Fractional doses of the yellow fever vaccine

Experts advise that a smaller dose of the vaccine can protect people from yellow fever. Studies show that the yellow fever vaccine given as one fifth of the regular dose, still provides full immunity against the disease for at least 12 months and likely longer.

Fractional dosing, or dose sparing, could be used to control an outbreak in cases where the vaccine supply is limited.

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After reviewing evidence, WHOꞌs Strategic Advisory Group of Experts (SAGE) on Immunization, determined that a fifth of a standard vaccine dose can provide full protection against the disease for at least 12 months and can be used to control outbreaks.

In a mass vaccination campaign in Democratic Republic of the Congo in 2016 the fractional dosing method was shown to be feasible and a promising approach to protect at-risk populations that would otherwise be left unprotected.

Fractional dosing is not proposed for routine immunization, as there is not enough data available to show that lower doses confer life-long protection. Studies are ongoing to determine the long-term protection provided by fractionated doses.

• Strategic Advisory Group of Experts on Immunization - yellow fever page 21

Using fractional dosing is the best way to stretch vaccine supplies and protect as many people as possible to stop the spread of yellow fever in emergency situations. Based on the available evidence, the Strategic Advisory Group of Experts (SAGE) on Immunization affirms that a fractional dose can be used as part of an exceptional response when there is a large outbreak and a shortage of vaccine.

A fractional dose does not entitle people to a yellow fever certificate valid for international travel. People, who want to travel internationally, require a full dose of the vaccine. The full dose entitles them to an international yellow fever certificate.

There is currently no available data to show that a fractional dose of the yellow fever vaccine in children under 2 years of age will provide the same protection as the full dose. Very young children may have a weaker immune response to the vaccine than older people. Therefore, children under 2 years of age should be offered a full dose.

WHO recommends that countries using fractional dosing keep good vaccination records of people who receive it so that they can be followed up later to assess how long the vaccine protection lasts and be revaccinated if necessary. These people will need to be informed that they have received the fractional dose and will require a full dose of the vaccine if they wish to travel.

The fractional dose comes from the same full dose vaccine. It has been given to millions of people to prevent yellow fever in the past. It is as safe and as effective as the full dose of the vaccine.

Serious adverse effects following a full dose of yellow fever vaccine are extremely rare (less than one per one million people). There is no evidence of increased serious adverse effects when using a fractional dose.

Fractional dosing is currently being used for inactivated polio vaccine (IPV), Rabies and Bacillus Calmette–Guérin (BCG), a vaccine primarily used against tuberculosis.