In this Policy Forum, Neil Ferguson et al. use results from a model of virus transmission to analyze the current Zika epidemic in Latin America, suggesting that it may have already peaked. Evidence increasingly suggests a causal link between Zika infection and microcephaly, as well as other serious congenital anomalies, prompting the World Health Organization to declare the Zika epidemic an international health concern in February 2016.

Here, using a model incorporating factors that determine the scale and speed of emerging viral infection in naïve populations, Ferguson and colleagues estimate that the current epidemic in Latin America will be over in three years; they base this estimate largely on the transmissibility of Zika and the time between cycles of infection.

After these three years have passed, herd immunity - a phenomenon by which a large percentage of a population becomes immune to an infection - will likely delay the next large Zika epidemic for more than a decade, the authors say. They also note that targeting mosquitos, the main culprit behind Zika spread, could actually be counterproductive, because this would interfere with herd immunity.

As a key means to reduce fetal complications associated with Zika, health authorities are advising women to delay having children for several years, which the authors note is not feasible for all women; they recommend detailed local monitoring of the epidemic, so that local advisories and delayed pregnancies are more relevant and feasible.

In a Review also on Zika, Justin Lessler et al. summarize research to date on the virus, particularly that relevant to the latest outbreak in Latin America. As of June 2016, more than 35 countries throughout the Americas have reported local circulation of Zika. While Zika symptoms tend to be mild, the greatest concern surrounding the virus is its effects on a growing fetus.

Microcephaly was the first fetal abnormality recognized, but there is increasing evidence that Zika may be responsible for other fetal complications, such as intracranial calcifications, ventriculomegaly, ocular impairment, brainstem hypoplasia, intrauterine growth restriction and fetal demise. The authors highlight studies in pregnant women infected with Zika who are symptomatic or asymptomatic, noting at what trimester they became infected; longer-term studies are ultimately required to understand Zika's impact on expectant mothers though, they say.
The authors also discuss ways in which Zika may be expected to spread, but note that this can be difficult to predict. Dengue, a virus that is also transmitted by the same type of mosquito, has caused epidemics throughout the Americas, but has not achieved sustained transmission in the continental U.S., despite widespread vector presence. The reasons for dengue's limited spread in America may include not only climate but also differences in built environments and social factors, all of which are likely to affect Zika transmission as well.

Human and mosquito genetics may play a role in how the current epidemic pans out. A Zika vaccine may be the best way to protect at-risk populations over the long term, the authors say.