Rapid risk assessment: Zika virus transmission worldwide – 9 April 2019
Symptoms associated with Zika virus (ZIKV) infection are generally mild and most people who become infected do not develop any symptoms. However, since the 2015‒2017 epidemic in the Americas, ZIKV has been recognised as being associated with severe neurological disorders, mainly Guillain-Barré syndrome in adults and congenital Zika syndrome in foetuses and infants, along with other complications including pre-term birth and miscarriage.
Since the epidemic in the Americas peaked in the early spring of 2016, a continuous decline in the number of reported ZIKV disease cases has been observed in the majority of countries throughout the Americas and the Caribbean. Moreover, virus transmission appears to have been interrupted in several island territories since 2017 and early 2018. In Asia, retrospective investigations and epidemiological surveillance suggest a wide geographical distribution of ZIKV. In Africa, information about ZIKV circulation remains limited.
The travel-related risk of infection primarily depends on the risk of mosquito-borne transmission at the destination, although sexual transmission is also a possible factor. The risk of infection may be high during epidemics, but ongoing virus circulation is expected to be lower in areas where ZIKV circulation is considered endemic. In such endemic areas the risk of exposure is low to medium. As a precautionary principle, areas where ZIKV circulation has been reported historically (but where there is limited capacity for ZIKV disease surveillance and therefore a lack of evidence concerning the current level of transmission) can be considered as having low-to-moderate transmission risk.
Most of the European Union (EU) Outermost Regions (OMRs) and Overseas Countries and Territories (OCTs), where the main mosquito vector, Aedes aegypti, is present, have reported autochthonous transmission in the past. In those areas where transmission has been interrupted, re-introduction of the virus may occur, but the probability of large outbreaks is currently low due to herd immunity in the population.
In EU OMRs and OCTs outside of the Caribbean with no previous ZIKV circulation, but where potentially competent vectors are present, such as Madeira and Mayotte with Aedes aegypti, or Réunion with Aedes albopictus, there is a low risk of local transmission if the virus were to be introduced, as local vector transmission has not been documented to date.
In continental parts of the European Union/European Economic Area (EEA), there are two mosquito vectors that have been shown to be competent for ZIKV in laboratory studies: Aedes albopictus and Aedes japonicus. Nevertheless, their vector competence has been demonstrated to be lower than Aedes aegypti. The probability of mosquito-borne transmission of ZIKV is therefore very low in the European parts of EU/EEA during the spring. However, during the summer and autumn, when temperatures and vector abundance are higher, autochthonous transmission in the European parts of the EU/EEA is possible, if the virus were to be introduced by a viraemic traveller.