A recently published study casts doubt on two major claims about Zika virus infection and microcephaly: first, that there was an outbreak of newborn microcephaly in Brazil in 2015; and second, that maternal Zika virus infection was responsible for the outbreak.

The Zika virus is a mosquito-borne agent that can cause fever, rash, headache, red eyes, and muscle and joint pain. Microcephaly is a birth defect in which a baby is born with a head smaller than expected, because the brain did not develop properly during pregnancy. A baby’s head becomes larger as the brain grows. Babies with very small heads are classified as cases of severe microcephaly.

The study appeared in Annals of Epidemiology, a peer-reviewed, international medical journal devoted to epidemiological research and method development.

“The goal was to carefully evaluate and make sense of the existing data,” said study author Leonelo Bautista, MD, MPH, DrPH, associate professor of population health sciences at the UW School of Medicine and Public Health. Specifically, the study aimed to evaluate whether an outbreak of microcephaly actually took place in northeast Brazil in 2015, and to assess the evidence supporting a causal link between Zika infection among pregnant women and microcephaly in their babies.

According to the study, no outbreak of microcephaly occurred in Brazil in 2015, because the number of all microcephaly and severe microcephaly cases in that year were similar or lower than in previous years. Also, changes in the number of Zika virus infections corresponding to 11 to 18 weeks of pregnancy were not followed by changes in the number of cases of microcephaly. According to Bautista, major factors leading to the perceived outbreak of microcephaly were overdiagnosis, due to heightened rumors and media attention, changes in the traditional definition of microcephaly, and low notification of cases to the regular Brazilian Live Birth Information System.

Moreover, several factors led Bautista to conclude that while there were cases of microcephaly among babies born from women who had a Zika virus infection during pregnancy, there wasn’t enough evidence to support that Zika virus causes microcephaly. Indeed, in observational studies the prevalence of microcephaly was only 30 percent higher in newborns of Zika-infected mothers, an increase that could be explained by chance and by systematic errors in the design of those studies.

The study makes clear that while responding to a perceived health crisis is sensible, the response should match the level of risk and must consider potential undesirable effects for Latin American countries that face larger public health problems, as scarce resources may be diverted from far more burdensome health threats. These countries may also experience significant economic losses in the tourist industry. Moreover, personal decisions based on fear of the consequences of Zika virus infection could lead to unnecessary suffering, deaths, and untoward lifetime consequences.