Yellow Fever Vaccination Response

In December 2015, a yellow fever outbreak started in Angola and quickly spread within the country and to its neighbor, the Democratic Republic of Congo (DRC). Laboratory testing confirmed 962 cases, but there were thousands of suspected cases, making this the largest reported outbreak in 30 years. A critical aspect to yellow fever outbreak response is vaccination campaigns in the affected areas. One dose of yellow fever vaccine is capable of providing lifelong protection. Thus the World Health Organization (WHO) keeps a stockpile of 6 million yellow fever vaccine doses for outbreak response. However, the 2016 outbreak response campaigns in Angola and DRC used almost 30 million vaccine doses, far more than had been needed in recent outbreaks.

A vaccination campaign scheduled for August 2016 targeted 8 million people in DRC’s capital city of Kinshasa. However, at that time, 8 million doses of yellow fever vaccine were not available in the global supply. Due to this vaccine shortage, WHO recommended a strategy that had worked well in a couple of small studies but had never been used during a large-scale response effort: administering 1/5 of the regular dose (also called a “fractional” dose). This meant that one standard dose of vaccine could vaccinate up to 5 people. Officials in DRC agreed to this strategy and the Kinshasa campaign administered fractional dose yellow fever vaccine to all non-pregnant adults and children age two years and over. Children under 2 years old and pregnant women received full doses because the fractional dose had never been studied in children or pregnant women. Since the target population and the campaign environment were very different from those in the controlled research settings where fractional dose vaccine had previously been used, DRC health authorities and yellow fever experts globally felt it was important to evaluate fractional dose vaccine during this campaign. Specifically, everyone wanted evidence that fractional dose vaccination caused an immune response in recipients that would be sufficient to protect them from yellow fever virus infection.

With only one month notice before the start of the campaign, the CDC and the DRC’s Institut National de Recherche Biomedicale quickly developed an evaluation plan, with USAID providing key financial support. Working at six of the more than 2000 vaccination sites in Kinshasa, the evaluation team enrolled 760 adults and children (aged >2 years) during the 10-day vaccination campaign. Participants provided blood samples before vaccination and one month after vaccination. These samples are currently being tested to see if participants developed antibodies that will protect them against yellow fever. One year after vaccination, evaluation staff will collect an additional blood sample from participants to see if the levels of antibodies are still high enough to provide protection against yellow fever. If the fractional dose is found to provide a sufficient immune response, fractional dose vaccination may be used more frequently in outbreak response campaigns. This would reduce the vaccine supply needs during outbreaks and significantly increase the global capacity to respond to large outbreaks like the recent one in Angola and DRC.

• Read more about Global Immunization: New and Underused Vaccines (GID page)
• Read more about yellow fever (NCEZID page)
• Q&A: Fractional doses of the yellow fever vaccine (WHO)
• Notes from the Field: Investigation of Patients Testing Positive for Yellow Fever Viral RNA After Vaccination During A Mass Vaccination Campaign — Angola, 2016.