Table 2. Risk categorization by major international bodies.

International authority	Risk classifications and travel advisories, 2016	Updated country classifications and advisories since March 2017 (current to 20 May 2017)
US CDC [25] (https://wwwnc.cdc.gov/travel/page/zika-information)	1st released interim travel guidelines on 15 Jan 2016, which, “out of an abundance of caution, “advised pregnant women to consider postponing travel to areas with ongoing local transmission, or to take precautions against mosquito bites if they must travel”. 2 categories, Epidemic and Endemic, with 3 levels of travel notices and 2 Alert levels: 1) Epidemic: active transmission, currently experiencing outbreaks of ZIKV. Travel notices may be posted for these countries, which are considered epidemic because mosquitoes that carry ZIKV are present, ZIKV has not been reported there in the past, and very little, if any, of the population is immune. Two possible associated travel alert levels associated with epidemic transmission: Level 3 alert: Avoid non-essential travel; Level 2 alert: Practice enhanced precautions. Increased risk in defined settings; certain high-risk populations may wish to delay travel to these destinations. 2) Endemic: past or sporadic cases (potentially ongoing). Large number of local residents are likely to be immune so the risk to travelers is likely much lower than in epidemic countries, and no travel alert is currently issued.	Risk levels changed in March 2017, with countries now categorized as “Risk of ZIKV” or “No known risk”. Within the two risk areas, the 3 levels of travel notices with 2 categories of Alerts have been retained: Level 3 alert: Avoid non-essential travel; Level 2 alert: Practice enhanced precautions (some increased risk so certain high-risk populations may wish to delay travel to these destination); Level 1: Watch only, practice usual precautions (limited impact to traveler). Additionally, some countries are listed as having a ZIKV risk but with no alert.
	Associated travel notice: Usual or slightly above baseline risk with limited impact to travelers. Watch, practice usual precautions for these destinations. Pregnant women should consult with their health care provider and, if they proceed with travel, strictly follow steps to prevent mosquito bites.	All countries designated as having a ZIKV risk have a travel recommendation such that pregnant women should avoid travel to these areas. CDC classifies areas with a risk of Zika virus according to WHO Categories 1 and 2, plus a subgroup of Category 4 countries (those that share a border and ecologic zone with another country which has endemic transmission [Category 4a countries as defined by ECDC]). Countries classified as no risk correspond to WHO Categories 3, 4-non-subgroup (Category 4b), and countries without a competent vector.
	Exception: “Special considerations” notice issued for 11 Southeast Asian countries on September 30th 2016 [38]: Countries in Southeast Asia with previously reported ZIKV or case exportations, but with limited data available to fully evaluate risks, were considered to have endemic disease with large numbers of immune local residents. Risks to non-immune travelers were thought to be lower than in areas where ZIKV is newly introduced and spreading widely. Travel notices were not issued for these destinations but pregnant women were advised to consult with health care providers and consider postponing non-essential travel	The Special Considerations notice for Southeast Asia was removed in March 2017, and most countries previously listed in this group were incorporated into those with ZIKV risk but with no associated Alert.
