Zika infection late in pregnancy may be not so risky

Early results from Zika cases tracked in Colombia suggest that the viral infection late in pregnancy is not linked to microcephaly, a birth defect that leaves infants with abnormally small heads and brains.

In a group of 1,850 pregnant women reportedly infected with the virus during the third trimester, more than 90 percent have given birth to babies without “apparent abnormalities,” researchers report online June 15 in the New England Journal of Medicine.

The finding offers the barest glimmer of hope in the ongoing Zika crisis. Since 2015, the mosquito-borne virus has spread to 46 countries and left more than 1,500 babies with severe birth defects, including microcephaly and other malformations.

Any optimism should come with caution, says Anna Durbin, an infectious diseases physician at Johns Hopkins Bloomberg School of Public Health. The absence of microcephaly is “a huge sigh of relief,” she says. But releasing the results now “is a little bit premature.” Thousands of pregnant women in Colombia reported to have been infected with the Zika virus have yet to give birth (SN: 4/2/16, p. 26).

“What we really want to know is what’s going to happen to women who were infected in their first trimester,” Durbin says.

A study last month estimated that the risk of delivering a baby with microcephaly after a first trimester Zika infection could be up to 13 percent.

Zika reached Colombia in October 2015, a year and a half after the virus broke out in Brazil. Data collected by Colombia’s national health institute has let researchers sketch a rough picture of pregnancy outcomes in a subset of infected women.

As of April 2, Colombia reported 11,944 cases of Zika virus disease in pregnant women. So far, 12 percent of these women have tested positive for Zika virus RNA.

These numbers, however, underestimate the total effect of the Zika outbreak, because they include only people with symptoms of infection, Margaret Honein of the U.S. Centers for Disease Control and Prevention in Atlanta and coauthors note in the new paper. Scientists estimate that as many as three out of four people infected with Zika are asymptomatic.

Among the women Honein and colleagues studied, none gave birth to babies with microcephaly. Four others in Colombia, though, have delivered babies with birth defects linked to Zika. (These mothers didn’t have symptoms, and it is unclear when in pregnancy they contracted the disease.)

The results aren’t all that surprising, says University of Pittsburgh public health researcher Ernesto Marques. Data from Pernambuco, a state in northeast Brazil that was swamped with cases of microcephaly last year, suggest that the majority of birth defects linked to Zika have occurred in women infected during their first trimester.

But the new findings are no guarantee that infection late in pregnancy is OK — or that the Colombian infants are out of the woods just yet, researchers caution. “The fact that these babies do not have an anatomical problem,” Marques says, “does not mean that they do not have any problem. We need to follow the development of these kids.”