By Amanda Vincent

Over a year after the global community declared Zika a public health emergency, researchers now have several vaccines in the safety trial phase, but a key element is needed to prove the effectiveness of any of the Zika vaccines—an active outbreak.

Zika virus originated from Uganda in the late 1940s and it was already known to infect humans. Zika is transmitted by mosquitoes, and its main transmission vectors are two Aedes species mosquitoes, Aedes aegypti and Aedes albopictus. Zika has only recently been mysteriously associated with a birth defect called microcephaly. Microcephaly is a birth defect that causes the infant to develop a small head. In comparison, adults tend to show mild symptoms with infection, such as rash, aches, and fever. However, in rare instances adults can develop Guillain-Barre syndrome, which is a neurological condition. A result of the 2015-2016 outbreak is that infection in pregnant women is now associated with increased instances of microcephaly in infants, although evidence supporting a cause and effect relationship is still up for debate.

The Pan American Health Organization reports that since late 2016, no new countries or territories have joined the existing 48 countries and territories with confirmed direct Zika transmission, meaning transmitted from mosquitos to humans. Countries and territories with direct (aka autochthonous) transmission can be viewed here. Transmission is still occurring, but has been dampened in some areas due to winter. In South America, transmission has stabilized to an average of over 6,600 cases a week, and Central America is averaging over 300 cases a week. Without an active Zika outbreak to measure vaccine effectiveness, serious vaccine development and funding consequences could occur. This creates a catch-22 for Zika control and prevention, as countries need an effective vaccine to protect vulnerable individuals, but vaccine development requires an active outbreak of cases to prove any vaccine’s effectiveness and weed out low performing vaccines.

In 2016, scientists created models to forecast the spread of Zika virus in the Americas, which have been fairly accurate, as well as model previous spread. Those forecast models haven’t yet been updated for 2017, however global temperatures are predicted to hit above average again this year. Higher temperatures are associated with promoting Aedes species mosquito population growth. However, even if 2017 presents high temperatures promoting Aedes population growth, there is no guarantee that the Zika virus epidemic will re-establish a high transmission rate in the Americas or Puerto Rico.

The global emergency that Zika presents is a significant public health issue, with anticipated long-term health and economic implications for individuals and infants affected. Areas hit the hardest (i.e. Brazil, Puerto Rico, etc.) are in dire need of an effective vaccine, but for a true test of efficacy, Zika transmission must initiate a surge in cases. It is important to note, that another key element will be successfully enrolling and maintaining follow-up with high numbers of pregnant and susceptible women, which has been tricky for trials in the past.