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Recent scientific findings: Association between Zika virus infection and microcephaly in Brazil, January to May, 2016: preliminary report of a case-control study

ECDC comment

​​A preliminary report by de Araújo et al. of a case-control study aiming to test the hypothesis of the association between the occurrence of microcephaly and Zika virus infection during pregnancy was published in The Lancet Infectious Diseases. This publication provides the preliminary results of a prospective case-control study conducted in eight public hospitals in Recife,Pernambuco, Brazil during the first half of 2016.

​A preliminary report by de Araújo et al. of a case-control study aiming to test the hypothesis of the association between the occurrence of microcephaly and Zika virus infection during pregnancy [1] was published in The Lancet Infectious Diseases. This publication provides the preliminary results of a prospective case-control study conducted in eight public hospitals in Recife, Pernambuco, Brazil during the first half of 2016. Early findings were presented at the International Zika Summit 2016 at Institut Pasteur (Paris, April 2016) and in the Cura Zika symposium at the University of Pittsburgh Graduate School of Public Health in May 2016 [2,3], as well as being cited in an ECDC risk assessment in May 2016 [4].

A total of 32 cases and 62 controls were recruited prospectively between January and May 2016. Cases were neonates with microcephaly (head circumference below two standard deviation smaller than the mean for sex and gestational age in the Fenton growth chart). Controls were live neonates without microcephaly, with no brain abnormalities identified by transfontanellar ultrasonography and no major birth defects detected by physical examination. For each case, two controls were selected from the first neonates born from the following morning in one of the study hospitals, matched by expected date of delivery and health region of residence.

Zika virus infection was confirmed by using RT-PCR and tests for Zika virus specific IgM antibodies (ELISA). The tests were performed on sera of mothers and neonates at birth (on umbilical cord blood, and if not available, on peripherical blood) for both cases and controls. For cases, the tests were also performed on cerebrospinal fluid samples. Neonates were considered to have laboratory-confirmed Zika if IgM detection or RT-PCR was positive.

A crude odds ratio of 31.7 (95% CI= 4.7 - ∞) was estimated for the association between microcephaly and laboratory confirmation of Zika virus infection considering serum results only. This preliminary analysis indicates that neonates exposed to Zika virus infection during pregnancy were almost 32 times more likely to present with microcephaly than neonates not exposed. Considering serum or cerebrospinal fluid samples for exposure classification, a crude odds ratio of 55.5 (95% CI= 8.6 - ∞) was estimated. The estimates remain of the same order of magnitude after adjustment on maternal education or maternal age.

ECDC comment

Despite the limitation of this preliminary report, these results re-enforce the evidence of a causal link between Zika virus infection during pregnancy and microcephaly, in line with the earlier statement from WHO about Zika causality [5]. In addition, the study provides a first estimate of the strength of this association. The inclusion of new cases and controls is ongoing and is expected to provide more accurate estimates of odds ratios, as well as to clarify the impact of possible confounding factors.

References

  1. de Araújo TVB, Rodrigues LC, de Alencar Ximenes RA, de Barros Miranda-Filho D, Montarroyos UR, de Melo APL, et al. Association between Zika virus infection and microcephaly in Brazil, January to May, 2016: preliminary report of a case-control study. Lancet Infect Dis. Epub 2016 Sep 15.
  2. Martelli CT. Oral communication: Congenital Zika syndrome: epidemiology and ongoing studies in Brazil. 2016. In: Cura Zika symposium 2016 [Internet]. .
  3. Thalia VBA, Rodrigues L. Microcephaly and Zika infection: preliminary report of a case-control study. 2016. In: International Zika Summit, 25-26 April 2016 [Internet]. Paris: Institute Pasteur. 
  4. European Centre for Disease Prevention and Control. Rapid risk assessment - Zika virus disease epidemic. Sixth update, 20 May 2016 [Internet]. Stockholm: ECDC; 2016. 
  5. World Health Organization. Zika causality statement - Zika virus infection: update on the evidence for a causal link to congenital brain abnormalities and Guillain-Barré syndrome, 7 September 2016 [Internet]. Geneva: WHO; 2016.

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