Disease Outbreak News: Yellow fever – African Region (AFRO)

Since the beginning of 2023, and as of 25 February 2024, a total of 13 countries in the WHO African Region have documented probable and confirmed cases of yellow fever (YF), namely Burkina Faso, Cameroon, the Central African Republic, Chad, Republic of the Congo, Côte d’Ivoire, the Democratic Republic of the Congo (DRC), Guinea, Niger, Nigeria, South Sudan, Togo and Uganda. Preliminary data for 2023 indicate a case fatality rate (CFR) of 11%. While the overall risk at the regional level was re-assessed as moderate and the global risk remains low, active surveillance is required due to the potential for onward transmission through travel and the presence of the competent vector in neighboring regions. The urban proliferation of Aedes spp. mosquitoes, which bite during the day, can also significantly amplify transmission risks, particularly in densely populated areas, leading to swift outbreaks. The WHO-led global Eliminate Yellow Fever Epidemics (EYE) secretariat coordinated preventive and reactive efforts and notably during 2023, substantially enhanced vaccination coverage. Approximately 62 million people have been vaccinated in Africa through preventive and reactive mass vaccination campaigns. Additionally, under catch-up campaigns in Sudan, approximately 4 million individuals received vaccination against YF.

Description of the Situation

Since the beginning of 2023, and as of 25 February 2024, a total of 13 countries in the WHO African Region have documented probable and confirmed cases of yellow fever (YF), namely Burkina Faso, Cameroon, the Central African Republic, Chad, Republic of the Congo, Côte d’Ivoire, the Democratic Republic of the Congo (DRC), Guinea, Niger, Nigeria, South Sudan, Togo and Uganda.

Preliminary data for 2023 indicate a case fatality rate (CFR) of 11% and a sex ratio of 1.7 (M: F). The median age of cases is 25 years, with two-thirds (69%) of cases aged over 15 years.  Suboptimal surveillance capacity exists in many affected countries, with data fragmentation, limited integration with routine surveillance and clinical systems, and a lack of standardized case definitions that contribute to underreporting and increased mortality rates.

During the last quarter of 2023 and as of 25 February 2024, a total of eight countries (Cameroon, Chad, Congo, DRC, Guinea, Niger, Nigeria, and South Sudan), have reported active YF transmission with confirmed YF cases (Table 1). These countries have initiated response planning activities.

Chad, Gabon, Niger, Nigeria, and Togo** are currently awaiting the final classification of the YF laboratory plaque reduction neutralization test (PRNT) positive samples.

Cameroon: Three YF cases were confirmed by polymerase chain reaction (PCR) testing, occurring in the weeks ending on 22 October and 13 November 2023. The country recorded probable and confirmed YF cases consistently throughout 2023, starting from the week ending 29 January 2023.

Chad: A yellow fever PCR-positive case with negative serology was reported on 18 January 2024 from a patient sampled on 10 October 2023, in the Léré district of the Mayo-Kebbi Ouest region.

Republic of the Congo: Cases pending classification with the national health authorities. No further information was available as of February 2024.

Democratic Republic of the Congo: One YF PCR-positive case with unknown vaccination status was reported on    18 December 2023. Additionally, nine PRNT positive cases were recorded, eight of which have unknown vaccination status and one lacks information in the medical charts.

Guinea: Three YF PCR-positive cases were confirmed at the Institute Pasteur Dakar (IPD). These cases were reported on 17 October and 23 December 2023 in a 6-year-old female from Faranah region, a 7-year-old male from the Koundara health district (sample collected on 6 December), and a 60-year-old female from the Guiéckédou district (sample collected on 15 December), representing three out of the seven country’s regions. Notably, this occurrence is less than three years since cases were confirmed in the Koundara health district in 2020 and 2021.

Niger: Case reported in the Tahoua city, Department of Tahoua during the first week of January 2024. No further information was available as of February 2024. Investigations are ongoing.

Nigeria: One YF PRNT-positive case was reported in January 2024 in Lagos state in a 49-year-old male residing in a rural area. Investigations are ongoing.

South Sudan: A confirmed YF case was reported on 24 December 2023, following the investigation of suspected cases and deaths of viral hemorrhagic fever. The case, a 24-year-old male, presented with fever, vomiting blood, and jaundice. Two additional cases were confirmed on 2 February 2024. As of 12 February 2024, South Sudan reported a total of 64 cases that fulfil the current outbreak case definition of YF, including 61 suspected cases and three confirmed cases from six out of 10 counties in Western Equatoria state: Yambio (33), Nzara (09), Tambura (12), Ibba (4), Ezo (5) and Maridi Counties (1). Among 61 suspected cases, there were six suspected deaths currently under investigation.