This site is intended for health professionals only

Yellow fever: prevalence and vaccination guidelines

Carolyn Driver
RGN RM RHV FPCert MSc(Travel Med)
Independent Travel Health Specialist Nurse
Cheshire

Historical texts refer to yellow fever as far back as 400 years ago. It is currently estimated to cause 200,000 cases and 30,000 deaths annually.(1) The disease is caused by a flavivirus, which is transmitted to man by an infected female Aedes or Haemogogus (in South America) mosquito (Figure 1). Transmission occurs in urban, rural and forested areas.

[[NIP01_fig1_37]]

Currently there are 34 countries in Africa, ten in South America and several Caribbean islands where the virus is present (Table 1, Figure 2). A country is regarded as infected if the virus is found in the monkey population as they act as intermediate hosts. The mosquito passes the infection to its offspring via its eggs and thus is the true reservoir for infection, ensuring that it is transmitted from one year to the next and making it virtually impossible to eradicate the disease.(2) Large-scale vaccination programmes are the key to controlling spread of the disease but they require population coverage of 80%. Currently only 17 of the 34 infected African countries have initiated vaccination programmes, and while two have achieved 50% coverage none has yet reached 80% coverage. Yellow fever has never been present in Asia, but the Aedes mosquito is indigenous so the potential exists, and thus anyone arriving in South-East Asia from an infected country requires a valid yellow fever vaccination certificate.

[[NIP01_table1_37]]

[[NIP01_fig2_37]]

The disease
Although there are three different transmission ­patterns to yellow fever - urban, forest (sylvatic) and intermediate (combination of the two) - the clinical disease is the same. After an incubation period of 3-6 days the disease can vary from mild to fatal. Mild cases may experience a short febrile illness with headache and myalgia, after which there is full ­recovery. Alternatively, a period of recrudescence is followed by a sudden deterioration with hepatorenal failure. Jaundice develops (hence the yellow in the name) and other symptoms of haemorrhagic fever appear - bleeding from the mouth, nose, bladder, rectum and other organs. Treatment is supportive only, and differential diagnosis relies on serology. In fatal cases death occurs between day 7 and day 10 - mortality can be as high as 50% in non-indigenous victims.(3) Lifelong immunity follows infection with yellow fever.

International certificate of vaccination
The purpose of International Health Regulations is to control the spread of disease rather than to protect the individual. There have been several deaths in recent years among non-vaccinated tourists to both Africa and South America. They may have been victims of the common misconception that if a country does not require a certificate of vaccination on entry, then there is no risk to the individual. This is far from the case. Countries that demand certificates are generally those that have the potential for the disease but currently have either no infection or a very low level. Countries where the disease is endemic are not likely to demand certificates of vaccination. It is beholden on all health professionals who advise the travelling public to understand this principle and to know how to check for endemicity of this disease and give appropriate ­recommendations.

The vaccination
The 17D strain live attenuated vaccination (cultivated on chick embryo) was first developed in 1937 and is the only vaccination currently authorised by the World Health Organisation.(1) The vaccine confers immunity in nearly 100% of recipients within 10 days of administration, and protection lasts for a minimum of 10 years.(4)

Contraindications
The only absolute contraindication is known hypersensitivity to egg protein. Vaccination is best avoided in HIV-positive individuals as not enough is known about their likely response. Pregnancy is a relative contraindication - if travel to high-risk destinations is essential then risk of disease outweighs ­theoretical risk of vaccination.
Anyone in whom the vaccination is contraindicated should be advised of the risks, instructed about personal protection measures and given a certificate of exemption.

Yellow fever centres
The WHO keeps a register of all centres authorised to administer yellow fever vaccine and to issue the internationally recognised yellow fever certificate. In the UK any general practice can apply to become a yellow fever centre - the address for enquiries is on page 267 of the current Green Book.(4) Practices wishing to do so need to have a dedicated vaccine refrigerator, but ­otherwise there are no complicated prerequisites.
From a professional viewpoint, nurses who agree to administer yellow fever vaccination should have a working knowledge of the disease, its endemicity, certificate requirements and vaccine recommendations, because patients referred to a "special" centre have a right to expect a level of knowledge from practitioners.

Current vaccine supply
Medeva Pharma Ltd is the only licensed manufacturer of yellow fever vaccine in the UK. Since mid-2000 they have been unable to produce their vaccine (Arilvax) because of a problem in their processing plant. Aventis Pasteur MSD manufactures yellow fever vaccine in France (Stamaril) and will supply it to the UK. The vaccine is the same as Arilvax but is unlicensed in the UK simply because it is not normally distributed here (it is licensed in Europe). It cannot be given under patient group directions but should be "prescribed" on an individual basis. Aventis Pasteur MSD can supply 10-dose ampoules only, but MASTA (Medical Advisory Services to Travellers) have been able to source a small supply of single-dose Stamaril - again it is unlicensed in the UK, but smaller centres could obtain vaccine from this outlet and maintain a service to their patients. 
Medeva Pharma Ltd is unable to say when this ­situation will be resolved. Until it is, practice staff should familiarise themselves with local centres offering the vaccine so that patients can be advised appropriately - it is not acceptable to say "we haven't got any vaccine".

[[NIP01_pp_39]]

References

  1. Vainio J, Cutts F. Yellow fever. Global programme for vaccines and immunization. Expanded programme on immunization. Geneva: WHO; 1998.
  2. Anker M, Schaaf D. Yellow fever. In: WHO report on global surveillance of epidemic-prone infectious diseases. Geneva: WHO; 2000.
  3. Simpson DIH. Arbovirus infections. In: Cook GC, editor. Manson's Tropical diseases. 20th edn. London: WB Saunders; 1996.
  4. Salisbury D, Begg N, editors. Immunisation against infectious disease [The Green Book]. London: HMSO; 1996.

Resources
TRAVAX
Online database run by Scottish Centre for Infection and Environmental Health
W:www.travax.scot.nhs.uk
MASTA - Medical Advisory Services to Travellers Subscription-based online database
W:www.masta.org
Also supplier of vaccines and other travel-related products

Further reading
WHO. International Travel and Health.
Geneva: WHO; 2001.
Book published annually - excellent quick reference source for yellow fever and malaria status of individual countries.Can also be viewed at:W:www.who.int/ith/english/index.htm.