WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Yellow fever
Description of the data
Yellow fever is one of the diseases reportable under the International Health Regulations (IHR). As such, countries are required to report cases and deaths to WHO within 24 hours of being notified of a case of yellow fever on their territory. Reporting of yellow fever cases and deaths to WHO began in 1948. Table 2.2 presents the total number of yellow fever cases and deaths reported to WHO from Africa and the Americas since 1950. WHO also collects data on immunization coverage, which is presented in Table 2.1 and Fig. 2.2.
Strengths and weaknesses of the data
As with other diseases under the International Health Regulations, only a small fraction of cases are reported to WHO. However, it is unlikely that major epidemics of yellow fever have been missed completely. (1)
There are often long delays in detection of yellow fever outbreaks. This is due to a number of reasons including the often remote epidemic sites, lack of diagnostic facilities, difficulties in clinical recognition of the disease by peripheral health workers (yellow fever shares its symptoms with many other diseases that are common in the tropics), delays in recognition of the epidemic, and sparse communication of reports to the central level. These cause delays in the implementation of control measures, such as mass vaccination and vector control.
Logistical capacity to collect specimens and ready access to laboratory testing is essential for the confirmation of suspect cases but it is not always available (particularly in Africa). Accurate laboratory analysis depends upon trained laboratory staff, appropriate equipment and supplies, the provision of reagents, and proficiency testing. WHO has recently recommended that every at-risk country has at least one national laboratory where basic yellow fever blood tests can be performed, (2) and is actively assisting countries to upgrade the capacity for laboratory testing.
Data on routine yellow fever childhood immunization is weak. Not many countries are reporting routine childhood yellow fever immunization coverage to WHO. Of those countries that did report, coverage usually failed to reach even a rate of 50% of eligible children (3) (Fig. 2.2).
Fig. 2.2 Yellow fever childhood immunization coverage, 1996-1998
(1) A recent literature search for all available reports of yellow fever outbreaks indicated that when epidemics were known about, they had been reported to WHO - although often fewer cases were reported than had occurred.
(2) District guidelines for yellow fever surveillance, World Health Organization, 1998, WHO/EPI/GEN/98.09.
(3) EPI information system: global summary, World Health Organization, 1998, WHO/EPI/GEN/98.10.