WHO, the United States Centers for Disease Control and Prevention and the European Centre for Disease Prevention and Control have developed a new Zika virus classification scheme. The classification serves to categorize the presence of and potential for vector-borne ZIKV transmission and to inform public health recommendations. Based on the defined criteria and expert review, some countries, territories and subnational areas were reclassified and some were classified for the first time.

Category 1: Area with new introduction or re-introduction with ongoing transmission

• A laboratory-confirmed autochthonous, vector-borne case of ZIKV infection in a country /territory/subnational area where there is no evidence of virus circulation before 2015, whether it is detected and reported by the country /territory/subnational area where infection occurred, or by another country by diagnosis of a returning traveller; or
• A laboratory-confirmed autochthonous, vector-borne case of ZIKV infection in a country/territory/subnational area where transmission has been previously interrupted, whether it is detected and reported by the country where infection occurred, or by another country by diagnosis of a returning traveller.

Category 2: Area either with evidence of virus circulation before 2015 or area with ongoing transmission that is no longer in the new or re-introduction phase, but where there is no evidence of interruption

This category takes into account those countries with known historical laboratory evidence of ZIKV circulation prior to 2015, based on the literature as well as all ZIKV surveillance data whether detected and reported by the country where infection occurred or by another country reporting a confirmed case in a returning traveller. Countries in this category may have seasonal variations in transmission. These countries may also experience outbreaks of ZIKV disease.

Laboratory criteria to ascertain the presence of ZIKV in past studies are:

• Detection of the virus in humans, mosquitoes or animals; and/or
• Serologic confirmation of ZIKV infection with tests conducted after 1980, and considered as confirmed infection on expert review based on testing for all appropriate cross-reactive flaviviruses and utilization of comprehensive testing methodologies. Because of testing and interpretation limitations with serological data antedating 1980, they were not used for classification purposes.

Category 3: Area with interrupted transmission and with potential for future transmission

The minimum timeline for determining transition to an interrupted state is 12 months after the last confirmed case, and no cases identified in travellers. For countries with a high capacity for diagnostic testing, consistent timely reporting of diagnostic results, a comprehensive arboviral surveillance system and/or a temperate climate or island setting, the interruption of vector-borne transmission is defined as the absence of ZIKV infection 3 months after the last confirmed case. Countries where interruption is epidemiologically likely to have occurred should provide surveillance data to WHO to support the assessment by expert review.

Category 4: Area with established competent vector but no known documented past or current transmission

All countries/territories/subnational areas where the main competent vector (A. aegypti) is established, but which have not had a documented, autochthonous, vector-borne case of ZIKV infection. This category also includes a subgroup of countries/ territories /subnational areas where ZIKV transmission may occur because of a shared border with a neighbouring Category 2 country, by belonging to the same ecological zone and having evidence of dengue virus transmission. In this subgroup, a first laboratory-confirmed, autochthonous vector-borne case of ZIKV infection may not necessarily indicate new introduction (Category 1), but rather previously unknown and undetected transmission (Category 2), and these countries/territories/subnational areas will be reclassified accordingly.