The number of Zika cases worldwide has dropped, but that doesn’t mean its days are numbered.
If Zika behaves like other arboviruses, such as chikungunya and dengue, it will probably stick around. Arbovirus diseases tend to be cyclical, says public health researcher Ernesto Marques of the University of Pittsburgh.
“You have big booms, then they drop,” he said. “Then a few years later, they come back again.”
The mosquito-borne virus staged a massive assault on the Western Hemisphere in 2015 and 2016.
It’s difficult to assess what percentages of populations across the Americas have been exposed to Zika. While the virus is linked to serious birth defects and to neurological problems in adults, most cases lead to mild or no symptoms. Some infected people probably don’t get medical assistance, leading to missed cases.
In the hardest-hit nations, data from each country’s department of health shows a striking drop in locally acquired cases — that is, ones caused by bites from local, infected mosquitoes.
- Brazil had more than 216,000 probable cases in 2016; as of early September, the new cases for 2017 were around 15,500.
- Colombia tallied more than 106,000 suspected and confirmed cases from 2015 to the end of 2016. This year, new cases have plummeted, with around 1,700 by mid-October.
- Mexico went from about 8,500 confirmed cases in 2015 and 2016 combined to around 1,800 by early October of this year.
A similar trend is happening in the United States and its territories.
In 2016, most cases in the 50 states were in travelers who had been to places where Zika was active, although 224 were locally acquired in Florida and Texas.
Now, local transmission seems to have come to a standstill, with one suspected case in Texas and one case confirmed in Florida.
Epidemiologist Albert Ko of the Yale School of Public Health said the dramatic decrease in Zika cases is likely due to herd immunity.
“Zika came in like a bulldozer,” he said, and many people in the Americas who coexist with Aedes mosquitoes, which transmit Zika, were infected. Now that “there are so many people who’ve already been exposed to the virus and are presumably immune, it kind of protects indirectly the people who haven’t been infected. So that’s probably happened.”
For every virus, a certain portion of the population must be infected before herd immunity takes hold, said David Morens, senior scientific adviser to the director of the National Institute of Allergy and Infectious Diseases.
Zika transmission is more complicated because mosquitoes are involved, Morens said. Other factors — such as how dense the human population is in an area, mosquito abundance and climate — also play a part.
“But it’s clear there is some level of herd immunity,” he added. “We see it with all of these arboviruses that cause epidemics. They burn out because the virus can’t find enough people to infect.”
A 2016 study found that reinfections are possible. So Zika immunity might wane over time, perhaps leading to reinfections, said pediatrician and microbiologist Peter Hotez of Baylor College of Medicine in Houston.