During a recent outbreak of the Zika virus in French Polynesia, roughly one in 100 women infected in the first trimester of pregnancy developed a fetus with an abnormally small head and brain damage, researchers reported on Tuesday.
The research, published in The Lancet, provides some reassurance that the complication is rare, experts said.
“It means you have a 99 percent chance of having a normal baby,” said Dr. Laura C. Rodrigues, a professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, who wrote an editorial accompanying the report.
By comparison, the risk of congenital rubella syndrome — which may include heart disease, hearing loss and developmental delays in infants — ranges from 38 percent to 100 percent when mothers are infected in the first trimester.
But because the Zika virus is carried largely by mosquitoes, it can infect broad sections of a population. “If you apply a 1 percent risk to a large number of women, it’s still a large public health problem,” said Simon Cauchemez, the study’s lead author and the director of mathematical modeling for the infectious diseases unit at the Pasteur Institute in Paris. “In French Polynesia, 66 percent were infected.”
French Polynesia has a population of roughly 270,000 and just one prenatal diagnosis center. “I really believe they picked up every event that occurred,” said Dr. David O. Freedman, professor of medicine and epidemiology at the University of Alabama at Birmingham. “You can’t do studies like this in Brazil.”
Dr. Cauchemez and his colleagues scoured medical records to find cases of microcephaly in French Polynesia and estimated the proportion of people infected with the Zika virus by using data from previous studies that looked for antibodies in blood.
The researchers established that the baseline prevalence of microcephaly in the territory was two cases per 10,000 newborns. Between September 2013 and July 2015, before and after the outbreak of Zika virus, there were eight cases of microcephaly.
Seven cases occurred around the end of the epidemic. “Such temporal clustering strongly supports the proposed association” between infection with the virus and microcephaly, Dr. Cauchemez and his colleagues wrote.
However, the researchers do not know if the mothers of the microcephalic fetuses were infected with the virus, because they were not tested for this study. Three of the microcephalic fetuses were carried to term. Five pregnancies were terminated between 26 and 31 weeks of gestation.
Dr. Freedman cautioned that the 1 percent absolute risk may not apply in other countries where Zika is spreading. Recent research suggests that the virus may be associated with other serious complications. This month, a study published in the New England Journal of Medicine found 29 percent of Brazilian women who had ultrasound exams after testing positive for Zika infections had fetuses with abnormally small heads or placentas and nerve damage.