Map of PNG provinces with case number and case rates per 100,000 population
Papua New Guinea (PNG) is the largest and most populous nation in the Pacific, and was declared polio-free by the World Health Organization (WHO) in 2000 [1]. However, a case of Vaccine Derived Polio Virus 1 (VDPV1) was reported in April 2018 in a 6-year-old child in Lae, Morobe Province [2]. This paper describes the epidemiology of poliomyelitis in PNG within the context of the 2018 VDPV1 epidemic to identify underlying predisposing factors.
Poliomyelitis is a highly infectious disease caused by poliovirus, an RNA enterovirus. Transmission occurs through the faecal-oral route, and most cases are asymptomatic or mild [3]. However, in approximately 0.5% of cases, poliovirus can spread to the spinal cord and cause acute flaccid paralysis (AFP) [3].
The widespread adoption of the oral polio vaccine (OPV) by the Global Polio Eradication Initiative (GPEI) has been successful in reducing the incidence of wild poliovirus (WPV) by >99.9% since 1988 [4]. OPV contains live but attenuated poliovirus and carries the risk of vaccine-derived poliovirus (VDPV), which has the potential to cause paralytic disease and threatens global polio eradication [4]. VDPVs emerge in settings where there is poor sanitation, immunity against polio is low, and the live Sabin virus in the OPV is allowed to mutate into a variant form that can cause paralysis [5]. Following OPV administration, some of the live Sabin virus may combine with native gut enteroviruses (e.g., Coxsackie virus) in the small intestine, subsequently undergoing mutation [6,7]. The divergent strain may then cause disease in unvaccinated children who are in contact with the faeces of vaccinated children and are therefore exposed to the virus [8,9]. Bivalent OPV remains in use in many countries, including PNG, due to the low cost and ease of administration [10].
Annual nonpolio AFP (NPAFP) rates are used to assess AFP surveillance performance [11], where stools of AFP cases are sent to the WHO reference laboratory to confirm the absence of polio virus. According to GPEI, countries are considered to have highly sensitive polio surveillance systems if NPAFP rates are ≥2 per 100,000 children <15 years old [11].
Primary outbreak alerts were extracted for analysis from EPIWATCH, a semi-automated outbreak data collection and analysis observatory that monitors and provides critical analysis of global outbreaks and epidemics of public health significance using only publicly available sources [12]. The EPIWATCH system uses artificial intelligence to scan huge amounts of open-source data globally, in multiple languages with geographic information in real time, to identify early warning signals of potential epidemics [12,13]. This may provide particularly useful data from countries with low diagnostic and surveillance capacities due to a lack of resources [13].
Data from EPIWATCH was extracted using the keywords, “Papua New Guinea + Polio + outbreak + epidemics”. Data from 14 relevant posts for poliomyelitis in PNG were collated and set the basis for additional resources required to summarize the epidemic, including journal papers and other public domains including the WHO [2,14-16].
Map data for PNG was sourced from the United Nations Office for the Coordination of Humanitarian Affairs Data Exchange (https://data.humdata.org) [17]. Maps were generated using the ggplot2 package in R version 4.1.1 (R Foundation for Statistical Computing).
Case numbers per province were collected from WHO [15]. Population data from the most recent population census was collected from the PNG National Statistical Office [18]. Data were extracted from WHO for NPAFP reporting in PNG from 2000 to 2018 [15].
The index case was a 6-year-old boy in Lae, Morobe Province who presented with AFP on 24 April 2018 [2]. Stool samples were sent to the WHO regional polio reference laboratory, which isolated VDPV1 [2]. In response to the index case, active case finding was conducted, and in June 2018, the regional polio reference laboratory reported the isolation of VDPV1 from the stools of two healthy community contacts of the index case. The samples were sent for analysis to the global polio reference laboratory at the United States Centers for Disease Control and Prevention (US CDC) and it was confirmed that the isolates were genetically linked to the index case [16]. The subsequent 310 cases were found through enhanced surveillance of AFP, of which 26 were confirmed to have cVDVP1 isolated in the stool [2].
On 20 June, WHO reported cVDPV1 was isolated from the stool of two household contacts of the index case and confirmed a polio outbreak [16]. Genetic analyses linked the two household contacts to the index case indicating prolonged viral replication and circulation [16].
The PNG government established a National Emergency Operations Centre to coordinate the outbreak response on 27 June 2018 [16]. This included nationwide supplementary immunization activities (SIA), a community-focused risk communication campaign, and enhanced surveillance for AFP [16]. Vitamin A supplementation was integrated into the second and third rounds of the nationwide polio campaign [19].
The 2018 cVDPV1 Polio epidemic comprised 26 confirmed cases from nine (41%) of the 22 provinces of PNG. The Eastern Highlands Province recorded the highest number of cases (Figure 1) [15]. Immunization coverage rates were lower in Morobe (where the initial case was identified), and the neighbouring provinces of Madang and Eastern Highlands, when compared to the national rate [20]. This was in part caused by poor health literacy, low socioeconomic status, and poor geographical accessibility [20]. Of the 26 total cases, 19 (73%) were male and seven (27%) were female; 19 (73%) cases were less than 5 years, six (23%) others were from 5 to 14 years while one (4%) was a 17-year-old [2]. Most cases developed onset of AFP symptoms from the month of June 2018, peaking in August 2018 [2]. Figure 2 shows the epidemiological curve of all confirmed cases.
