Current state of the Outbreak
The current pattern of Zika-associated disease observed in Brazil represents a significant public health risk. Based on estimated incidence of Zika infection in Brazil, and the confirmed incidence of excess microcephaly, Brazilian mothers infected with Zika during pregnancy are between 3,700 to 11,000 times more likely to deliver infants with primary microcephaly than uninfected mothers. The current best estimates for the incidence of Zika virus infection in Brazil range between 440,000 and 1,300,000 cases, resulting in an estimated annual (2015) attack rate between 0.30% and 0.88%. 3,530 confirmed cases of Zika-associated primary microcephaly were reported between October 22, 2015 and January 9, 2016, a rate predicting approximately 16,000 cases of Zika-associated primary microcephaly in Brazil during 2016 assuming constant incidence. The estimated risk that a mother infected with Zika during pregnancy will deliver a baby with primary microcephaly ranges from 18% to 53%. Incidence estimates for Zika-associated Guillain-Barre syndrome (GBS) in adults and other types of birth defects other than microcephaly are not yet available. In some Brazilian hospitals adult GBS mortality during 2015 may be as high as 50%.
Methods and policies designed to delay the spread of the virus into uninfected regions will buy critical time to develop medical countermeasures. Rapid development and deployment of diagnostic tests for Zika to clinical laboratoriesshould be a top priority. Cases acquired abroad will continue to be identified in non-endemic countries and must be differentiated from autochthonous outbreaks. Strategies for rapidly developing specific medical countermeasures may involve drugs and biologicals with antiviral and immunotherapeutic activities for use by pregnant women in their first and second trimesters. Surges in GBS may require augmented availability of intensive care and ventilator support (including trained personnel), equipment for plasma exchange, stockpiles of intravenous immunoglobulin, and development of novel therapeutic strategies. In contrast, development of a general use prophylactic vaccine for Zika virus may require considerable time and careful evaluation to mitigate typical vaccine-associated risks in previously healthy unexposed general populations for which Zika infection poses modest morbidity and mortality (GBS) risks.