WHO [17, 39] (see http://www.who.int/csr/disease/zika/information-for-travelers/en/ and http://www.who.int/csr/resources/publications/zika/classification/en/)	Numerous changes in WHO categories between 1st warnings on 5 Feb 2016 and 14 April 2016, but generally captured 3 categories: i) Reported autochthonous transmission; ii) Indication of viral circulation; iii) Countries previously affected by ZIKV transmission/outbreaks terminated. Categories changed on 14 April 2016 to 7 July 2016: Category 1. Countries experiencing a first outbreak of ZIKV since 2015, with no previous evidence of circulation, and with ongoing transmission by mosquitos. Category 2. Countries with evidence of ZIKV transmission prior to 2015, with or without ongoing transmission; or countries where outbreak is over. From 7th July 2016 to March 2017, 3 risk category levels were in place, with travel advisory issued for Categories 1 and 2: Category 1: Countries with reported outbreak from 2015 onwards. Laboratory confirmed, autochthonous, mosquito-borne case of ZIKV infection in an area with no evidence of circulation of the virus in the past (prior 2015), OR in an area where transmission has been previously interrupted (assumes that there is sufficient population susceptibility to allow transmission);OR Increase in incidence of laboratory confirmed, autochthonous, mosquito-borne ZIKV infection in areas where there is on-going transmission (> two standard deviations of baseline rate, or doubling of case numbers over a 4-week period). Category 2: Countries with possible endemic transmission or evidence of local mosquito-borne Zika infections in 2016. Countries reporting an outbreak with consistent presence of confirmed, autochthonous, mosquito-borne cases of ZIKV infection 12 months after the outbreak OR Countries where ZIKV has been circulating for several years with consistent presence of confirmed, autochthonous, mosquito-borne cases of ZIKV infection or evidence of local mosquito-borne ZIKV in 2016. Category 3: Countries with evidence of local mosquito-borne Zika infections in or before 2015, but without documentation of cases in 2016, or outbreak terminated. Absence of confirmed cases over a 3-month period in areas with climatic conditions suitable for year-round arbovirus transmission, or over a 12-month period in areas with seasonal vector activity	In March 2017, WHO reclassified countries into 4 epidemiologic Categories, with travel advisory issued for Categories 1 and 2: Category 1: Area with new introduction or re-introduction with ongoing transmission. Laboratory confirmed locally acquired case in either local population or in a traveller returning to a second country. 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. Includes countries with known historical laboratory evidence of ZIKV circulation prior to 2015, based on the literature or on surveillance data (whether detected and reported by the country where infection occurred or by another country reporting a confirmed case in a returning traveler). Countries in this category may have seasonal variations in transmission. These countries may also experience outbreaks of ZIKV disease. 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 travelers. 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. Category 4: Area with established competent vector but no known documented past transmission or current transmission. All 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 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).
	Associated travel advisory for categories 1 and 2: WHO advises pregnant women not to travel to areas with ongoing ZIKV outbreaks because of increased risk of microcephaly and other congenital malformations. Healthcare practitioners advising travelers should: i) Provide travelers to areas with ongoing ZIKV outbreaks with up-to-date advice on reducing infection risks (preventing mosquito bites and practicing safer sex); ii) Advise travelers to practice safer sex or consider abstinence for at least 6 months and not to donate blood for at least 1 month after return; iii) Advise pregnant women not to travel to areas with ongoing ZIKV outbreaks; iv) Advise pregnant women whose sexual partners live in or travel to areas with ongoing ZIKV outbreaks to ensure safer sexual practices or abstain from sex for the duration of their pregnancy	Categories 1 and 2 are associated with travel advisory: Pregnant women should not to travel to these countries
