Disease Outbreak News

Circulating vaccine-derived poliovirus type 2 – Global update

26 March 2021

Description of the situation

A global update on circulating vaccine-derived poliovirus type 2 (cVDPV2) outbreaks and events is summarized below, as of February 2021.

In 2020, 959 human cases of circulating vaccine-derived poliovirus type 2 (cVDPV2) and 411 cVDPV2-positive environmental samples were reported globally from 27 countries, of which 21 countries were from the African Region and six countries from reported from the regions of the Eastern Mediterranean, Europe and the Western Pacific. The number of cVDPV cases and environmental samples increased in 2020 compared to 2019 when 366 cVDPV2 cases and 173 cVDPV2-positive environmental samples were reported.

The latest epidemiological information on circulating vaccine-derived poliovirus is updated on a weekly basis.

African Region

Since 2017, several genetically-distinct cVDPV2 outbreaks continue to be reported across the Region. In total, 21 countries are affected by ongoing cVDPV2, and outbreak response activities continue to be implemented in Angola, Benin, Burkina Faso, Cameroon, Central African Republic, Chad, Côte d’Ivoire, Democratic Republic of the Congo, Ethiopia, Ghana, Guinea, Kenya, Liberia, Mali, Niger, Nigeria, Republic of Congo, Senegal, Sierra Leone, South Sudan and Togo.

Eastern Mediterranean Region

Afghanistan has reported co-circulation of wild poliovirus type 1 (WPV1) and cVDPV2. In 2020, 56 WPV1 cases and 35 environmental samples positive for WPV 1 were reported; 307 cases of cVDPV2 and 172 environmental samples positive for cVDVP2 were reported. Of the 307 cVDPV2 cases reported in acute flaccid paralysis (AFP) cases in 2020, 199 (65%) genetically derived from the Pakistan- Gilgit-Baltistan -1 emergence which originated in Gilgit-Baltistan, Pakistan. The remaining 108 cases were classified as a new emergence from Afghanistan; 105 cases were classified as AFG-NGR-1 and 3 as AFG-HLD-1. As of February 2021, 11 cVDPV2 cases and nine environmental samples positive for cVDVP2 were reported in 2021.

Pakistan is one of the two endemic countries for polio, and has reported co-circulation of WPV1 and cVDPV2. In 2020, 84 WPV1 cases and 407 environmental samples positive for WPV1 were reported. Among the 84 WPV1 cases, 60 were reported in the first half of 2020 while the remaining 24 cases were reported in the second half (compared to 103 cases during the same period in 2019). As of 28 February 2021, Pakistan has reported one case of WPV1 in 2021.

In 2020, 135 cVDPV2 cases and 135 environmental samples positive for cVDPV2 were reported from Pakistan. Since the beginning of 2021, there have been six cVDPV2 cases and ten cVDPV2 positive environmental samples reported to date. The cVDPV2 outbreak poses increased risk of spread to neighboring countries, and cVDPV2 has been detected from Iran and Tajikistan.

Sudan continues to report a cVDPV2 outbreak, which originated in Chad, with 57 cases reported in 2020.

Somalia continues to report a cVDPV2 outbreak, with three cVDPV2 cases in 2019 and 14 cVDPV2 cases in 2020. In 2021, cVDPV2 linked to Somalia was detected in Kenya.

European Region

In Tajikistan, the first cVDPV2 case with onset of AFP on 22 November 2020 was reported from Khatlon province. cVDPV2 detected from this case was linked to cVDPV2 currently circulating in Pakistan. In February 2020, cVDPV2 was identified in another AFP case in Khatlon province, as well as in a healthy child from Vahdat district close to the capital, Dushanbe.

On 8 March 2021, two more AFP cases with cVDPV2 were confirmed in Hisor district. Three environmental samples from the newly established surveillance site in Dushanbe were reported cVDPV2 positive.

Western Pacific Region

In the Philippines, outbreak response to a cVDPV2 outbreak, which emerged in 2019, is continuing. A total of 13 cases have been associated with this outbreak in the country; 12 in 2019 and one in 2020. No cVDPV2 cases have been reported since January 2020. However, after the use of type 2 monovalent oral polio vaccine (mOPV2) in large-scale polio supplementary immunization activities (SIAs), one case of ambiguous vaccine-derived poliovirus type 2 (aVDPV2) was detected in 2020. Separately, the country is also affected by a circulating vaccine-derived poliovirus type 1 (cVDPV1) outbreak, with two cases reported in the country in 2019. No cVDPV1 cases have been reported since December 2019.

The cVDPV2 outbreak response in Malaysia is also continuing. No cases of cVDPV2 have been reported since March 2020 while several environmental samples tested positive for cVDPV2. All of them were genetically linked with cVDPV2 in the Philippines. Malaysia is also affected by the cVDPV1 outbreak and the response is ongoing. Four cases have been associated with this outbreak in the country, three in 2019 and one in 2020. All cases were genetically linked with the cases detected in the Philippines. No cases of cVDPV1 have been detected since March 2020.

According to the WHO-UNICEF routine immunization estimates, inactivated polio vaccine (IPV) coverage was estimated to be 41% and 99% in Philippines and Malaysia respectively in 2019.

