International travel and health

Yellow fever risk mapping and recommended vaccination for travellers

The scientific and technical advisory group on geographical yellow fever risk mapping (GRYF)

Risk mapping

The scarcity of well documented and consistent methods in YF risk assessment and the changing global epidemiology of the disease emphasised the need to revise classification and standardise the geographical risk assessment for YF.

For instance, in late 2007 and early 2008, the disease re-emerged in Paraguay and Argentina after more than 30 years. Furthermore, increased numbers of cases were reported from many countries in central Africa that had previously reported cases only rarely.

Criteria for classification

In 2008 WHO convened an Informal Working Group on Geographic Risk mapping for Yellow Fever :

  • to review factors important for the transmission of yellow fever virus and country specific yellow fever information;
  • to establish criteria for additions to or removal from the list of countries with risk for yellow fever virus transmission;
  • to update yellow fever risk maps, and to revise the recommendations for vaccination for international travel.

The working group met by teleconference regularly from September 2008 to May 2010 and then on an ad hoc basis. This working group outlined four levels of yellow fever risk and classified geographical areas into four corresponding categories: endemic, transitional, low risk, and no risk, see Table 1.

The revised global yellow fever risk map and recommendations for vaccination, 2010: consensus of the Informal WHO Working Group on Geographic Risk for Yellow Fever, Lancet Infect Dis 2011; 11: 622–32

The factors identified in Table 1 were adopted as the criteria for the addition or removal of countries and geographical regions in annex 1 of the International Travel and Health (ITH) publication.

The risk of YF was determined on the basis of the virus circulation in humans, nonhuman primates and vectors; the distribution of YF vectors and animal reservoirs; and the ecological factors.

See background for the Consultation on Yellow Fever and International Travel, 2010

Within certain countries, and where data exist, it is possible to stratify areas according to the epidemiological risk of YF virus transmission. The factors identified by the group which can provide useful information on risk of YF virus transmission were:

  • Periodicity of reported human or animal YF cases (active or passive surveillance);
  • Presence and distribution of mosquito vectors and non-human primate hosts involved in the YF virus transmission cycle (field research);
  • Ecological factors (proxy indicators for presence, abundance, and activity of vectors and primates): Vegetation, rainfall, elevation, temperature (satellite imagery);
  • Historical serological surveys of the human population;
  • Detection of YF virus or antibodies in non-human primates and in vector mosquitoes (field studies).

YF Risk Mapping 2015 updates

Further information on yellow fever risk assessment is available from here: www.who.int/csr/disease/yellowfev/en/