Scientists studying the 2015 Zika outbreak in Brazil have discovered that people previously exposed to dengue might have been protected from the virus.
Three-quarters of the inhabitants of a favela in the country’s northeast caught the mosquito-borne Zika virus during the epidemic.
The outbreak left more than 3,000 babies across Brazil with microcephaly, a birth defect caused by mothers catching the virus during pregnancy.
Federico Costa, a professor of epidemiology at the Federal University of Bahia, one of the authors of the study in the favela in Bahia’s state capital, Salvador, published in Science magazine, said it was the first time scientists had been able to conduct tests on one specific population before and after an outbreak.
Costa said it was the most accurate research of this kind done to date.
“It is the first [study] to really give a clue as to what happened when Zika came into the region in Brazil, which was hardest hit months later by microcephaly,” said Albert Ko, a professor of medicine at the Yale School of Public Health and another of the report’s authors.
Led by the Yale school, the research team found that 73 percent of residents of the Pau da Lima favela in Salvador caught Zika between March and October 2015.
They examined 1,453 residents and found infection rates varied from 29 percent in some areas of the densely populated favela to 83 percent in others.
They also discovered that previous exposure to dengue protected people against Zika infection, and now believe that those who catch the virus — many of whom show no symptoms — subsequently become immune to it.
Before 2015, Zika had been unknown in Brazil and was often misdiagnosed as dengue.
Less than a year after the epidemic, a wave of newborn babies with abnormally small heads caused panic among Brazilians.
As of November 2018, 3,279 cases of “growth and development alterations possibly related to infection by the Zika virus and other infectious aetiologies” had been confirmed by the Brazilian Ministry of Health, including 2,079 in the northeast. Numbers later faded away.
“[The northeast] has all the perfect conditions to have a huge epidemic,” said Isabel Rodriguez-Barraquer, assistant professor of medicine at the University of California in San Francisco, the lead author.
First identified in monkeys in Uganda in 1947, Zika circulated for decades, but was virtually ignored until an outbreak on the island of Yap in 2007.
Brazilian authorities first confirmed the virus in the northeastern states of Rio Grande do Norte and Bahia in May 2015.
Doctors in the region began noticing an increase in newborns with microcephaly — until then, a rare condition. Cases mushroomed and, in December 2015, the Pan-American Health Organization issued an epidemiological alert.
The Aedes aegypti mosquito, which spreads Zika and related viruses like dengue and Chikungunya, thrives in areas where sanitation is poor and garbage is left uncollected.
Ko said blood tests of favela residents dating from before the 2015 outbreak helped researchers discriminate between dengue and Zika.
“People who were exposed to dengue before had antibodies and those antibodies protected them from Zika,” he said.
Rodriguez-Barraquer believes many people in the Americas are now immune to Zika.
“The epidemic just burned throughout the continent,” she said.
However, the virus is still circulating in other parts of the world and the researchers believe their study will help those struggling to understand it.
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