Only one case of locally transmitted Zika virus has been documented in the United States this year, a reflection of the fact that cases have plummeted in Latin America and the Caribbean. Experts say the cause of the drop has nothing to do with efforts to control the mosquitoes that spread the disease or the impact of climate on their populations. Rather, the rapid spread of the virus through the Americas—which had its first reported case in Brazil in April 2015—left large swaths of the population immune. This so-called “herd immunity” has made it increasingly difficult for the Zika virus to find susceptible people, breaking the cycle of transmission between humans and mosquitoes. The drop in cases has not lessened the push to develop a vaccine against Zika, which caused widespread alarm after thousands of babies in Brazil were born with brain deformities that studies have shown are linked to the mothers being infected by the virus while they were pregnant. But with transmission in the region becoming scarce, it does complicate efforts of clinical trials that hope to show lower rates of transmission and disease in vaccinated people. One such trial is underway right now.
