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Researchers predicted 1,133 cases of microcephaly would occur between May and December 2016, but only 83 cases were reported by local health officials, said senior researcher Christopher Dye. He is director of strategy, policy and information for the World Health Organization in Geneva, Switzerland.
Brazil served as the epicenter of the 2015 Zika outbreak in South America, and it was the country that endured the highest rates of microcephaly and other Zika-related neurological birth defects that year.
Zika reemerged in Brazil early in 2016, and so researchers expected more cases of microcephaly to crop up as the months passed.
"We expected to see microcephaly cases increasing from May onwards," Dye said. Instead, few cases of microcephaly occurred.
The researchers think the supposed Zika resurgence early last year might have been due to doctors mistakenly diagnosing it when a patient's illness actually was caused by another tropical virus.
"Most likely, we think, is that cases reported as Zika were actually due to another virus, Chikungunya, which causes fever and rash -- similar symptoms to Zika -- but not microcephaly as a result of infections in pregnancy," Dye said.
This is possible, the study authors said, because the 2015 Zika outbreak likely created "herd immunity" against the virus among Brazilians. Most people in that country have endured infection and now are immune against the virus, providing little opportunity for mosquitoes to spread Zika person-to-person through their bite, the researchers suggested.
"It is likely that Zika is now endemic in the Americas and that, once the susceptible population builds up again -- mainly through the births of children who have not previously been exposed -- then we will see new outbreaks, unless the mosquito population is reduced substantially or we have a vaccine," Dye said. "The outbreaks we saw in 2015 may not happen again for a few years -- perhaps a decade."
Another possibility is that Zika may interact with some other factors -- possibly other tropical disease viruses -- during pregnancy to cause microcephaly in developing fetuses, the researchers added.
Dr. Amesh Adalja is an affiliated scholar at the Johns Hopkins University Center for Health Security in Baltimore. He said, "This is an intriguing possibility and it would be interesting to study the Zika-infected women who did not give birth to microcephalic infants, in order to determine what exactly is at play."
For instance, evidence has suggested that women previously infected with dengue fever -- and thus carrying antibodies against that tropical virus -- might be more likely to have a baby with microcephaly caused by Zika, Adalja said.
A third possibility is that fear of potential birth defects may have led more Brazilian women to either not conceive or to terminate their pregnancy last year if they became infected with Zika, the researchers suggested.
However, this explanation is not likely because Brazilian officials do not expect a large change in the number of live births for 2016, the study authors concluded.
Though largely confined to Latin America and the Caribbean in 2016, Zika appeared in the United States last summer, with local transmission occurring in a couple of Miami-area neighborhoods.
However, so far, most U.S. cases of Zika have occurred in travelers who contracted the virus in other countries. Of 5,158 cases reported, only 222 occurred due to local transmission in Florida, according to the federal Centers for Disease Control and Prevention.
There have been 54 cases of babies born with Zika-related birth defects and seven pregnancies lost to Zika in the United States, but, again, these cases have been attributed to people who contracted the virus overseas, the CDC said.
The new report on Zika in Brazil was published online March 29 in the New England Journal of Medicine.
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SOURCES: Christopher Dye, D.Phil, director of strategy, policy and information, World Health Organization, Geneva, Switzerland; Amesh Adalja, M.D., affiliated scholar, Johns Hopkins University Center for Health Security, Baltimore; March 29, 2017, New England Journal of Medicine, online