27 April 2011

Changes to geographic risk areas for yellow fever and recommendations for vaccination

The World Health Organization (WHO) has published their 2011 edition of International travel and health. With publication of this book, new guidelines for vaccination against yellow fever (YF) come into effect.

Vaccination against yellow fever

There are three categories for vaccination:

• vaccination recommended: following a risk assessment, vaccination recommended for all travellers 9 months and older.
• vaccination generally not recommended:  because of the low potential for exposure to YF during travel to these areas, e.g. Tanzania, vaccination is generally not recommended, unless the traveller will have prolonged travel or residence, and heavy exposure to mosquitoes with an inability to protect against bites.
• vaccination not recommended: vaccination is not recommended for any traveller to the area or country.

The new vaccination maps for the Americas and Africa can be found on the WHO website. Vaccination recommendations for each country are available on the NaTHNaC Country Information Pages.

Background

In 2008 the WHO convened a Consultation on Yellow Fever and International Travel with the aim of reviewing criteria for inclusion of countries in the list of those with a risk of YF transmission (Annex 1 of ITH). This was prompted by:

• the recognition of rare but serious adverse events associated with YF vaccine
• the changing epidemiology of YF disease
• the need to harmonise country-specific YF vaccination recommendations amongst global travel health advisory bodies 

The consultation’s broad goals were to:

• provide a more accurate definition of geographic risk areas
• have transparency of recommendations
• inform country policy around International Health Regulations  (2005) and country requirements for YF vaccination [1]

The 2008 consultation agreed on the evidence that would be used for determining geographic risk of YF and then created four classifications of risk. Following this consultation, an informal working group (WG) consisting of representatives from WHO, the US Centers for Disease Control and Prevention, the European Centre for Disease Control and Prevention, NaTHNaC and individual experts, was formed to [2]:

• systematically apply the evidence to each country at risk for YF
• define the categories of risk for each country
• develop country YF risk and vaccination maps

The evidence used to categorise each country was as follows:

• human and non-human primates: cases, clusters and outbreaks
• human serology obtained prior to country YF vaccination programmes. Most of this data was generated in the 1950s and earlier.
• vegetation and altitude
• vector distribution

The four risk categories that were applied to an entire country or to an area of a country are:

• endemic: mosquito vectors and hosts are present, yellow fever cases occur at regular intervals in humans and primates, and there is evidence for transmission of YF on serology.
• transitional: areas or countries that border endemic areas, mosquito vectors and hosts are present, there is evidence for yellow fever activity in the past, based on serology, but cases of yellow fever occur at long intervals, sometimes as long as several decades.
• low potential for exposure to YF: mosquito vectors and hosts are present, but no human or primate YF cases have ever been recognised; there may be evidence of low levels of transmission in the remote past.
• no risk: no risk of transmission of YF.

These risk categories have been translated into vaccination recommendations (see above).

These changes represent an entirely new approach to protection of travellers against YF, and will require healthcare providers to consider carefully the traveller’s risks for exposure to YF and the safety of the vaccine [3].

References

1. International Health Regulations (2005). 2nd Edition. Geneva. World Health Organization. [Accessed 27 April 2011]. Available at: http://www.who.int/ihr/en/index.html

2. World Health Organization. Yellow fever risk mapping: Background for the Consultation on Yellow Fever and International Travel, 2010 (update September 2010). [Accessed 27 April 2011]. Available at: http://www.who.int/ith/YFrisk/en/index.html

3. Lindsey NP, Schroeder BA, Miller ER, et al. Adverse event reports following yellow fever vaccination. Vaccine 26:6077-82, 2008.