Infection-related microcephaly after the 2015 and 2016 Zika virus outbreaks in Brazil: a surveillance-based analysis

Abstract

Background: On Nov 11, 2015, the Brazilian Ministry of Health declared a Public Health Emergency of National Concern in response to an increased number of microcephaly cases, possibly related to previous Zika virus outbreaks. We describe the course of the dual epidemics of the Zika virus infection during pregnancy and microcephaly in Brazil up to Nov 12, 2016, the first anniversary of this declaration.

Methods: We used secondary data for Zika virus and microcephaly cases obtained through the Brazilian Ministry of Health's surveillance systems from Jan 1, 2015, to Nov 12, 2016. We deemed possible Zika virus infections during pregnancy as all suspected cases of Zika virus disease and all initially suspected, but later discarded, cases of dengue and chikungunya fever. We defined confirmed infection-related microcephaly in liveborn infants as the presence of a head circumference of at least 2 SDs below the mean for their age and sex, accompanied by diagnostic imaging consistent with an infectious cause, or laboratory, clinical, or epidemiological results positive for Zika virus or STORCH (infectious agents known to cause congenital infection, mainly syphilis, toxoplasmosis, cytomegalovirus, and herpes simplex virus). We excluded cases of congenital anomalies or death without microcephaly. We analyse the spatial clustering of these diseases in Brazil to obtain the kernel density estimation.

Findings: Two distinct waves of possible Zika virus infection extended across all Brazilian regions in 2015 and 2016. 1 673 272 notified cases were reported, of which 41 473 (2·5%) were in pregnant women. During this period, 1950 cases of infection-related microcephaly were confirmed. Most cases (1373 [70·4%]) occurred in the northeast region after the first wave of Zika virus infection, with peak monthly occurrence estimated at 49·9 cases per 10 000 livebirths. After a major, well documented second wave of Zika virus infection in all regions of Brazil from September, 2015, to September, 2016, occurrence of microcephaly was much lower than that following the first wave of Zika virus infection, reaching epidemic levels in all but the south of Brazil, with estimated monthly peaks varying from 3·2 cases to 15 cases per 10 000 livebirths.

Interpretation: The distribution of infection-related microcephaly after Zika virus outbreaks has varied across time and Brazilian regions. Reasons for these apparent differences remain to be elucidated.

Funding: None.