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Marini et al provide a short commentary on their research article First outbreak of Zika virus in the continental United States: a modelling analysis published in September 2017 in Eurosurveillance. This work was supported by the European Union’s Horizon 2020 Research and Innovation Programme under ZIKAlliance Grant Agreement no. 734548.


During the summer of 2016, an outbreak of Zika virus (ZIKV) occurred in Wynwood, a neighbourhood of Miami-Dade County, Florida, leading to twenty-one locally transmitted symptomatic cases. Researchers from the Fondazione Edmund Mach (Trento, Italy), ZIKAlliance partner working in Work Package 1 (Clinical Studies), developed a mathematical model to investigate the outbreak dynamics and to answer the following questions: When was the virus introduced in the area? How many individuals, and especially pregnant women, were infected, considering that many Zika infections are asymptomatic, and not all symptomatic infections are reported? How effective were the mosquito control measures implemented by the authorities?

The authors of the study found that the transmission chain started with an infected person who had likely arrived in Wynwood in March, four months prior to the epidemic being detected. Following detection, public health authorities carried out interventions using both larvicides and adulticides to reduce the number of mosquitoes. Mathematical modelling predicted that these actions halved the mosquito abundance. However, since ZIKV had circulated in the population undetected for a long time, reactive control measures were insufficient to prevent a large number of infections. When unreported infections are taken into account, it is predicted that more than 1000 people were infected with ZIKV during the outbreak and an estimated 15 pregnant women to have been infected. Therefore, a stronger effort and a more effective approach through the application of preventive measures would be required to maintain a low mosquito density.

Model simulations indicate a likely extinction of transmission during winter even in the absence of interventions. Nonetheless, given the predicted sustained presence of infected mosquitoes, undetected infections might have occurred even after the authorities declared an end to the outbreak in September. Locally transmitted cases were recorded in other Miami-Dade areas until the end of December and the researchers’ findings indicate that they probably originated from Wynwood commuters rather than from imported, travel-related, cases.

The implications of this study for control are expected to be robust with respect to some unknowns on ZIKV epidemiology, such as the role of asymptomatic infections, sexual transmission and spatial dynamics, along with uncertainties in parameter values and possible drifts in vector competence following adaptations of the viral genome, since they depend on the silent transmission of Zika virus in the early months after importation, which is now a well-established trait of this emerging infection.

Marini Giovanni, Guzzetta Giorgio, Rosà Roberto, Merler Stefano. First outbreak of Zika virus in the continental United States: a modelling analysis. Eurosurveillance 2017;22(37).

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