Yellow fever: a major threat to public health

The Lancet

Jan 27, 2018 Volume 391 Number 10118 p281-400  e2-e4

Yellow fever: a major threat to public health
The Lancet | 3 February 2018
The world’s largest fractional-dose vaccination campaign for yellow fever started on Jan 25 in Brazil, with the support of WHO. The campaign attempts to avoid the urban transmission cycle, not seen in the country since 1942. 33 people have died due to yellow fever between Jan 14 and 23, while the number of confirmed cases in the country has reached more than 130.

Although more cases of yellow fever were recorded in Brazil in 2016–17, the recent outbreak has extended into a much larger area, including highly populated cities, making it more threatening to public health. These large cities are infested by Aedes aegypti, the urban yellow fever vector, which can transmit the disease from person to person. The number of people at risk is also increasing in other tropical regions, such as South America and Africa.

Due to a global shortage of the vaccine, in outbreak emergencies WHO recommends fractional dosing to protect more people by using less antigen in each dose. Whereas studies have shown that a fifth of the standard dose can provide the same immunity as the standard full dose for at least 12 months, it is not clear how long the immunity ultimately lasts.

For routine immunisation, the standard full dose, recommended by WHO since 2013, is thought to confer lifelong protection, although supporting evidence for this view is not strong. Some Brazilian experts believe that a booster vaccine 10 years after the primary vaccination should be administered to guarantee lifetime protection.

As a zoonotic disease, with a reservoir in non-human primates, it is unlikely that yellow fever will be eradicated. However, epidemics can be prevented if populations are protected by routine immunisation and if mass vaccination campaigns are implemented quickly in response to an outbreak. A coalition of partners led by WHO, UNICEF, and Gavi, the Vaccine Alliance, aims to eliminate yellow fever epidemics worldwide by 2026. To achieve this goal, there is an urgent need for research to clarify the duration of protective levels of immunity provided by fractionated and full-dose yellow fever vaccines to support development of effective vaccination programmes.