Map of PNG provinces with case number and case rates per 100,000 population
Epicurve of confirmed cases by date of symptom onset
OPV3 coverage is used as an indicator to measure country-level performance in providing immunity against poliomyelitis [1]. Data extracted from WHO revealed that the coverage rate in PNG of OPV3 remained consistently below 70% between the years 2000 to 2018, with a downward trend after 2009 [15]. SIA were conducted in PNG in 1997,1999, 2003, 2008, 2010 and 2012 due to persistently low OPV3 coverage, however it was observed that most of the SIA did not meet their target rate of 95% [14].
In response to the cVDPV1 epidemic in 2018, the PNG National Department of Health and its partners, including WHO, aimed to immunize 3.2 million children [16,20]. There were five rounds of immunization in total. The first two rounds were conducted sub-nationally, covering children <5 years in affected provinces in July, August and September 2018 [2,15]. The third and fourth rounds were conducted at the national level covering children <15 years in October and November 2018 [2,15]. The fifth round was conducted in selected provinces in December 2018. A total of 7.8 million vaccines were administered, with the WHO reporting 97% vaccination coverage by the end of 2018 [15,16].
NPAFP rates used to assess surveillance performance in PNG was consistently below the required annual rate of 2 per 100,000 children <15 years old, from 2000 until the 2018 epidemic [16].
During the 2018 epidemic, the response team strengthened the AFP surveillance system. Health care workers including surveillance officers, field epidemiologists and clinicians were trained to identify, collect, and transport samples, and report suspected cases [16]. The Ministry of Health along with provincial health departments conducted surveillance in health clinics and registered households, while humanitarian agencies such as PNG Red Cross Society targeted high risk and unregistered children [20]. By the end of 2018, the number of AFP cases reported from the surveillance was 310, of which 26 cases had cVDVP1 isolated in the stool [2]. The annual NPAFP rate was more than 7.0 per 100,000 persons aged <15 years in 2018 due to improved surveillance, compared with 0.8 in 2017 [2]. To complement the AFP surveillance, environmental sampling was undertaken in Morobe Province and the National Capital District (NCD). Of 27 samples taken, seven tested positive, mainly in the NCD [16]. No further cases of polio have been reported since 2018 [15].
VDPV is an emerging threat to the global polio eradication effort. PNG was declared polio-free in 2000, however the years that followed were characterised by very low routine immunization coverage and poor polio surveillance [1]. Findings from a 2005 survey identified that weaknesses in the health system, low vaccine knowledge, and barriers around accessibility and transportation were linked with low immunization coverage in PNG [21].
While the use of OPV3 in routine and supplementary immunization programmes confers protection against polio for the PNG population, it also increases the number of viral strains available for replication and genome mutation, thus increasing the risk of a cVDPV outbreak [5,9].
Adequate AFP surveillance is necessary to accurately identify WPV and cVDPV cases in the community [16]. However, in PNG, the non-AFP rate remained under the required annual rate of 2 per 100,000 children <15 years old, from 2000 until the 2018 epidemic [16].
The response by the WHO, UNICEF and PNG authorities to the 2018 epidemic was done at both the national and provincial level, reaching above the targeted 3.2 million children and improving the national coverage rate to 97% [16]. These initiatives also improved the non-AFP surveillance rate to 7 per 100 000 children <15 years old compared to 8 per 100 000 children <15 years old in 2017 [16]. Given that no further cases of polio have been reported since 2018, these interventions have been successful in halting transmission. To ensure the long-term eradication of poliomyelitis in PNG, human and other resources must be directed towards maintaining high vaccination coverage along with ensuring a robust surveillance system is maintained [16]. Tracking unvaccinated people, improving the reach and efficiency of outreach services, and improving staff and community knowledge about vaccination are all areas that have been identified to improve the performance of the national immunization strategy in PNG [22].
There are some limitations to the current research. Firstly, inadequate surveillance of AFP can lead to underreporting and impact the accuracy of any inferences made. Secondly, primary outbreak alerts were extracted from the EPIWATCH system. Although data from EPIWATCH provides early signals that trigger further investigation, the number of reports is not equivalent to case numbers. However, the data were supplemented by other sources to provide a comprehensive picture of the 2018 epidemic.
The 2018 cVDPV1 epidemic in PNG resulted in 26 AFP cases across nine provinces. Factors leading to this epidemic included low population vaccination rates, routine use of OPV, and inadequate surveillance, in the context of a poorly resourced health system. The outbreak response improved the national OPV3 coverage rate to 97% and strengthened the AFP surveillance and monitoring system. To ensure sustainability of these gains, resources directed at maintaining high vaccination coverage, improving surveillance systems, and improving the capacity of outreach services will provide the nation with the best protection against not just poliomyelitis, but also future threats from other vaccine-preventable diseases.
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