Polio Re-emergence in Papua New Guinea: Understanding the 2025 cVDPV2 Outbreak
Introduction
Poliovirus, a highly contagious virus primarily affecting children under five, has long been a global public health concern. While wild poliovirus cases have significantly declined thanks to widespread immunization campaigns, the virus continues to pose a threat through rare strains such as circulating vaccine-derived poliovirus (cVDPV). The global initiative to eradicate polio has made tremendous progress, with most regions declared polio-free. However, the emergence of vaccine-derived strains reveals that the fight is not over.
In May 2025, Papua New Guinea reported the detection of cVDPV type 2 (cVDPV2) from stool samples of two healthy children in Lae City, Morobe Province. This marks a significant public health event, especially given the country’s historically low immunization coverage.
This article explores the details of the outbreak, how cVDPV emerges and spreads, risk factors, the public health response, and the global context—highlighting why sustained vigilance and vaccination remain critical in preventing polio’s resurgence.
What is cVDPV2?
Circulating vaccine-derived poliovirus type 2 (cVDPV2) is a mutated form of the weakened poliovirus strain originally used in the oral polio vaccine (OPV). While OPV has played a crucial role in reducing global polio cases due to its ability to induce strong gut immunity and community protection, it contains live, attenuated (weakened) virus that can replicate in the intestines for a short period. In under-immunized populations, however, this virus can continue to circulate and, over time, genetically mutate into a more virulent form capable of causing paralysis—much like the wild poliovirus.
Vaccine-derived poliovirus (VDPV) becomes classified as circulating (cVDPV) when there is evidence of person-to-person transmission in the community, supported by genetic sequencing. If virus isolates from different sources are genetically linked and collected over at least two months, this constitutes a cVDPV outbreak.
Type 2 (cVDPV2) is of particular concern because it has been responsible for the majority of cVDPV cases globally in recent years. Its emergence highlights critical immunity gaps in populations where routine vaccination is inadequate. The presence of cVDPV2 in the environment or healthy individuals, as seen in Papua New Guinea, signals ongoing transmission and poses a serious threat to public health—particularly in communities with low immunization coverage.
Situation Overview in Papua New Guinea
The detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in Papua New Guinea follows a concerning sequence of events that began in early April 2025. On April 4, an environmental surveillance site in Lae City, Morobe Province, reported the presence of cVDPV2 in a sewage sample. This prompted a targeted investigation and the collection of stool specimens from 25 healthy children in the area on April 10.
Laboratory analysis conducted at the WHO Polio Regional Reference Laboratory in Australia confirmed on May 8 that two children—both asymptomatic and from separate villages within Lae City—had tested positive for poliovirus type 2. The World Health Organization (WHO) was officially notified of these findings on May 9, elevating the concern to an international level.
Genetic sequencing of the virus revealed 18–19 nucleotide differences from the original Sabin 2 vaccine strain. These isolates were genetically linked to each other and to the INO-PAP-2 strain that previously caused a cVDPV2 outbreak in Indonesia, indicating ongoing transmission and cross-border spread.
Importantly, Lae City is no stranger to polio resurgence. In 2018, the same location experienced an outbreak of cVDPV1, further emphasizing its vulnerability due to consistently low immunization coverage in the region.
Given this context and the confirmed environmental and human cases, WHO has officially classified this event as a polio outbreak. The presence of cVDPV2 in asymptomatic children underscores the silent and stealthy nature of the virus’s transmission, especially in under-immunized communities, reinforcing the urgent need for aggressive surveillance and immunization efforts.
Epidemiological Risk Factors
Poliovirus is a highly infectious pathogen transmitted primarily through the fecal-oral route, meaning it spreads via contact with contaminated hands, water, or food. Less commonly, it can also spread through respiratory droplets. Poor sanitation, overcrowding, and inadequate hygiene create an environment where the virus can easily circulate, especially in under-immunized communities.
A key challenge in controlling polio is its asymptomatic nature—up to 90% of infected individuals show no symptoms, allowing the virus to spread undetected. In symptomatic cases, the infection can lead to acute flaccid paralysis in approximately 1 out of every 200 cases. Among those paralyzed, 5–10% may die due to respiratory muscle failure, underscoring the seriousness of the disease.
In Papua New Guinea (PNG), low routine immunization coverage greatly increases vulnerability. As of 2024, national coverage for the third dose of oral polio vaccine (OPV3) stood at only 44%, with Morobe Province showing even lower coverage of 28–37% for bivalent OPV (bOPV). Such gaps leave a significant portion of the population without mucosal immunity, which is critical for halting person-to-person transmission.
Though Lae City has relatively higher immunization rates—73% bOPV and 90% IPV1—its status as a major commercial hub and seaport raises the risk of virus importation and exportation. Combined with weak public health infrastructure and sanitation challenges in surrounding regions, the conditions are primed for further local and regional spread of cVDPV2 if not addressed promptly.
Global Context and Comparative Data
Between 2024 and May 2025, circulating vaccine-derived poliovirus type 2 (cVDPV2) has continued to pose a global health challenge. In 2024 alone, 297 cases of acute flaccid paralysis (AFP) linked to cVDPV2 were reported across multiple countries. The highest numbers came from Nigeria (98 cases), Ethiopia (43), Chad (39), Yemen (37), Niger (16), and the Democratic Republic of the Congo (15).
