Vaccine-derived Poliovirus Strains Research Articles

Wide circulation of type 1 vaccine-derived poliovirus strains in clinical specimens from suspected cases of poliomyelitis, their contacts and in wastewater in Madagascar since late 2020

Madagascar has faced three major outbreaks of vaccine-derived polioviruses (VDPVs) in recent decades, with VDPV type 1 reemerging in late 2020. Here, we report the molecular characterization of these cVDPV1 strains. WHO protocols were used for poliovirus detection in stool and wastewater samples. Molecular genotyping was based on the 5′ non-coding (5′NC), VP1, and 3Dpol regions. From 2020 to 2022, 92 of 5690 stool samples and 129 of 1046 wastewater samples tested positive for cVDPV1. Genetic analysis of the VP1 gene revealed 1.3%–6.1% variability compared to the Sabin strain. Most sequences showed mutations at neurovirulence attenuation sites. Phylogenetic analysis distributed strains into four genogroups originating from Southern Madagascar. All analyzed cVDPV1 strains were recombinant, containing mutated oral polio vaccine sequences in VP1 and type C enterovirus sequences in other regions. This study demonstrated that all strains were closely related during this epidemic.

Polio outbreak response, Yemen.

A decrease in vaccine coverage in conflict-affected areas has placed Yemen at higher risk of polio outbreaks caused by vaccine-derived poliovirus strains. In response to polio outbreaks, the Yemeni health ministry and partners initiated multiple vaccination campaigns to deliver vaccines to children. We also implemented several measures to enhance communication, education, health promotion and hygiene, especially in camps for internally displaced people. In 2009, Yemen achieved polio-free status and maintained it until 2019. However, the ongoing political conflict since 2015, coupled with challenges in delivering the polio vaccine to conflict-affected areas, resulted in two polio outbreaks: 35 cases caused by vaccine-derived poliovirus strain 1 between 2019 and 2021, and 230 cases due to vaccine-derived poliovirus strain 2 between November 2021 and December 2022. In response to the first outbreak, by the end of 2020, we vaccinated 7.2 million children through nationwide vaccination campaigns, except in Sa'ada governorate due to a ban by the authorities. By the end of 2021, 3 800 313 children younger than 5years had received polio vaccines. For the second outbreak, by the end of 2022, 4 463 389 vaccines had been given to children younger than 10years, and 1 217 423 to those younger than 5years. Vaccination campaigns in conflict-affected areas with low vaccine coverage remain crucial in eradicating polio. Efforts are needed to reach vulnerable groups such as displaced populations. Advocacy, communication and social mobilization actions help ensure broader public inclusion and participation in vaccination efforts to prevent polio outbreaks.

Immunogenicity of Catch-Up Immunization with Conventional Inactivated Polio Vaccine among Japanese Adults.

Most Japanese adults are vaccinated twice with the Sabin trivalent oral polio vaccine. Booster vaccination is recommended for Japanese travelers to polio-endemic/high-risk countries. We assessed the catch-up immunization of healthy Japanese adults aged ≥20 years with two doses of standalone conventional inactivated polio vaccine (cIPV). Immunogenicity was evaluated by serum neutralization titers (pre-booster vaccination, 4-6 weeks after each vaccination) against type 1, 2, and 3 poliovirus strains. The participants were 61 healthy Japanese adults (26 men/35 women; mean age ± standard deviation age 35.8 ± 8.0 years). Seropositivity rates (percentage of participants with anti-poliovirus antibody titers ≥1:8) pre-vaccination were 88.5%, 95.1%, and 52.5% for Sabin strains (type 1, 2, and 3); 72.1%, 93.4%, and 31.1% for virulent poliovirus strains (type 1: Mahoney; type 2: MEF-1; and type 3: Saukett); and 93.4%, 93.4%, 93.4%, and 88.5% for type 2 vaccine-derived poliovirus strains (SV3128, SV3130, 11,196, and 11,198). After one cIPV dose, all seropositivity rates increased to 98.4-100.0%. After two cIPV doses, the seropositivity rates reached 100% for all strains. cIPV was well tolerated, with no safety concerns. Catch-up immunization with standalone cIPV induced robust immune responses in Japanese adults, indicating that one booster dose boosted serum-neutralizing antibodies to many strains.

