O

fficial reports on the impact of Zika virus in Colombia appear to dramatically underestimate the number of infants born there with associated birth defects, a new study suggests.

The actual number may be at least two times higher than the official estimate and perhaps even higher still, according to the research.

The study was published Friday in Morbidity and Mortality Weekly, a journal of the Centers for Disease Control and Prevention. It was conducted by scientists from the CDC and its Colombian equivalent, the National Institute of Health.

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The research found there was a fourfold increase in microcephaly cases — babies born with a smaller than normal heads and often under-formed brains — in Colombia from March to November of this year compared with last year. While there were 110 in that period in 2015, there were 476 in 2016.

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Colombia has reported only 60 cases of Zika-linked microcephaly to the World Health Organization.

The peak in affected births came about 24 weeks after the peak in reported infections in pregnant women — and corresponds with the theory that maternal infection in the first trimester or early in the second is the period of highest risk for the developing fetus, said one of the senior authors, Margaret Honein, chief of the CDC’s birth defects branch.

“I think this gives us a better indication of the impact that Zika has had in Colombia and … demonstrates that the increase in microcephaly is not specific to Brazil,” Honein said.

“We expect that every country that has a large Zika virus outbreak will have an increase in microcephaly and severe brain abnormalities following that,” she said.

Asked why the official microcephaly figure differs from the numbers in the study, Honein deferred to Colombia. But she said researchers there are conducting thorough investigations of each suspected case and reporting on those in which there is strong evidence of maternal Zika infection during pregnancy.

Her fellow senior author, Martha Ospina Martinez, insisted there is no discrepency between what Colombia is reporting to the WHO and the figures in the study. The 476 number represents all reports of microcephaly, each of which is being investigated.

“Up to this report only 60 babies with microcephaly have a confirmation of Zika virus infection as the only reason that will explain microcephaly. All other cases are either discarded or under investigation,” Ospina Martinez, the institute’s director general, said in an email.

Public health experts studying the Zika outbreak have been puzzled by the fact that Brazil — where the link between the virus and microcephaly was first noted — has seen so many infants with Zika-induced birth defects and other countries have seen, in comparative terms, so few.

A report on Zika released by the WHO on Thursday showed 29 countries have reported a total of 2,436 affected births, with 2,211 of them from Brazil.

Colombia’s approach to proving a link between a microcephaly case and Zika infection may in part explain why its confirmed case tally remains so low.

The study noted that samples were not taken for testing in all cases, and even when samples were taken, sometimes they were not taken quickly enough to find evidence of infection.

In other parts of the country, there has been a definite increase in microcephaly cases. But the condition is difficult to diagnose, and countries are using different definitions, complicating efforts to compare what is happening across borders, Honein said.

Additionally, after Brazil raised the alarm about the potential link between Zika and microcephaly, the government of Colombia advised women to consider delaying pregnancy. The number of live births in the country dropped by approximately 18,000 over a nine-month period after that warning was issued, the study authors reported.

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