Scientists studying the 2015 Zika outbreak in Brazil have discovered that people previously exposed to dengue may have been protected from the virus.

Three-quarters of the inhabitants of a favela in the country’s north-east 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.

Albert Ko, a professor of medicine at the Yale School of Public Health and another of the report’s authors, said: “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.”

Led by the Yale School of Public Health, the research team found that 73% of residents of the Pau da Lima favela in Salvador caught Zika between March and October 2015. Researchers examined 1,453 residents and found infection rates varied from 29% in some areas of the densely-populated favela to 83%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 Brazil’s health ministry, including 2,079 in the north-east. Numbers later faded away.

With a population of roughly 57 million people, Brazil’s impoverished north-east was completely unprotected against Zika. “[The region] 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.

Facebook Twitter Pinterest An Aedes aegypti mosquito, known to carry the Zika virus, is seen through a microscope at the Fiocruz institute in Recife, Pernambuco state, Brazil. Photograph: Felipe Dana/AP

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 north-eastern 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 like Pau da Lima, where sanitation is poor and garbage is left uncollected.

“There has been disordered growth, so people build on any land they can find,” said Laiara dos Santos, 34, a nurse living in the community. “Pau da Lima has areas with no sanitation … they end up with open sewers.”

Dos Santos, who also caught Zika at the time, remembers the first baby she saw with microcephaly in the community – a little girl. “The mother did not know what was happening,” she said. “She took her daughter and hugged her, she had a lot of love for the child, but she cried a lot.”

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Ko said blood tests of favela residents dating from before the 2015 outbreak helped researchers discriminate between dengue and Zika, which can be hard to tell apart. “People who were exposed to dengue before had antibodies and those antibodies protected them from Zika.”

Rodriguez-Barraquer believes many people in the Americas are now immune to Zika. “The epidemic just burned throughout the continent,” she said. But the virus is still circulating in other parts of the world and the researchers believe their study will help those struggling to understand it.

In December, the US Centers for Disease Control and Prevention issued a warning over “an unusual increase in the number of Zika cases in Rajasthan and surrounding states”, and advised pregnant women not to travel to the Indian state even though transmission by mosquitoes is more likely from August to November.

Whether that leads to a rise in microcephaly cases depends on how much Zika has circulated previously. “The big question is: how bad was the transmission before and was it enough to essentially immunise women?” said Ko.