Brazil’s Missing Infants: Zika Risk Changes Reproductive Behavior

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The Zika Virus gained global notoriety in 2015 when it infected large parts of Brazil and spread rapidly to other countries in the Americas. Disturbing images of infants born with microcephaly filled the newspapers, and early studies raised concerns about the virus’s effect on human reproductivity as it was proven to affect the fertility of rats and nonhuman primates. Upon detecting the virus in human amniotic fluid samples, Brazil declared a national medical emergency, and the government began issuing statements cautioning pregnant women to take preventative action against disease-spreading mosquitoes. Around this time, Brazil began to see a decrease in birth cohort sizes, hinting at a possible reduction in fertility due to the Zika virus. Researchers sought to understand the cause of this birthing reduction, hoping to understand whether the decrease resulted from the direct health effects of the virus or from a conscious decision among women to delay having children due to awareness of its potential consequences. 

The nature, timing, and spatial distribution of the Zika virus epidemic in Brazil provide an unusual vantage point into likely causal effects. If population fertility were to fall because of Zika’s effect on maternal and infant health, researchers expected missing births to be detectable either 6-8 months after the outbreak (due to Zika’s impact on spontaneous pregnancy loss) or 8–10 months after the outbreak peak (due to Zika’s impact on conception rates). By contrast, fertility decline resulting from behavioral attempts to delay fertility and avoid pregnancies (including elective termination) would be expected at a later time, at least 6–10 months after the announcement of the epidemic, which, in some locations, occurred well after the peak in cases. 

While there is evidence of a real biological effect due to Zika, the data in Brazil showed strong evidence of a much larger behavioral response to learning about the disease. In the areas most affected by the disease, birth rates were reduced by as much as 25% of their expected value 10-15 months after the emergency declaration, not earlier, as they would have if health effects were the primary drivers. In the city of Recife, the center of the epidemic, reductions peaked (at 19.8 %) in November 2016, one year after the national emergency declaration, not December 2015 (nine months after the peak infection rate). Both cases indicate that birth reduction was primarily driven by a behavioral response to learning about the disease rather than by the disease itself.  

Methods and Results

Researchers used information from the Brazilian Birth Registry on all births in Brazil between January 2011 and December 2017. They aggregated this information to generate monthly birth counts by microregion (defined as areas containing common employment and goods markets). Because this study is centered on a health emergency associated with a mosquito-transmitted disease, they retrieved reports published by the Ministry of Health’s Secretaria de Vigilancia em Saude on the Larval Index Rapid Assessment for Aedes aegypti (Levantamento de Indice Rapido do Aedes aegypti (LIRAa)). The results were used to construct an index indicating the percentage of households visited that tested positive for Aedes mosquitos.  

Researchers then compared the plausible biological effects with the changes in fertility observed after information about Zika was widely distributed, using two complementary quasi-experimental design approaches to rule out competing explanations for behavioral change. By unpacking Brazilian demographic change accompanying the Zika epidemic, researchers were able to analyze a central aspect of fertility: whether, when, and which people time the birth of their offspring to align with reduced risks to maternal and infant health, i.e., behavioral timing, and demonstrate evidence of public response to explicit policy recommendations from the federal administration to slow down the fertility rate. This particular case is all the more striking because realized fertility reductions occurred in a region of Brazil where the majority of births are unintended, and abortion is illegal and difficult to access safely, raising questions about access to family planning as a fundamental human right. 



Marcos Rangel (Duke University), Jenna Nobles (University of Wisconsin-Madison), Amar Hamoudi (University of Wisconsin-Madison)


  • National Institute of Child Health and Human Development (NICHD)
  • University of Wisconsin–Madison Graduate School
  • Center for Demography and Ecology at Wisconsin
  • Sanford School of Public Policy Pilot Project Fund 

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Human Development, Health, South America