From 1 January to 12 May 2025, an additional 49 AFP cases were confirmed, notably 21 from Ethiopia, 14 from Nigeria, and 9 from Chad, among others. Furthermore, 57 environmental cVDPV2 isolates were detected in regions including Algeria, Djibouti, and the occupied Palestinian territory.
These widespread detections underscore the global nature of poliovirus risks. The interconnectedness of nations through international travel and trade significantly increases the chances of virus importation and exportation, even in previously polio-free regions. As such, sustained vigilance, high immunization coverage, and surveillance remain critical worldwide.
Public Health Response in PNG
Following the detection of cVDPV2 in Lae City, Papua New Guinea (PNG) swiftly initiated a coordinated public health response. Both the National and Provincial Emergency Operations Centers were activated to manage the outbreak and implement containment measures.
Surveillance efforts were significantly enhanced, particularly in the area surrounding the environmental surveillance (ES) site. Acute flaccid paralysis (AFP) surveillance has been intensified to detect potential symptomatic cases, while environmental sample collection frequency in Lae City was increased from monthly to twice per month, with plans to expand to additional sites.
In response to the outbreak, technical preparations are underway for a targeted immunization campaign using oral polio vaccine type 2 (OPV2). This will be complemented by nationwide IPV (inactivated polio vaccine) catch-up activities, aiming to boost individual immunity and reduce the risk of paralysis.
PNG is also working closely with Global Polio Eradication Initiative (GPEI) partners to ensure alignment with international protocols. Cross-border coordination efforts are being strengthened to prevent regional spread, given Lae City’s status as a commercial hub and seaport.
The government is providing regular updates through the International Health Regulations (IHR) mechanism, while the World Health Organization (WHO) continues to offer technical guidance, support, and global monitoring of the situation.
WHO Risk Assessment & Recommendations
The World Health Organization (WHO) continues to classify poliovirus as a Public Health Emergency of International Concern (PHEIC), reaffirmed in November 2024. This reflects the ongoing threat posed by both wild and vaccine-derived polioviruses worldwide.
In Papua New Guinea (PNG), WHO has assessed the risk of local transmission as high, particularly due to low immunization coverage and suboptimal sanitation infrastructure. While Lae City does not border another country, its role as a major commercial hub and seaport increases the risk of international exportation of the virus.
WHO strongly recommends intensifying acute flaccid paralysis (AFP) and environmental surveillance, especially in and around Lae City. Additionally, boosting routine immunization and expanding targeted vaccination campaigns are essential to close existing immunity gaps.
Public health messaging and community awareness are also critical to ensure cooperation and early detection. WHO does not recommend any travel or trade restrictions based on the current information but emphasizes global vigilance.
Surveillance Strategy
Effective surveillance is critical for early detection and containment of poliovirus outbreaks. Acute flaccid paralysis (AFP) surveillance remains the cornerstone for identifying symptomatic cases, while environmental surveillance detects poliovirus circulation in communities, including asymptomatic carriers.
The WHO recommends expanding environmental surveillance beyond the current sites in Lae City to improve virus detection sensitivity. Sampling frequency has been increased from monthly to bi-monthly at three existing locations, with plans to add more sites in the city.
Community engagement plays a vital role in encouraging public cooperation with surveillance efforts and raising awareness about polio symptoms and prevention. Alongside this, healthcare workers are being rigorously sensitized on AFP case detection protocols and reporting requirements to ensure no cases go unnoticed.
This strengthened surveillance network aims to quickly identify and respond to new infections, helping to prevent further spread of cVDPV2 within Papua New Guinea and beyond.
Vaccination Measures & Immunization Strategy
The use of oral polio vaccine type 2 (OPV2) is central to Papua New Guinea’s outbreak response, aiming to rapidly boost immunity against the circulating vaccine-derived poliovirus type 2 (cVDPV2). Since the global withdrawal of OPV2 from routine immunization programs in 2016, population mucosal immunity has declined, creating vulnerabilities that cVDPV2 can exploit.
To contain the outbreak, targeted immunization campaigns will focus on vaccinating children with incomplete or no previous polio immunization, particularly in affected areas like Lae City and Morobe Province. These efforts are carefully coordinated with the national routine immunization schedule to ensure sustainable immunity levels.
Additionally, the inactivated polio vaccine (IPV) plays a key role by providing strong individual-level protection against paralysis, although it offers less impact on intestinal immunity than OPV.
Closing immunity gaps through both vaccination strategies is essential to prevent further transmission, protect vulnerable populations, and move closer to complete polio eradication.
Conclusion
The detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in Papua New Guinea is a serious public health concern that demands urgent action. Rapid response, robust surveillance, and coordinated vaccination efforts are critical to contain the outbreak and prevent further spread. This event underscores the importance of maintaining high routine immunization coverage to close immunity gaps and protect communities. Papua New Guinea’s focused approach, combined with strong cross-border collaboration, offers a clear path forward. Ultimately, global polio eradication depends on collective vigilance—no country is truly safe until every country is polio-free.