Circulating vaccine-derived poliovirus type 2 (cVDPV2) – Kenya

28 July 2023

Situation at a glance

On 11 July 2023, WHO received an official report regarding the detection of a circulating vaccine-derived poliovirus type 2 (cVDPV2) in two acute flaccid paralysis (AFP) cases and two asymptomatic healthy children community contacts from Hagadera refugee camp, in Kenya, the second largest refugee camp in the world with over 100 000 refugees.

The genetic sequencing analyses showed that all four isolates are genetically linked to the cVDPV2 circulating in Banadir, Somalia.

WHO assesses the overall risk at the national level to be high due to the overcrowded living conditions in the refugee camp, high rate of malnutrition, poor water and sanitation facilities, and frequent population movements with Somalia.

Description of the situation

On 11 July 2023, WHO received an official report regarding the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in Kenya. The detection was reported from Hagadera refugee camp which is the second largest refugee camp in the world with over 100 000 refugees.

Four genetically-linked cVDPV2 were isolated in samples from two acute flaccid paralysis (AFP) cases, and two asymptomatic healthy children community contacts, from Fafi district, Garissa county. The cases and community contacts are all from Hagadera refugee camp, associated with frequent population movement with neighboring Somalia.

The first case is a 2-year-old male, vaccinated with three doses of oral poliovirus vaccine (OPV), with no travel history, who developed paralysis on 26 May, and stool samples were collected on 1 June and 2 June. The second case is a 2.7-year-old female with a travel history to Afmadhow, Lower Juba, southern Somalia, vaccinated with three doses of oral polio vaccine (OPV) who developed paralysis on 27 May, and stool samples were collected on 2 June and 3 June 2023.

A community contact, a 2-year-old male asymptomatic healthy child was identified with a stool sample collected on 25 May 2023. The child has zero-dose vaccination. A second asymptomatic healthy child community contact was also identified from Hagadera refugee camp with the same emerging strain from Somalia and case investigation is being conducted.

The genetic sequencing analyses showed that all four isolates have undergone between 65 and 73 nucleotide changes from Sabin and are genetically linked to the cVDPV2 circulating in Banadir, Somalia.

According to the WHO UNICEF estimates of national immunization coverage, oral polio vaccine third dose (OPV3) and inactivated poliovirus vaccine first dose (IPV) was 91% in Kenya in 2021. However, the coverage in Hagadera camp is 77% for both OPV3 and IPV as of May 2023.

Epidemiology of Poliomyelitis

Polio is a highly infectious disease that largely affects children under five years of age, causing permanent paralysis (approximately 1 in 200 infections) or death (2-10% of those paralyzed).

The virus is transmitted from person-to-person, mainly through the fecal-oral route or, less frequently, by a common vehicle (e.g., contaminated water or food) and multiplies in the intestine, from where it can invade the nervous system and cause paralysis. The incubation period is usually 7-10 days but can range from 4-35 days. Up to 90% of those infected are either asymptomatic or experience mild symptoms and the disease usually goes unrecognized.

Vaccine-derived poliovirus is a well-documented strain of poliovirus mutated from the strain originally contained in OPV. OPV contains a live, weakened form of poliovirus that replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. On rare occasions, when replicating in the gastrointestinal tract, OPV strains genetically change and may spread in communities that are not fully vaccinated against polio, especially in areas where there is poor hygiene, poor sanitation, or overcrowding. The lower the population’s immunity, the longer this virus survives and the more genetic changes it undergoes.

In very rare instances, the vaccine-derived virus can genetically change into a form that can cause paralysis as does the wild poliovirus – this is what is known as a vaccine-derived poliovirus (VDPV). The detection of VDPV in at least two different sources and at least two months apart, that are genetically linked, showing evidence of transmission in the community, is classified as ‘circulating’ vaccine-derived poliovirus type 2 (cVDPV2).

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