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Maternal-fetal transmission and adverse perinatal outcomes in pregnant women infected with Zika virus: prospective cohort study in French Guiana

BMJ 2018; 363 doi: (Published 31 October 2018) Cite this as: BMJ 2018;363:k4431

Re: Maternal-fetal transmission and adverse perinatal outcomes in pregnant women infected with Zika virus: prospective cohort study in French Guiana

Pomar et al.[1] reported a 26% risk of maternal-fetal transmission of Zika virus (ZIKV), and a greather than 4-fold increase in the risk of adverse outcomes in ZIKV-infected fetuses/neonates, in a cohort of women from French Guiana. These findings are likely a consequence of methodological flaws in their study.

They recruited women with ZIKV infection symptoms admitted to the prenatal unit (PU) or the Department of Obstetrics and Gynaecology (DOBGYN) of the CHOG. Admission diagnoses were not reported, making it uncertain to whom study findings might apply. Women with pregnancy complications, abnormal fetal ultrasound, and at high risk of being ZIKV-infected were likely oversampled. This explains a prevalence of maternal ZIKV infection (MZVI) 7.60 times higher than in the general population (29.5% vs. 3.9%),[1] and a prevalence of central nervous system (CNS) fetal ultrasound abnormalities in non-infected women (4.3%)[2] at least six times higher than in other developing countries.[3, 4, 5] Concerns about the role of MZVI on fetal abnormalities made women with both conditions more likely admitted and included in the study than those with only one condition.[2] Consequently, the risk of congenital ZIKV infection (CZVI) and the association between MZVI or CZVI and fetal abnormalities were necessarily overestimated in the cohort, even if these conditions were not associated in the whole population.

Investigators used an in-house MAC-ELISA assay with a sensitivity of 94.4% (95% confidence interval: 86.2 to 98.4) and a specificity of 60.0% (48.4 to 70.8) for serologic diagnosis of ZIKV infection.[6] Re-testing individuals with other flavivirus infections for neutralizing antibodies could have increased specificity up to a maximum of 80% (65.4 to 90.4).[6] Therefore, even if 30% to 50% of the participants were true cases, 1/4 to 1/6 of serologically-diagnosed cases should have been false positive. Though this limitation did not apply to RT-PCR tests, RT-PCR was used only in mothers admitted to the DOBGYN.

Unfortunately, the impact of errors in ZIKV infection diagnosis is uncertain, because authors failed to report the distribution of participants by recruitment source (PU/ DOBGYN) and diagnostic test (IgM/RT-PCR).

CZVI was defined as a positive RT-PCR test in placenta, amniotic fluid, or fetal/neonatal cerebrospinal fluid, urine, or blood. However, RT-PCR in amniotic fluid and placenta is unreliable for a diagnosis of CZVI.[7, 8, 9] CZVI cases were also detected through IgM for ZIKV in umbilical cord/neonatal blood or cerebrospinal fluid. Positive cases were retested at age 3-days, to exclude maternal contamination of umbilical cord blood. Nevertheless, 20% (10 to 35) of ZIKV IgM positive neonates, without other flavivirus infections, would still be false positive.[6]

In a previous report based on the first 110 births in this cohort[2] the risk of CZVI was 10.9% (5.8 to 18.3). Therefore, 64 new CZVI cases from 181 mothers were added in the current report[1], for a risk of 35.4% (28.4 to 42.8). The probability of this increase in risk of CZVI happening by chance is about 1 in 241,000. Authors provided no explanation for this inconsistency.

The authors’ definition of fetal/neonatal adverse outcome rested solely on the assumption that a congenital ZIKV syndrome (CZVS) exists and is identifiable through fetal ultrasound.[2, 10] However, it is uncertain which clinical findings constitute the syndrome,[11] and no study has shown that among fetus/neonates of ZIKV-infected mothers those findings cluster more frequently than in fetuses/neonates of non-infected mothers.[12] Moreover, claims that neuroimaging findings are highly suggestive of CZVS come from studies in women with rash, but without confirmed ZIKV infection.[13, 14, 15, 16, 17] More important, an association between specific neuroimaging findings and CZVS was untestable in those studies, because none included a control group.

Ultrasound findings used to define “any adverse outcome” (AAO), the main study outcome, frequently occur as isolated conditions, resolve after a variable period, and have good prognosis and no obvious cause.[18, 19, 20, 21] Authors provided no rationale for combining clinical findings to define AAO. It seems findings were labeled as "major" or "minor" depending on their prognostic value. However, findings with good prognostic value may contribute little to an accurate diagnosis of CZVS/AAO, or to identifying their etiology, particularly if they tend to concur. Indeed, there is no reason to expect cases with three "minor signs" to have similar prognosis or be more likely caused by CZVI than those with one "major sign". Additionally, it seems authors departed from their own definition of “severe symptoms”, since cases 185, 216, 205, 266, and 297 were considered severe, but had none of the major signs in Appendix 1.

Investigators reviewed and classified all fetal/neonatal outcomes blinded to MZVI status. However, blinding was not mentioned in two previous reports,[2, 10] and the approach used for blinding was not described. More important, blinded review of findings was inconsequential, because investigators themselves ascertained those findings during the follow-up and, as treating physicians, were ethicaly bound to know the mother’s infection status. Moreover, blinding could have been broken just by counting ultrasound exams, since ZIKV-infected mothers had monthly ultrasound exams (an average of four)[2], while other women “were monitored as clinically indicated”. In that regard, it is unclear why only one ultrasound exam, and which one, was reviewed, since only two cases in Appendix 4 have more than one exam.

Authors reported fetal findings in 69 RT-PCR positive mothers (Appendix 4). The association between CZVI and AAO could be estimated from these data while avoiding selection bias, because selection was independent of CZVI (request details and analysis code to When fetal IgM and RT-PCR in any tissue/fluid were used to define CZVI, the odds ratio (OR) for AAO was 0.95 (0.21 to 4.28). When RT-PCR tests in placenta and amniotic fluid were not used to define CZVI, the OR was 1.54 (0.23 to 10.56). Corresponding ORs for “any major CNS finding” (Appendix 1) were 1.86 (0.67 to 5.12) and 2.17 (0.60 to 8.33), respectively. Thus, study data provides little evidence of a CZVI-AAO association. Finally, uncertainty about which one happened first made it impossible to know if CZVI was a cause or a consequence of fetal abnormalities.

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Competing interests: No competing interests

03 December 2018
Leonelo E Bautista
Associate Professor
Department of Population Health Sciences, University of Wisconsin at Madison
610 Walnut Street, WARF 703 Madison, WI 53726