If the mosquito-borne Zika virus can’t find enough people to infect, it can’t easily spread. (Felipe Dana, AP)

While the Zika virus mostly faded from the public's consciousness last year, it's still a hot topic among scientists — and for good reasons.

After Zika emerged in the Western Hemisphere, it shook the Americas as reports of infections and devastating birth defects swept through Brazil and Colombia, eventually reaching the United States in 2016. In a welcome turn, the number of Zika cases in the hemisphere last year dropped dramatically in the hardest-hit areas.

But few scientists are naive enough to think we've seen the last of Zika. "The clock is ticking for when we will see another outbreak," says Andrew Haddow, a medical entomologist at the U.S. Army Medical Research Institute of Infectious Diseases in Frederick, Md.

Researchers' to-do list for tackling this once-unfamiliar virus is daunting. But progress has been made, especially in learning more about Zika's biology and interactions with its hosts, and in developing a safe and effective vaccine.

In 2017, the epidemic lost steam because many areas have probably developed herd immunity to the virus. Zika infected a large number of people, who are now presumably immune, and those exposed provide indirect protection to people who haven't yet encountered Zika. If the mosquito-borne virus can't find enough people to infect, it can't easily spread.

But Zika doesn't rely only on mosquitoes to get around. Last year, researchers learned more about how the virus spreads through sexual intercourse.

In humans, Zika can persist in semen for close to three months, researchers learned. And Haddow and colleagues reported in the August issue of Emerging Infectious Diseases that four of eight macaques exposed to the virus vaginally developed infections, as did seven of eight macaques that received the virus via the rectum.

In the wild, animals can act as reservoirs for Zika between human outbreaks. A small number of black-striped capuchin monkeys and common marmosets in a region of Brazil with high numbers of human cases were found to carry the virus already, the first such report among Western Hemisphere monkeys.

But there's still a lot to learn about how the virus is maintained long-term in nature, Haddow says.

Convinced that Zika is here to stay and especially concerned about its effects during pregnancy, researchers have rushed to develop vaccines.

In its first test in humans, reported online Oct. 4 in the New England Journal of Medicine, one vaccine based on DNA from the virus elicited an immune response, with 100 percent of participants developing antibodies after a three-dose regimen. Another DNA vaccine, developed by the National Institute of Allergy and Infectious Diseases, is in a second round of human testing.

Attention to the virus may wane further. But the risk to public health remains, and interventions are still needed, such as ongoing monitoring that tracks Zika and other infectious diseases in pregnancy, says Denise Jamieson, an obstetrician and gynecologist at Emory University School of Medicine in Atlanta.

"We're not ready for another emerging infectious disease that may disproportionately affect pregnant women or their fetuses or babies," she says. "And we need to be."

Science News