 85, 65%) have positive results, p = 0.98. Earlier maternal trimester of infection is associated with positive results (p = 0.04) but not clinical disease (p = 0.98). ZIKV vertical transmission is frequent but laboratory confirmed infection is not necessarily associated with infant abnormalities.

Background: In utero transmission of Zika virus (ZIKV) can lead to adverse infant outcomes, but vertical transmission rates are unknown.Methods: Antenatally ZIKV-exposed children were followed prospectively since the time of the Rio de Janeiro epidemic in 2015-16. Serum and urine specimens were collected from infants from birth throughout the first year of life. Specimens were tested by quantitative reverse transcriptase polymerase chain reaction (PCR) and/or IgM antibody capture Zika MAC-ELISA.Infants had neurodevelopmental evaluations, brain imaging, eye examinations, and hearing assessments.Results: Over time 130 in utero ZIKV-exposed (mothers PCR+) children were tested with 407 specimens evaluated: 161 sera were tested by PCR and IgM assays, 85 urines by PCR; 84 children (65%) were positive in at least one assay. Among 94 children tested within 3 months of age, 70% were positive (39% serum PCR, 48% urine PCR, 39% IgM). After 3 months, 33%were positive by any laboratory method. Five children were intermittently PCR+ beyond 200 days of life. Concordance between IgM and PCR results was 52%, sensitivity 65%, specificity 40% (with any positive PCR result as the gold standard); IgM and serum PCR were 61% concordant; serum and urine PCR 55%. Most children (65%) were clinically normal. Positive results were seen in 29 of 45 children (64%) with abnormal findings and 55 of 85 normal children (65%), p=0٠98. Earlier maternal trimester of infection was associated with positive infant laboratory results but not infant clinical disease (p=0٠04).Conclusions: ZIKV has a high in utero transmission rate. Laboratory confirmed infection is not necessarily associated with abnormal infant findings.