Molecular evolution and antigenic drift of type 3 iVDPVs excreted from a patient with immunodeficiency in Ningxia, China.

A 2.5-year-old pediatric patient with acute flaccid paralysis was diagnosed with primary immunodeficiency (PID) in Ningxia Province, China, in 2011. Twelve consecutive stool specimens were collected from the patient over a period of 10 months (18 February 2011 to 20 November 2011), and 12 immunodeficiency vaccine-derived poliovirus (iVDPV) strains (CHN15017-1 to CHN15017-12) were subsequently isolated. Nucleotide sequencing analysis of the plaque-purified iVDPVs revealed 2%-3.5% VP1-region differences from their parental Sabin 3 strain. Full-length genome sequencing showed they were all Sabin 3/Sabin 1 recombinants, sharing a common 2C-region crossover site, and the two key determinants of attenuation (U472C in the 5' untranslated region and T2493C in the VP1 region) had reverted. Temperature-sensitive experiments demonstrated that the first two iVDPV strains partially retained the temperature-sensitive phenotype's nature, while the subsequent ten iVDPV strains distinctly lost it, possibly associated with increased neurovirulence. Nineteen amino-acid substitutions were detected between 12 iVDPVs and the parental Sabin strain, of which only one (K1419R) was found on the subsequent 10 iVDPV isolates, suggesting this site's potential as a temperature-sensitive determination site. A Bayesian Monte Carlo Markov Chain phylogenetic analysis based on the P1 coding region yielded a mean iVDPV evolutionary rate of 1.02 × 10-2 total substitutions/site/year, and the initial oral-polio-vaccine dose was presumably administered around June 2009. Our findings provide valuable information regarding the genetic structure, high-temperature growth sensitivity, and antigenic properties of iVDPVs following long-term evolution in a single PID patient, thus augmenting the currently limited knowledge regarding the dynamic changes and evolutionary pathway of iVDPV populations with PID during long-term global replication.

Poliomyelitis is a current challenge: long-term sequelae and circulating vaccine-derived poliovirus.

For more than 20years, the World Health Organization Western Pacific Region (WPR) has been polio-free. However, two current challenges are still polio-related. First, around half of poliomyelitis elderly survivors suffer late poliomyelitis sequelae with a substantial impact on daily activities andquality of life, experiencing varying degrees of residual weakness as they age. The post-polio syndrome as well as accelerated aging may be involved. Second, after the worldwide Sabin oral poliovirus (OPV) vaccination, the recent reappearance of strains of vaccine-derived poliovirus (VDPV) circulating in the environment is worrisome and able to persistent person-to-person transmission. Such VDPV strains exhibit atypical genetic characteristics and reversed neurovirulence that can cause paralysis similarly to wild poliovirus, posing a significant obstacle to the elimination of polio. Immunization is essential for preventing paralysis in those who are exposed to the poliovirus. Stress the necessity of maintaining high vaccination rates because declining immunity increases the likelihood of reemergence. If mankind wants to eradicate polio in the near future, measures to raise immunization rates and living conditions in poorer nations are needed, along with strict observation. New oral polio vaccine candidates offer a promissory tool for this goal.

Recombination Analysis of Non-Poliovirus Members of the Enterovirus C Species; Restriction of Recombination Events to Members of the Same 3DPol Cluster.

