On 3 March 2020, the Ministry of Health of South Sudan reported two presumptive positive cases of yellow fever in Kajo Keni county, Central Equatoria State, South Sudan. Both the cases were subsequently confirmed positive by plaque reduction neutralization testing (PRNT) at the regional reference laboratory, Uganda Viral Research Institute (UVRI) on 28 March.

The cases were identified through a cross-border rapid response team investigation mounted in response to the recently declared outbreak in bordering Moyo district, Uganda. During the investigation, the team collected 41 blood samples from five villages which were in close proximity to the bordering Moyo district, Uganda. Of the 41 individuals whose samples were collected, nine (22%) had history of fever, but none had history of jaundice. The individuals represented a spectrum of occupations typical for the area (farming, forestry, homemaker, soldier). Most of the individuals investigated were between 20-45 years of age, and 18 (44%) of these individuals were female.

In addition, a rapid entomology survey in the villages found evidence of multiple mosquito breeding sites and abundant Aedes species mosquitos (e.g. Aegypti , Albopictus, Simpsoni).

As of 28 March 2020, these are the only two cases (no deaths) that have been confirmed from Kajo Keji county.

South Sudan has experienced several yellow fever outbreaks in the past. The last outbreak was declared on 29 November 2018, in Sakure payam, Nzara County, Gbudue State when three laboratory confirmed cases with no associated deaths was reported. To respond to the outbreak, a targeted reactive vaccination campaign was mounted in the affected area. Prior to this outbreak, in May 2003, a total of 178 cases with 27 deaths were reported in Imatong region, Torit Country, South Sudan. A reactive vaccination campaign was mounted to respond to the outbreak in 2003.

Public health response

  • The country rapidly mounted an in-depth multi-disciplinary investigation in Kajo Keji county in response to the cross-border notification of the outbreak in Moyo, Uganda. The investigations included enhanced surveillance, active case finding and entomological surveys from 12-18 February, supported by WHO country office and headquarters.
  • Surveillance has been enhanced within the context of integrated disease surveillance and response (IDSR) through training of health workers and disseminating the yellow fever case definition to the health facilities to enhance case detection and reporting.
  • The Ministry of Health, with support from the WHO Country office, has planned a reactive vaccination campaign in Kajo Keji country for which an International Coordination Group (ICG) request has been submitted.
  • The Ministry of Health has also proposed implementation of preventive mass vaccination campaigns and introduction of yellow fever vaccination into the routine immunization schedule by 2022.

WHO risk assessment

Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes and has the potential to spread rapidly and cause serious public health impact. Vaccination is the most important means of preventing the infection, which provides immunity for life.

The risk of ongoing and further transmission of yellow fever is concerning in South Sudan due to :

  • Confirmed outbreak in the bordering area of Uganda, coupled with the continuous savannah and forest biome with common mosquito vectors and non-human primates;
  • Gaps in surveillance and weaknesses in health services could delay rapid detection of additional yellow fever cases;
  • Negligible population immunity;
  • Ongoing resettlement in Kajo Keji including over 13,200 returnees to the county primarily from Uganda in recent weeks represents an influx of new susceptible population to the county;
  • The ongoing COVID-19 related controls on borders (air/land) is anticipated; however, as the borders between Democratic Republic of the Congo, Uganda and South Sudan are highly porous with substantial cross border social and economic activities, risk of transmission cannot be excluded;
  • The current rainy season which started in early March is anticipated to increase the vector burden in coming weeks, thereby increase the risk of ongoing yellow fever transmission.

Close monitoring of the situation with active cross-border coordination and information sharing is needed.

Based on available information, WHO assesses the overall risk to be high at the national level and regional level.

WHO advice

South Sudan is classified as a high-risk country in the “Eliminate Yellow Fever Epidemics” (EYE) initiative. Epidemic spread of Yellow Fever is a risk in South Sudan as the estimated overall population immunity is negligible with nearly 0% immunity in Kajo Keji County.

Vaccination is the primary means for prevention and control of yellow fever and provides immunity for life. In urban centres, targeted vector control measures are also helpful to interrupt transmission. The country plans to introduce Yellow Fever vaccination into the routine immunization program and complete preventive mass vaccination activities to rapidly boost population immunity. Expedited planning and implementation of these activities to protect the population will help avert risk of future outbreaks.

WHO recommends vaccination against yellow fever for all international travellers aged 9 months and above going to South Sudan as there is evidence of persistent or periodic yellow fever virus transmission. Yellow fever vaccination is safe, highly effective and provides life-long protection. However, yellow fever vaccination is not recommended for infants aged 6 to 8 months, except during epidemics when the risk of yellow fever virus transmission may be very high. The risks and benefits of vaccination in this age group should be carefully considered before vaccination. The vaccine should be used with caution during pregnancy or breastfeeding. However, pregnant or breastfeeding women may be vaccinated during epidemics or if travel to a country or area with risk of transmission is unavoidable. South Sudan also requires, as a condition of entry, a valid yellow fever vaccination certificate for travellers aged 9 months and above.

In accordance with the International Health Regulations (IHR 2005), Third Edition, the validity of the international certificate of vaccination against yellow fever extends throughout the life of the person vaccinated. The International Certificate of Vaccination becomes valid 10 days after vaccination. A single dose of WHO approved yellow fever vaccine is sufficient to confer sustained immunity and life-long protection against yellow fever disease. A booster dose of the vaccine is not needed and is not required of international travellers as a condition of entry.

WHO encourage its Member States to take all actions necessary to keep travellers well informed of risks and preventive measures including vaccination. As a general precaution, WHO also recommends avoidance of mosquito bites. The highest risk for transmission of yellow fever virus is during the day and early evening. Travellers should be made aware of yellow fever symptoms and signs and instructed to rapidly seek medical advice if presenting signs and symptoms suggestive of yellow fever infection.
Travelers returning to South Sudan who may be infected with possible high levels of the virus in the blood may pose a risk for the establishment of local cycles of yellow fever transmission in areas where the competent vector is present.

WHO does not recommend any restrictions on travel and trade to Republic of South Sudan on the basis of the information available on this outbreak.

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