European CDC [26, 40] (see http://ecdc.europa.eu/en/healthtopics/zika_virus_infection/zika-outbreak/Pages/Zika-countries-with-transmission.aspx and http://ecdc.europa.eu/en/publications/Publications/21-03-2017-RRA%20UPDATE%209-Zika%20virus-Americas,%20Caribbean,%20Oceania,%20Asia.pdf)	1st issued notices on 21 Jan 2016. Initially determined if country was “affected”, then changed to listing according to “autochthonous in prior 9 months” or “prior 2 months”, then changed to two risk categories with 2 corresponding levels of travel advisories. Widespread transmission: > 10 locally transmitted cases of ZIKV in one area, OR local transmission of ZIKV in two or more areas, OR ZIKV transmission ongoing for > 3 months; Sporadic transmission: no more than 10 locally transmitted cases reported in a single area in past 3 months	Risk categories changed in April 2017 to 4 categories of risk (corresponding to the WHO categories listed above) plus two additional subcategories: 1) Areas with virus transmission following virus new/re introduction (corresponds to WHO Cat. 1); 2a) WHO category 2 areas with virus transmission following previous virus circulation; 2b) WHO category 2 subgroup: areas with new documented intense transmission (10 or more confirmed/probable/suspected cases documented in last three months, or two or more confirmed/probable/suspected cases documented in the last three months in at least two locations); 3) Areas with interrupted transmission (corresponds to WHO Cat.3); 4a: Areas bordering a WHO category 2 area, indicating a higher risk of transmission because of the proximity with a category 2 area, sharing the same ecological characteristics or experiencing virus transmission following past virus circulation (sub-category of WHO Cat. 4). Other countries and areas have been listed as category 4b; 4b: Areas with potential for transmission
	Travel notices: For widespread transmission, all travelers to affected areas are at risk of ZIKV infection unless they have immunity due to a previous infection. Pregnant women and travelers with immune disorders or severe chronic illnesses should seek pre-travel advice and postpone non-essential travel. Women of childbearing age who travel to affected areas should take measures to prevent mosquito bites and follow recommendations for prevention of sexual transmission while in affected areas. For sporadic transmission, all travelers to affected areas are at risk of ZIKV infection unless they have immunity due to previous infection. Pregnant women should seek pre-travel advice and consider postponing non-essential travel. Travelers should take measures to prevent mosquito-borne and sexual transmission of ZIKV.	Category 1 and Category 2 intense transmission subgroup are considered high risk; Category 2 is considered moderate risk, Category 4a is considered low risk. Categories 3 and 4b are considered very low risk and have no associated travel advisory.
Public Health England/NaTHNaC [27] (https://www.gov.uk/guidance/zika-virus-country-specific-risk)	1st published warnings 25 July 2016. Four risk categories for ZIKV transmission: High, moderate, low and very low risk, with 4 corresponding travel advisories. High risk (active transmission in the last 3 months): Subcategories: High risk (a): All countries that have reported active and increasing or widespread ZIKV transmission in past 3 months or High risk (b): Countries within main outbreak regions that have reported active but sporadic ZIKV transmission in past 3 months. Moderate (sporadic transmission in the last 6 months): Subcategories: Moderate risk (a): Countries outside main outbreak regions reporting active but sporadic ZIKV transmission in past 6 months or Moderate risk (b): Countries within main outbreak regions but no recorded cases in past 6 months. Low (no cases in the last 3 months): Countries with evidence of recent ZIKV transmission since 2007 but no recorded cases in past 6 months. Very Low (Zika prior to 2007 only): Countries with historical evidence of ZIKV transmission (pre-2007)	No change between Dec 2016 and Aug 2017. (Note: risk categories changed in Aug 2017 to better correlate with WHO and ECDC. There are still are 4 risk categories: high, moderate, low or very low risk.)
	Travel advisories: High risk: Pregnant women should postpone non-essential travel; Moderate risk: Pregnant women should consider postponing non-essential travel; High and moderate risk: All travelers to high and moderate risk countries should be given advice on ZIKV prevention, including avoidance of mosquito bites and sexual transmission; Low risk: Individual risk assessment advised pre-travel to discuss low risk; Very low risk: No advisory

CDC: US Centers for Disease Control and Prevention (CDC); ECDC: European Center for Disease Control and Prevention; WHO: World Health Organisation; NaTHNaC: National Travel Health Network and Centre (NaTHNaC), Public. Note: As of April 2017, countries with a Level 2 alert per CDC largely correspond to the Category 1 list (new introduction or re-introduction and ongoing transmission). Other countries listed as having ZIKV risk per CDC (with corresponding advice for pregnant women to avoid travel to these areas) but with no Level 2 alert correspond to Category 2 (past or ongoing transmission) or Category 4 subgroup (competent vector exists, no past or current transmission, but bordering a WHO category 2 area). By contrast, WHO has no travel advisory for Category 4 countries. Correlation with NaTHNaC categories, which additionally, provides a level of risk intensity, has been somewhat variable, although in Aug 2017 NaTHNaC updated the categories to be more in line with WHO and ECDC