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Disease Outbreak News: Circulating vaccine-derived poliovirus type 2 (cVDPV2) - Papua New Guinea, 20 May 2025

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Situation at a glance

On 9 May 2025, the International Health Regulations (IHR) National Focal Point (NFP) for Papua New Guinea (PNG) notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) from stool specimens of two healthy children from Morobe province, Papua New Guinea (PNG). The detection of wild poliovirus (WPV) or vaccine-derived poliovirus (VDPV), including from samples taken from healthy children, is considered a serious public health event. Given the country's suboptimal routine immunization coverage, especially at the subnational level, the risk of potential spread locally is considered high. WHO advises all countries—especially those with frequent travel and connections to polio-affected areas—should strengthen acute flaccid paralysis (AFP) and environmental surveillance while maintaining high level of immunization coverage to quickly detect and respond to virus importation and prevent further spread by closing immunity gaps.

Description of the situation

On 9 May 2025, the IHR NFP for Papua New Guinea (PNG) notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) from stool specimens of two healthy children from Lae city, Morobe province, Papua New Guinea (PNG). Stool samples from 25 healthy children were collected on 10 April 2025 as part of an in-depth epidemiological investigation in the catchment area of the Environmental Surveillance (ES) site, which reported cVDPV2 on 4 April 2025. On 8 May, the WHO Polio Regional Reference Laboratory in Australia, the Victorian Infectious Diseases Reference Laboratory (VIDRL) confirmed that two of these children tested positive for poliovirus type 2. The children who tested positive for the poliovirus are from two separate villages within the ES catchment area in Lae city and were asymptomatic at the time of sample collection.

All isolates were classified as cVDPV2 and were genetically linked to the environmental sample detected on 4 April 2025. Genetic sequencing revealed 18-19 nucleotide differences from the Sabin 2 vaccine strain. All isolates are closely related to each other and genetically linked to the INO-PAP-2 emergence that previously caused an outbreak in Indonesia.

The detection of circulating type 2 poliovirus is classified as a “polio outbreak”, in accordance with the Global Polio Eradication Initiative (GPEI) Standard Operating Procedures for responding to a poliovirus Event or Outbreak.

In Papua New Guinea, vaccination coverage is low, which may facilitate the spread of cVDPV2. As of 2024, national vaccination coverage for the third dose of oral polio vaccine (OPV3) was 44%. In Morobe province, where the virus is currently detected, coverage with bivalent OPV (bOPV) remained below 40% over the past five years, ranging from 28-37%. Coverage for the first dose of inactivated polio vaccine (IPV1) has remained between 52-54%. However, Lae city has higher coverage, with 73% for bOPV and 90% for IPV1.

Epidemiology

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 contaminated water or food. The virus 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 can 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 vaccine-derived poliovirus 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 (cVDPV). Similar to wild poliovirus, cVDPVs can be of three types (1,2 or 3), the current outbreak in Papua New Guinea is due to cVDPV type 2 (cVDPV2).

Poliovirus is a pathogen targeted for global eradication and Papua New Guinea was certified polio-free in 2000, along with the rest of the countries in the WHO Western Pacific Region, following the last reported case of wild poliovirus (WPV) in 1996.

However, in 2018, an outbreak of circulating vaccine-derived poliovirus type 1 (cVDPV1) was declared in Lae City, in Morobe province, the same city now considered to be experiencing a new poliovirus outbreak. The current detection of cVDPV2 in an environmental sample and in healthy children in Lae city is both unusual and unexpected, as it is not related to the previous cVDPV1 polio outbreak in 2018.

Public health response

Following the laboratory result, the following actions have been taken:

  • Continued activation of the National and Provincial Emergency Operations Centers.
  • Enhanced acute flaccid paralysis (AFP) and environmental surveillance to detect further transmission.
  • Accelerated IPV catch-up activities nationwide to mitigate the risk of paralysis.
  • Technical preparation underway for targeted immunization response using OPV2, in collaboration with GPEI partners.
  • Cross-border coordination efforts to align with regional risk mitigation strategies.
  • Provision of updates through the International Health Regulations (IHR) mechanism.