Public health response

In 2019 and early 2020, the Global Polio Eradication Initiative (GPEI) developed the Strategy for the Response to type 2 circulating vaccine-derived poliovirus 2020–2021 to more effectively address the evolving cVDPV2 epidemiology, including as appropriate through the roll-out of an improved vaccine, novel oral polio vaccine type 2 (nOPV2), through the WHO Emergency Use Listing. This strategy is in line with Executive Board Decision from 7 February 2020, which also called for the mobilization of domestic financial resources to complement international financial and political commitments, to address the evolving cVDPV2 emergency.

African Region: The African Region completed assessments of existing outbreaks and based on the findings and recommendations, and in accordance with global guidelines, this led to the closure of 18 genetically-distinct outbreaks in 2020 in six countries: Angola, Central African Republic, Democratic Republic of the Congo, Ethiopia, Nigeria and Zambia. Countries across the region are continuing to implement response in line with international outbreak response guidelines, supported by the Regional Rapid Response Team.

Eastern Mediterranean Region: The regional Incident Management Support Team (IMST), jointly coordinated by WHO and UNICEF, is supporting countries in their outbreak response and field investigation efforts. To ensure greater engagement of government leadership and provide additional support to both affected countries in the Region, WHO established the Ministerial Regional Subcommittee on Polio Eradication and Outbreaks, which is planned to convene in mid-March 2021.

Afghanistan: The national programme is adapting operational approaches, to rapidly stop both WPV1 and cVDPV2 transmission. Efforts are focusing on adapting vaccination campaign approaches to the current COVID-19 situation.
Pakistan: Pakistan polio program has successfully resumed vaccination activities amid the COVID-19 pandemic since July 2020, which started with a small-scale round and later on expanding to nationwide campaigns. Due to the dual threat of different strains, trivalent oral poliovirus vaccine (tOPV) was reintroduced as SIAs. Since resuming, all planned SIA activities and enhanced outreach activities for routine immunizations has resulted in declines in the number of cases of both WPV1 and cVDPV2. No isolation of cVDPV2 was reported in 72% of affected districts after completion of two rounds of type 2 containing vaccines.

The national Polio eradication programme in Pakistan is determined to interrupt the circulation of cVDPV2 by June 2021 and WPV1 by June 2022 as a key component of the broader health and economic COVID-19 recovery process.

European Region: In Tajikistan, local health authorities are preparing to fast-track response with nOPV2 campaigns. According to the WHO-UNICEF routine immunization estimates, IPV coverage was estimated to be 96% in 2019.

WHO risk assessment

African Region: WHO considers there to be very high risk of international spread and/or emergence of cVDPV2 due to the low population immunity against poliovirus type 2 (PV2) and low IPV vaccination coverage. This in conjunction with the existence of known viruses that have persisted for over one year, large-scale population movements, and ongoing challenges for country’s public health systems due to the COVID-19 epidemic and other infectious disease outbreaks in the Region.

Eastern Mediterranean Region: WHO considers there to be a high risk of international spread and/or emergence of cVDPV2, based on existence of known polioviruses that have persisted in the population/environment for over one year, large inaccessible populations in some countries, where large pockets of low population immunity arise as a result.

European Region: WHO considers there to be a low to moderate risk of international spread and/or emergence of cVDPV2 in the Region.

Western Pacific Region: WHO considers there to be a low to moderate risk of international spread and/or emergence of cVDPV2.

In all instances, the continued spread of existing outbreaks as well as the emergence of new outbreaks of cVDPV2 point to gaps in routine immunization coverage in addition to insufficient reactive vaccination as an outbreak response, with monovalent oral polio vaccine type 2 (mOPV2). The risk of further spread of such strains, or the emergence of new strains in 2021 is magnified by an increasing gap in global mucosal-immunity to type 2 poliovirus and dropping immunization rates related to the ongoing COVID-19 pandemic.

In its report from its most recent meeting in February 2021, the Emergency Committee under the International Health Regulations (2005) on the international spread of poliovirus expressed concern at the continued rapid spread of cVDPV2, and noted that the risk of international spread of cVDPV2 is currently very high and urged the implementation of advice aimed at reducing this risk.

WHO advice

It is important that all countries, in particular those with frequent travel and contacts with polio-affected countries and areas, strengthen surveillance for AFP cases and commence planned expansion of environmental surveillance in order to rapidly detect any new virus importation and to facilitate a rapid response. Countries, territories and areas should also maintain uniformly high routine immunization coverage at the district level to minimize the consequences of any new virus introduction.

WHO’s International Travel and Health recommends that all travellers to polio-affected areas be fully vaccinated against polio. Residents (and visitors for more than 4 weeks) from infected areas should receive an additional dose of OPV or IPV within 4 weeks to 12 months of travel.

As per the advice of an Emergency Committee convened under the International Health Regulations (2005), the risk of international spread of poliovirus remains a Public Health Emergency of International Concern (PHEIC). Countries affected by poliovirus transmission are subject to Temporary Recommendations. To comply with the Temporary Recommendations issued under the PHEIC, any country infected by poliovirus should declare the outbreak as a national public health emergency, ensure the vaccination of residents and long-term visitors and restrict at the point of departure travel of individuals, who have not been vaccinated or cannot prove the vaccination status.

As of 26 March 2021, updates on polioviruses will no longer be published on the Disease Outbreak News website. Instead, a Disease Outbreak News will serve as an alert when a change in polio the epidemiological situation worldwide is noted by GPEI, who closely monitor the situation on a weekly basis. For further information, see the GPEI weekly report.