SÃO PAULO, Brazil — Remember Zika?
With measles and Ebola grabbing headlines, it is easy to forget the health panic of 2016, when Zika was linked to severe birth defects in thousands of Brazilian newborns whose mothers were infected while pregnant, striking fear across the country and much of the Americas.
As health officials struggled to halt its spread, the virus galloped through Latin America and the Caribbean that spring and summer and eventually reached the United States, sickening more than 200 people in Florida and Texas and prompting countless travelers to cancel vacations in the tropics.
Then, seemingly overnight, the epidemic evaporated, and public attention moved on.
But Zika, it turns out, did not vanish.
“Zika has completely fallen off the radar, but the lack of media attention doesn’t mean it’s disappeared,” said Dr. Karin Nielson, a pediatric infectious disease specialist at UCLA who studies Zika’s impact in Brazil. “In some ways, the situation is a bit more dangerous because people aren’t aware of it.”
The virus, which is mostly spread by mosquitoes but also through sex with an infected person, is still circulating in Brazil and other countries that were at the center of the epidemic, and two years ago the same strain from the Americas arrived in continental Africa for the first time. That strain, researchers recently discovered, had been causing birth defects in Asia long before the Zika epidemic of 2016.
Another concern is over places where the mosquito that spreads the virus — the female Aedes aegypti — is endemic but have so far been spared locally transmitted cases of Zika. On Tuesday, the World Health Organization issued a report on Zika that listed 61 such countries, among them densely populated behemoths like China, Egypt and Pakistan as well as much of Africa.
Even Brazil remains vulnerable: The 2016 epidemic largely sparred the country’s south and most notably São Paulo, its biggest city. Warming temperatures associated with climate change are expected to expand the range of Aedes, according to a recent study, putting tens of millions more people at risk for Zika and other mosquito-borne diseases.
“The next outbreak is not a matter of if, but when,” said Dr. Ernesto T.A. Marques, a public health researcher at the Oswaldo Cruz Foundation in Rio de Janeiro who is also an assistant professor at the University of Pittsburgh.
In the United States, the Aedes mosquito can be found across significant swaths of the country during summer, though epidemiologists say the potential for large-scale American outbreaks is limited by the near ubiquity of air conditioning, window screens and local mosquito control efforts.
“It also helps that people in the U.S. tend to live fairly far apart in single-family homes,” said Dr. Lyle R. Petersen, who oversees vector-borne diseases at the Centers for Disease Control and Prevention. “This is a mosquito that doesn’t fly very far.”
While the number of new cases of Zika so far is small — last year there were nearly 20,000 infections in Brazil compared to more than 200,000 during the epidemic’s peak — countries like Angola, Thailand, Vietnam and Cape Verde have reported newborns with Zika-related microcephaly, the condition that leaves babies with the misshapen heads and profound neurological damage that stoked global anxiety.