28 July 2023
Situation at a glance
On 4 July 2023, the Ministry of Health of the United Republic of Tanzania notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in the country. The virus was isolated from a case of acute flaccid paralysis (AFP) in the Rukwa region, southwestern Tanzania bordering Lake Tanganyika to the west and Zambia to the south. Gene sequencing of the isolated virus has indicated close linkage with the cVDPV2 circulating in South Kivu, Demographic Republic of the Congo (DRC).1
The public health authorities of the Ministry of Health are conducting further field investigations including strengthening the AFP surveillance for the detection of additional AFP cases and subnational level immunity gap analysis to identify potential un-or under-immunized populations and/or areas to guide public health response activities.
WHO assesses the overall risk at the national level to be high due to the sub-optimal surveillance performance in some districts, sub-optimal vaccination coverage resulting in low population immunity and the ongoing population movement across neighbouring countries.
Description of the situation
On 4 July 2023, the health authorities of the United Republic of Tanzania confirmed and notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in the country. The case is a child under two years old and has received three doses of bOPV vaccine, one dose of IPV vaccine for routine immunization and two doses of bOPV during supplementary immunization activities (SIA) in 2022 with no documented travel history. The child was initially reported as a case of AFP from Rukwa region of southwestern Tanzania who experienced paralysis in late May 2023.
Two stool samples were collected from the case on 30 and 31 May 2023 respectively and were confirmed to be cVDPV2 on 30 June 2023. Gene sequencing results showed that the isolated virus has undergone 15 nucleotide changes and is closely related to the strain circulating in South Kivu, Demographic Republic of the Congo in 2023.1
Since 2022, Tanzania has been actively participating in a multi-country outbreak response across south-east Africa, in response to detection of different strains of poliovirus in the sub-region, including boosting immunity levels through mass vaccination campaigns and strengthening subnational surveillance capacity.
According to the WHO-UNICEF estimates of national immunization coverage, the oral polio vaccine third dose (OPV3) and the inactivated polio vaccine first dose (IPV1) was 88% in 2022 in Tanzania.
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).
The last recorded case of indigenous wild poliovirus (WPV) in Tanzania was in 1996, and the cVPDV2 case in 2023 is the first case detected in the country.