Enteroviruses (EVs) are highly prevalent viruses worldwide. Recombination is known to occur frequently in EVs belonging to species Enterovirus A, Enterovirus B, and Enterovirus C. Although many recombinant vaccine-derived poliovirus (VDPV) strains have been reported, our knowledge on recombination in non-polio EVs in the species Enterovirus C is limited. Here, we combined a dataset consisting of 11 newly generated full-length Enterovirus C sequences and 180 publicly available sequences to study recombination dynamics in non-polio EVs. To identify recombination patterns, maximum likelihood phylogenetic trees of different genomic regions were constructed, and segregation analyses were performed. Recombination was observed between members of the same 3DPol cluster, but was rarely observed between members of different clusters. We hypothesize that this restriction may have arisen through their different compartmentalization in respiratory and enteric tracts related to differences in cellular tropisms so that the opportunity to recombine may not be available.

Human IgA Monoclonal Antibodies That Neutralize Poliovirus, Produced by Hybridomas and Recombinant Expression.

Poliovirus (PV)-specific intestinal IgAs are important for cessation of PV shedding in the gastrointestinal tract following an acute infection with wild type or vaccine-derived PV strains. We sought to produce IgA monoclonal antibodies (mAbs) with PV neutralizing activity. We first performed de novo IgA discovery from primary human B cells using a hybridoma method that allows assessment of mAb binding and expression on the hybridoma surface: On-Cell mAb Screening (OCMS™). Six IgA1 mAbs were cloned by this method; three potently neutralized type 3 Sabin and wt PV strains. The hybridoma mAbs were heterogeneous, expressed in monomeric, dimeric, and aberrant forms. We also used recombinant methods to convert two high-potency anti-PV IgG mAbs into dimeric IgA1 and IgA2 mAbs. Isotype switching did not substantially change their neutralization activities. To purify the recombinant mAbs, Protein L binding was used, and one of the mAbs required a single amino acid substitution in its κ LC in order to enable protein L binding. Lastly, we used OCMS to assess IgA expression on the surface of hybridomas and transiently transfected, adherent cells. These studies have generated potent anti-PV IgA mAbs, for use in animal models, as well as additional tools for the discovery and production of human IgA mAbs.

Type I vaccine-derived polioviruses in China from 1995 to 2019

A nation-wide case surveillance was conducted in China since 1995 for the objective of identifying acute flaccid paralysis (AFP) in children so that potential wild polioviruses and vaccine-derived poliovirus (VDPV) could be identified on time. Two outbreaks associated with type I circulating VDPVs, eight native independent type I ambiguous VDPVs (aVDPV), and one imported aVDPV were identified during the AFP case surveillance in China from 1995 to 2019. The VP1 coding region of the Chinese type I VDPVs differed from the polio vaccine strain by 1.00%–3.75% (9–34 substitutions in 906 nucleotides). Most of the Chinese type I VDPV strains shared 4 amino acid substitutions in the neutralizing antigenic (NAg) sites: 3 located at the BC loop, which formed the NAg site 1, and another at NAg site 3a. All of the Chinese type I VDPVs identified during the AFP case surveillance were young VDPVs, which indicated a limited viral replication resulted from the administration of the initiating oral polio vaccine (OPV) dose. VDPVs can emerge and spread in isolated communities with immunity gaps and the circulation ceases following a mass immunization with OPV. As such, high-quality surveillance permitted very early detection and response and it played a key role in stalling the widespread circulation of the emergent cVDPV strains in China.

Successes and challenges of expansion of environmental poliovirus surveillance in the WHO South-East Asia Region.

