French Polynesia study gauges Zika microcephaly risk in early pregnancy

Pregnant woman with ultrasound
Pregnant woman with ultrasound

Milan Zokic / iStock

Scientists who scrutinized data from French Polynesia's Zika virus outbreak found more evidence supporting the virus’s association with microcephaly and estimated that the risk is about 1 in 100 women infected in the first pregnancy trimester.

French Polynesia's outbreak occurred in 2013 and 2014, and the territory's strong health and surveillance systems offered Pasteur Institute researchers a good opportunity to look for a connection between maternal Zika infections and the birth defect. The group published its findings online today in The Lancet.

French Polynesia and Brazil are the only two countries with documented microcephaly increases in the wake of Zika virus transmission, and global health officials are closely watching for similar patterns to emerge in countries hit by the virus after Brazil. Recent studies also suggest a link between maternal Zika infection and other pregnancy complications, such as fetal growth restriction or stillbirth, and hint that the virus could cause a range of problems during any trimester.

First trimester stood out as highest-risk period

French Polynesia's outbreak started in October 2013, peaked 2 months later, and ended in April 2014. According to the study, about 31,000 people sought medical care for suspected Zika virus infections. They estimated that 66% of the territory's population was infected.

To sift out the microcephaly risk, the team used serologic and surveillance data to gauge the probability of infection for each week of the outbreak and looked at medical records to identify all cases of microcephaly reported between September 2013 and July 2015. They also used mathematical and statistical models to estimate the expected number of microcephaly cases, based on a variety of assumptions about the association between Zika virus and microcephaly.

They identified eight microcephaly cases that occurred during the outbreak—five in pregnancies that were terminated and three in births. Seven of the cases occurred within a 4-month period near the end of the outbreak.

Comparing the different models with actual cases revealed that the first pregnancy trimester was associated with the greatest risk, which investigators estimated at 95 in 10,000 women, or about 1 in 100 infected in early pregnancy.

The risk of Zika-linked microcephaly—which researchers put at 1%—appears to be lower than that for birth defects tied to other maternal infections such as cytomegalovirus (13%) and congenital rubella syndrome (38% to 100%). However, the Zika association is still an important public health issue, because the risk of infection is high during outbreaks.

Risk looks much higher in Brazil

In a Lancet press release, the authors said the risk assessment may be useful for guiding public health responses to the outbreak. Arnaud Fontanet, MD, DrPH, said the French Polynesia data are especially important because the outbreak is already over. "This provides us with a small, yet much more complete dataset than data gathered from an ongoing outbreak. Much more research is needed to understand how Zika might cause microcephaly," he said, adding that the team's findings support the World Health Organization (WHO) recommendations for protecting pregnant women from mosquito bites.

In a commentary in the same Lancet issue, Laura Rodrigues, MD, PhD, of the London School of Hygiene and Tropical Medicine, wrote that the highest risk during the first trimester is biologically plausible, based on brain development timing and the severity of the neurologic condition. She is also part of the Microcephaly Epidemic Research Group, based in Recife, Brazil.

However, Rodrigues said the absolute risk of 1% that the group estimated is perhaps lower than expected, given findings from Brazil suggesting the microcephaly risk may be as high as 22% after symptomatic Zika virus infection during the first trimester.

Though it's not surprising that the risk estimates differ, because they are based on different study approaches, it's still not clear if they relate to a single underlying risk or if they might involve other factors, such as clinical symptoms or earlier dengue infections, she wrote.

Rodrigues said the fast pace of data gathering during the Zika epidemic is an opportunity to watch science as it develops, observing, "I expect we will teach our students about the production of science using examples from this Public Health Emergency of International Concern for many years to come."

Other developments

  • Health officials in Cabo Verde, an island nation off the West African coast, today announced the country's first suspected Zika-linked microcephaly case. In a foreign-language media report translated and posted by Avian Flu Diary, the country's health director addressed the case at a media briefing, but added that it has not yet been lab confirmed. The WHO last week said the outbreak in Cabo Verde appears to have peaked toward the end of November and early indications suggest its outbreak was caused by the African strain of the virus, with no neurologic abnormalities seen so far.

  • The New York Times today pressed Congress to approve President Obama's emergency request for $1.8 billion to fund the Zika outbreak response. In an editorial the paper said the request was modest and called Republican demands to transfer Ebola-earmarked funding to the Zika virus efforts senseless and dangerous. The Times said the Obama administration is trying to avoid mistakes that led to a slow Ebola response and that lack of Zika funds is prompting federal health officials to divert resources from other diseases such as dengue.

  • The WHO today issued interim guidance for entomologists on surveillance for Aedes mosquitoes in Zika virus settings. Developed by its Neglected Tropical Diseases and Global Malaria Program groups, the document covers selected sampling methods for doing surveillance of Aedes mosquitoes, pupae, and eggs. It also suggests priorities for countries, depending on Aedes populations and evidence of Zika virus circulation.

See also:

Mar 15 Lancet report

Mar 15 Lancet commentary

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