The World Health Organization has revealed that the risk of vaccine-derived polio in Kenya remains high.

Following the revelation, the Ministry of Health has started a campaign to vaccinate hundreds of thousands of children against polio in Nairobi and Garissa due to an outbreak.

WHO revealed that two children have been paralysed by a vaccine-derived poliovirus while two others remain asymptomatic.

All four are refugees from Somalia, and are at the Hagadera refugee camp, in Garissa county.

The children will be immunised with the oral polio vaccine type 2 (nOPV2), which is less likely to revert into a form which can cause paralysis.

“The first round of the vaccination campaign in August will vaccinate all children under five years of age targeting Garissa county, the epicenter of the outbreak, and neighbouring counties with a Somali community as well as Nairobi, due to population movements. A second and third round with the expansion of targeted areas is being planned to be conducted in September and October,” WHO said in a statement.

The WHO said the risk of the vaccine-derived poliovirus outbreak in Kenya remains high.

“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, mass and frequent population movements with Somalia,” WHO said.

It also noted there was influx of new arrivals to the refugee camp, late identification of the newly arrived children, high prevalence of unvaccinated dose children among the new arrivals.

The first case involved a two-year-old boy who had received three doses of oral poliovirus vaccine (OPV) and showed no travel history. He developed paralysis on May 26, and stool samples were collected on June 1 and June 2.

The second case was a 2.7-year-old girl who had travelled to Afmadhow, Lower Juba, in southern Somalia. She had received three doses of OPV and developed paralysis on May 27, with stool samples collected on June 2 and June 3.

Additionally, two asymptomatic healthy children from the community were identified with the same emerging strain from Somalia. One child, a 2-year-old male, had never been vaccinated, while the other child was a contact from Hagadera refugee camp.

“The continued spread of existing outbreaks including the genetically linked cVDPV2 cases detected in Kenya which is genetically linked to the one circulating in Somalia as well as the emergence of new outbreaks of cVDPV2 indicates significant gaps in routine immunization coverage and inadequate outbreak response vaccination,” WHO said.

The vaccine-derived poliovirus is a mutated strain of the poliovirus originally present in the oral polio vaccine. In rare instances, the virus can genetically change and spread in under-immunized communities with poor hygiene and sanitation.

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The lower the population’s immunity, the longer the virus survives and undergoes genetic changes, potentially causing paralysis as with the wild poliovirus. When such vaccine-derived virus spreads and is genetically linked in the community for at least two months, it is classified as a “circulating” vaccine-derived poliovirus type 2 (cVDPV2).

The Ministry of Health said it has already conducted risk assessments and field investigations.

“The MoH has activated a Technical Coordination Committee to prepare for emergency outbreak response, which includes large-scale supplementary immunization activities using type-2 containing polio vaccine,” MoH said.

To address potential under-immunized populations, active surveillance for acute flaccid paralysis (AFP) cases is being strengthened, and cross-border surveillance is being intensified. The WHO also recommends maintaining high routine immunization coverage, especially among vulnerable populations such as refugees and returnees.

“The risk of international spread and emergence of cVDPV2 is considered high, particularly in areas with low population immunity, suboptimal surveillance, and inadequate routine immunization levels,” WHO said.

“The detection of genetically linked cVDPV2 cases in Kenya and Somalia indicates significant gaps in vaccination coverage and outbreak response, which may have been exacerbated by disruptions in vaccination programs during the COVID-19 pandemic.”