Persistent Transmission of Circulating Vaccine-Derived Poliovirus — Somalia, January 2017–March 2024

Results

Immunization Activities and Coverage

During the reporting period, 28 SIAs using OPV type 2 (OPV2) or IPV were conducted, including seven national immunization days, 12 subnational immunization days, and nine smaller, targeted campaigns.^†† Twelve SIAs occurred in 2021, six in 2022, eight in 2023, and two in early 2024. Vaccines used included monovalent Sabin-strain OPV2 (mOPV2) (15 SIAs), novel OPV2 (nOPV2, further attenuated version of Sabin mOPV2 with enhanced genetic stability) (eight), tOPV (three), and IPV (two). During 2017–2020, 17 bOPV campaigns were conducted.

Among 1,921 children aged 6–59 months with nonpolio AFP reported during January 2017–March 2024, caretakers reported that 730 children (38%) had received ≥3 OPV doses through routine immunization, and that 538 (28%) had received ≥1 IPV dose; 1,364 (71%) children had received ≥3 OPV doses during SIAs. Overall, 231 (12%) children with nonpolio AFP were reported to have received no OPV doses through routine immunization or SIAs (i.e., zero-dose children); among these children, 219 (95%) were from districts in the South-Central region and 139 (60%) whose accessibility status was recorded were from inaccessible districts. As of December 2023, an estimated 472,743 children (representing 17% of children in Somalia aged <5 years) remain unreached for vaccination in the South-Central region because of security reasons (Figure 1).

AFP Surveillance

As of March 2024, the Somalia AFP surveillance system comprised 983 active surveillance sites and included a total of 796 village polio volunteers situated in all regions; 71% of these volunteers are in districts in the South-Central region. During 2017–2024, Somalia’s national nonpolio AFP rate consistently reached or exceeded three cases per 100,000 persons aged <15 years per year (annual range during 2021–2024 = 3.8–5.2). However, Banadir, the state reporting the highest number of cVDPV2 cases, consistently missed this target (range = 1.8–2.9) (Table).

The 80% stool specimen adequacy target was met each year during 2017–2023 (annual range = 92.4%–99.1%) and in each state except Galmudug (72.2%) in the first quarter of 2024. However, the proportion of stool specimens that arrived at the WHO-accredited laboratory in Kenya within the recommended 3 days after collection decreased from 44% in 2017 to 9% in 2022 and 2023.

Environmental Surveillance

During January 2017–March 2024, a total of 73 cVDPV2 isolates were detected across four of the six states with ES sites; Banadir state accounted for 66 (90%) detections. Collection sites in Somalia increased from four in 2017 to 17 in 2023.

Epidemiology of cVDPV Cases

cVDPV2 cases. During 2017–2024, 39 cVDPV2 cases were reported from all seven states, including 34 (87%) from districts in the South-Central region^§§ (Figure 2) (Supplementary Table, https://stacks.cdc.gov/view/cdc/157501) (Village accessibility survey, WHO Somalia, unpublished data, 2024). The remaining five cases were reported from Somaliland (four) and Puntland (one) during 2019–2020. Overall, the mean age of patients was 36 months (range = 3–108 months) and 49% were female. Among the 39 cVDPV2 patients, 20 (51%) reportedly had received zero routine immunization or SIA OPV doses.

cVDPV3 cases. In 2018, states Hirshabelle and Jubbaland reported five and two cVDPV3 cases, respectively (Figure 2). Among these cases, three patients had received no routine immunization or SIA OPV doses, and four had received >3 SIA doses. No cVDPV3 has been isolated since September 2018.

Genomic Sequence Analysis of cVDPV Isolates

In 2017, genomic sequence analysis of circulating cVDPV2 strains indicated protracted circulation of the SOM-BAN-1 cVDPV2 emergence occurred ≥3 years before detection. The most recent case in this emergence group was detected in Jubbaland in March 2024. During January 2021–March 2024, 17 (59%) of 29 isolates were detected in Banadir, including 10 of 16 orphan viruses,^¶¶ which indicates substantial gaps in surveillance.

In December 2023, a second cVDPV2 emergence (SOM-BAY-1) was identified in southern Somalia and was most recently detected in January 2024. In addition, a cVDPV2 isolated from an ES sample collected in Banadir in May 2022 was from the Yemen cVDPV2 emergence group YEM-TAI-1; no further detections have occurred in Somalia to date.