The last decade has witnessed an exponential expansion of environmental surveillance (ES) of poliovirus in sewage samples in the World Health Organization (WHO) South-East Asia Region. This has grown from only three sites in Mumbai, India in 2001 to 56 sites in 2017 in Bangladesh, India, Indonesia, Myanmar, Nepal and Thailand. ES is critical to the region in providing evidence of silent transmission of vaccine-derived poliovirus and Sabin-like poliovirus type 2 - especially since the global "switch" to cease use of oral polio vaccine type 2 - and for monitoring the effectiveness of containment activities. This targeted expansion of ES to supplement surveillance for acute flaccid paralysis (AFP) required quality assurance in ES procedures, improvements in the sensitivity of the laboratory-based surveillance system, and establishment of real-time data analysis for evidence-based programmes. ES in the region has provided documentary evidence for the absence of indigenous wild poliovirus in circulation and no importations via international travellers. Post-switch, while no vaccine-derived poliovirus was detected from AFP cases, ES identified five ambiguous vaccine-derived polioviruses in 2016 and early 2017, with no evidence of circulation. Future challenges include monitoring for vaccine-derived poliovirus strains shed for a prolonged time by immunodeficient individuals, and expanding ES to areas lacking sewage networks. To maintain the polio-free status of the WHO South-East Asia Region and achieve a world free of poliomyelitis, critical evaluation of immunization coverage, continued performance of surveillance for acute flaccid paralysis, and enhanced analysis of sewage samples to detect any breach in containment are essential.

Isolation and characterization of a highly evolved type 3 vaccine-derived poliovirus in China

In this study, we report the identification and characterization of a highly evolved type 3 vaccine-derived poliovirus (VDPV) strain designated as WIV14, isolated in 2014 from a 4-year-old child suspected of having an enteroviral infection in China. Complete genome sequence of WIV14 revealed multiple nucleotide substitutions when compared with the attenuated poliovirus (PV) Sabin 3, including the reversion of three major attenuation sites to wild type. From the nucleotide divergence for the P1/capsid region, we estimated that the evolution time of WIV14 was more than 7 years, indicating the possible long time of replication. WIV14 strain seemed to have differences in biological characteristics compared with attenuated PV strains, such as being non-temperature-sensitive and producing large plaques. The current isolation of a highly divergent type 3 VDPV gives an idea of the risk of emergent VDPV strains, and emphasizes the importance of maintaining high vaccination coverage and herd immunity against PVs in China.

Circulating vaccine-derived poliovirus type 2 outbreak in Democratic Republic of Congo 2011-2012

According to the WHO records of 2013, the incidence of poliomyelitis was reduced by more than 99%, the number of endemic countries decreased from 125 in 1988 to 3 in 2013 and over 10 million cases were prevented from poliomyelitis thanks to the intensive use of Oral polio vaccine (OPV). However, the emergence of circulating vaccine-derived poliovirus strains (cVDPV), causing serious epidemics like the wild poliovirus, is a major challenge on the final straight towards the goal of eradication and OPV cessation. This paper describes the cVDPVoutbreak that occurred in the Democratic Republic of Congo (DRC) from November 2011 to April 2012. All children under 15 years of age with acute flaccid paralysis (AFP) and confirmed presence of cVDPV in the stool samples were included. Thirty (30) children, all from the administrative territories of Bukama and Malemba Nkulu in the Katanga Province (south-east DRC), were reported. The virus responsible was the cVDPV type 2 (0.7% -3.5% divergent from the reference Sabin 2 strain) in 29 children (97%) and the ambiguous vaccine-derived poliovirus strain (0.7% divergent) was confirmed in one case (3%), a boy seventeen months old and already vaccinated four times with OPV. Twentyfive children (83%) were protected by any of the routine EPI vaccines and 3 children (10%) had never received any dose of OPV. In reaction, DRC has conducted five local campaigns over a period of 10 months (from January to October 2012) and the epidemic was stopped after the second round performed in March 2012. As elsewhere in similar conditions, low immunization coverage, poor sanitation conditions and the stop of the use of OPV2 have favoured the emergence of the third cVDPV epidemic in DRC. The implementation of the Strategic Plan for Polio eradication and endgame strategic plan 2013-2018 will prevent the emergence of cVDPV and set up the conditions for a coordinated OPV phase out.