WHO risk assessment

The international spread of poliovirus was declared a Public Health Emergency of International Concern (PHEIC) by the Director-General of the WHO on 5 May 2014, and most recently, the PHEIC declaration was extended on 6 November 2024.

Following the global withdrawal of type 2 containing OPV from routine immunization programmes in April and May 2016, the risk of poliovirus transmission has increased due to declining mucosal immunity in children.

In Papua New Guinea, the recent detection of cVDPV2 in both healthy children and in the environment has led to a classification of a poliovirus outbreak. Given the country's suboptimal routine immunization coverage, especially at the subnational level, the risk of potential spread locally is considered high. While poliovirus has the potential to spread rapidly across large distances through human movement, the affected province does not share an international border. However it is a major commercial hub and a designated seaport of entry, which increases the risk of virus importation and exportation and spread especially in the case of VDPVs and in communities with low or incomplete vaccination coverage.

It is important to note that the Western Pacific Region has been free of endemic poliomyelitis (polio) since 2000, and there are currently no other ongoing poliovirus outbreaks in the region. However, as long as any form of poliovirus continues to circulate anywhere in the world, there remains a risk of importation. Such importations can lead to new outbreaks if the virus is introduced into an area with low population immunity.

Globally from 1 January to 12 May 2025, a total of 49 cases of acute flaccid paralysis (AFP) caused by cVDPV2 have been confirmed including 21 from Ethiopia, 14 from Nigeria, nine from Chad and one case each reported from Angola, Burkina Faso, Djibouti, Niger and Sudan. Environmental surveillance reported an additional 57 cVDPV2 isolates, mainly from Algeria, Chad, Djibouti, the occupied Palestinian territory including east Jerusalem, and Nigeria. In 2024, a total of 297 AFP cases were confirmed to be associated with cVDPV2, with majority of cases were reported from six countries: Nigeria 98 cases (33%), Ethiopia 43 cases (14. 5%), Chad 39 cases (13.1%), Yemen 37 cases (12.5%), Niger 16 cases (5.4%) and the Democratic Republic of the Congo 15 cases (5.1%). The occurrence of cVDPV outbreaks, which emerge and circulate due to a lack of polio immunity in the population, shows the potential risk for further international spread. WHO will continue to closely monitor the global situation and stands ready to provide assistance, as required.

WHO advice

It is important that all countries, particularly those with frequent travel and contacts with polio-affected countries and areas strengthen AFP surveillance and initiate a planned expansion of environmental surveillance to rapidly detect 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 and to fill residual immunity gaps causing further spread.

Surveillance:

To limit the potential spread of poliovirus, characterized by asymptomatic infections, contact tracing undertaken by public health authorities is essential to identify potentially infected individuals.

Surveillance needs to be intensified in the local area contributing to the manhole site where any positive environmental sample is collected. The genetic analysis of this virus indicates it has been circulating for a considerable period before detection. Hence it is essential to strengthen poliovirus surveillance nationwide and ensure the health system is on high alert to promptly detect any new cases or ongoing transmission.

Immediate action should include: notifying all national and subnational surveillance units about the poliovirus event. Rigorously sensitize all health care workers on acute flaccid paralysis (AFP) surveillance requirements. Conduct community engagements and awareness activities to increase awareness of AFP and polio.

Increase the frequency of environmental surveillance sample collection in Lae City, Morobe province, from monthly to twice monthly in three existing sites and to consider expanding surveillance to additional sites within the city.

Vaccination:

In accordance with the Global Polio Eradication Initiative (GPEI) Standard Operating Procedures, preparations are underway in the country to launch a vaccination response using the oral polio vaccine type 2 (OPV2). All efforts should be implemented to increase population immunity and prevent poliovirus transmission to protect vulnerable populations, by identifying and vaccinating children with no history or incomplete immunization.

Routine immunization remains the cornerstone of polio eradication in alignment with the national childhood immunization schedule. The IPV provides a high level of individual immunity and effective protection against paralysis.

WHO does not recommend any restriction on travel and/or trade to the Papua New Guinea on the basis of the information available for the current event.

Further information

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