Zika Virus and Microcephaly in French Polynesia

Summary and Comment |
March 28, 2016

Zika Virus and Microcephaly in French Polynesia

  1. Mary E. Wilson, MD

The estimated risk for microcephaly in the fetus was approximately 1% for women infected in the first trimester of pregnancy.

  1. Mary E. Wilson, MD

Zika virus (ZIKV) caused a large outbreak in 2013–14 in French Polynesia, where it was estimated to infect 66% of the general population. In a retrospective study, investigators estimated the probability of ZIKV infection during each week of the epidemic and identified all cases of microcephaly from September 2013 to July 2015. They developed a simple model to assess periods for risk during pregnancy and to estimate the associated risk for microcephaly.

Using a strict and standardized definition, the researchers identified eight cases of microcephaly during the study period, seven of which occurred during a 4-month period in 2014. Other data sets used in the analysis were the number of weekly consultations for suspected ZIKV infections, ZIKV seroprevalence at the start and end of the epidemic, and the number of births.

The mathematical models with satisfactory fit all included ZIKV infection in the first trimester of pregnancy as a period of risk for microcephaly. The model with the best fit overall included only the first trimester. Among women infected during the first trimester, an estimated 1% had infants with microcephaly.


The estimated rate of microcephaly in this population is substantially above the historical baseline of about 2 per 10,000 neonates (0.02%). The authors note that the 1% risk for microcephaly post–Zika infection is low compared with other viruses, such as rubella, that cause congenital abnormalities after first-trimester infections. Despite the low risk for microcephaly per pregnancy, the extremely high attack rate of Zika infection in a susceptible population can produce a high burden. The editorialist comments that other studies have suggested higher rates of post–Zika infection microcephaly. As yet unknown is whether risk is influenced by other factors, such as genetics, symptomatic (vs. asymptomatic) infection with Zika, and infection with dengue virus.

Editor Disclosures at Time of Publication

  • Disclosures for Mary E. Wilson, MD at time of publication Consultant / Advisory board GeoSentinel Surveillance Network (Special Advisor) Editorial boards UpToDate; Clinical Infectious Diseases; International Health; Infectious Diseases in Clinical Practice; Travel Medicine and Infectious Diseases


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