The releases will reduce the population of the mosquito species that carries the Zika virus.
"Genetically altered male mosquitoes, which don't bite but are expected to mate with females to produce offspring that die before reaching adulthood, were released in the West Bay area of Grand Cayman Island, according to a joint statement from the Cayman Islands Mosquito Research and Control Unit and British biotech firm Oxitec."
"The mosquitoes will be released over nine months in an area known to be a hot spot for the Aedes aegypti species, which are not native to the Cayman Islands and are the main vector for Zika as well as other viruses, including chikingunya and dengue."
http://www.wral.com/genetically-modified-mosquitoes-released-in-cayman-islands/15885969/
Releases of the modified mosquitoes are planned for the Florida Keys.
Some background information:
http://www.theatlantic.com/technology/archive/2016/04/genetically-modified-mosquitoes-zika/479793/