Yellow fever Outbreak Toolbox

Update | November 2019

Key references documents

Yellow fever fact sheet (Geneva: World Health Organization).
Yellow fever information page (Geneva: World Health Organization).
Yellow fever health topic page (Brazzaville: World Health Organization).
Yellow fever (Washington (DC): Pan American Health Organization; 2019).

Case definitions

WHO suggested outbreak case definition¹

Suspected case:

Any person with acute onset of fever, with jaundice appearing within 14 days of onset of the first symptoms.

Probable case:

A suspected case; and one of the following:

presence of yellow fever IgM antibody in the absence of yellow fever immunization within 30 days before onset of illness; or
positive post-mortem liver histopathology; or
epidemiological link to a confirmed case or an outbreak.

Confirmed case:

A probable case; and
Absence of yellow fever immunization within 30 days before onset of illness; and one of the following:

detection of yellow fever-specific IgM;²or
detection of fourfold increase in yellow fever IgM, or IgG antibody titres between acute and convalescent serum samples, or both; or
detection of yellow fever-specific neutralizing antibodies.

Absence of yellow fever immunization within 14 days before onset of illness; and one of the following:

detection of yellow fever virus genome in blood or other organs by polymerase chain reaction (PCR); or
detection of yellow fever antigen in blood, liver or other organs by immunoassay; or
isolation of yellow fever virus.

WHO surveillance case definition³

Case definitions for public health surveillance (Geneva: World Health Organization; 2015).
Control of yellow fever: Field guide (page 10) (Washington (DC): Pan American Health Organization; 2005).

WHO other definitions

Outbreak definitions:

Presence of at least one confirmed case, as interpreted in a context.

¹ Yellow fever. Case definitions for public health surveillance. (Geneva: World Health Organization; 2015)

² Yellow fever-specific means that the results of antibody tests (such as IgM or neutralizing antibody) for other prevalent flavivirues are negative or not sigificant. Testing should include at least IgM for dengue fever and West Nile but may include other flaviviruses according to local epidemiology (for example, Zika virus).

³ Yellow fever confirmation is complex:

Any case classification should account for vaccination status
Serological testing cannot distinguish between vaccine- and naturally-acquired antibodies.
Yellow fever diagnostic is both positive and differential. It is essential to rule out cross-reactivity with other arboviruses such as dengue, zika, chikungunya or West Nile virus This cross-reactivity can be seen on serology testing (IgM) as well as plague reduction neutralization assays (PRNT).
Results should be interpreted considering local epidemiology of other flaviviruses.

Data collections tools

Case investigation form(s):

Ready reference card n°3 - alert form: suspected yellow fever. In Yellow fever – Investigation of yellow fever epidemics in Africa: Field guide (Geneva: World Health Organization; 2008).
Annex 2 - Epidemiological investigation form for yellow fever. Control of Yellow Fever: Field guide (Annex 2) (Washington (DC): Pan American Health Organization; 2005).
Line list(s): Not available.
Electronic tools: Not available.