Dynamics of a two-strain vaccination model for polio

A new deterministic model for the transmission dynamics of two strains of polio, the vaccine-derived polio virus (VDPV) and the wild polio virus (WPV), in a population is designed and rigorously analysed. It is shown that Oral Polio Vaccine (OPV) reversion (leading to increased incidences of WPV and VDPV strains), together with the combined effect of vaccinating a fraction of the unvaccinated susceptible and missed susceptible children, could induce the phenomenon of backward bifurcation when the associated reproduction number of the model is less than unity. Furthermore, the model undergoes competitive exclusion, where the strain with the higher reproduction number (greater than unity) drives the other (with reproduction number less than unity) to extinction. In the absence of OPV reversions (leading to the co-existence of both strains in the population), it is shown that the disease-free equilibrium of the model is globally-asymptotically stable whenever the associated reproduction number is less than unity. Numerical simulations of the model suggest that the model undergoes the phenomenon of competitive exclusion, where the strain with the higher reproduction number (greater than unity) drives the other to extinction. Furthermore, co-existence of the two strains is feasible if their respective reproduction number are equal or approximately equal (and greater than unity).

A single chimpanzee-human neutralizing monoclonal antibody provides post-exposure protection against type 1 and type 2 polioviruses

BackgroundDevelopment of anti-poliovirus therapies to complement vaccination is an urgent priority. A number of antiviral drugs are in development. Recently we have developed human monoclonal antibodies that could be used for treatment of chronically infected individuals and emergency response to potential reappearance of polioviruses after eradication. ObjectiveThe aim of this study was to characterize neutralizing activity of anti-poliovirus monoclonal antibody A12 against wild type, vaccine-derived, and drug-resistant poliovirus strains, evaluate in vivo pre- and post-exposure protective properties of the antibody against polioviruses of serotypes 1 and 2, and to determine whether it interferes with response to immunization with poliovirus vaccine. Study designImmunogenicity studies were performed in CD1 mice. Poliovirus neutralizing titers were determined in poliovirus microneutralization assay. Poliovirus immunization-challenge experiments were performed in poliovirus-susceptible TgPVR21 mice. ResultsWe show that monoclonal antibody A12 effectively neutralizes in vitro a broad range of type 1 and type 2 wild and vaccine-derived polioviruses, provides effective pre- and post-exposure protection of TgPVR21 mice from challenge with a lethal dose of poliovirus. Treatment of animals with the antibody concurrent with IPV immunization does not prevent immune response to the vaccine. ConclusionsAnti-poliovirus antibody A12 effectively neutralizes a range of wild and VDPV strains and protectstransgenic mice susceptible to poliovirus against lethal challenge upon pre- and post-exposure administration. This suggests that the antibodies could be used in combination with drugs and/or vaccine to improve their efficacy and prevent emergence of resistant variants, and provides a justification for initiating their clinical evaluation.

Isolation and Characterization of a Type 2 Vaccine-Derived Poliovirus from Environmental Surveillance in China, 2012

Environmental surveillance of poliovirus on sewage has been conducted in Shandong Province, China since 2008. A type 2 vaccine-derived poliovirus (VDPV) with 7 mutations in VP1 coding region was isolated from the sewage collected in the city of Jinan in December 2012. The complete genome sequencing analysis of this isolate revealed 25 nucleotide substitutions, 7 of which resulted in amino acid alteration. No evidence of recombination with other poliovirus serotypes was observed. The virus did not lose temperature sensitive phenotype at 40°C. An estimation based on the evolution rate of the P1 coding region suggested that evolution time of this strain might be 160–176 days. VP1 sequence analysis revealed that this VDPV strain is of no close relationship with other local type 2 polioviruses (n = 66) from sewage collected between May 2012 and June 2013, suggesting the lack of its circulation in the local population. The person who excreted the virus was not known and no closely related virus was isolated in local population via acute flaccid paralysis surveillance. By far this is the first report of VDPV isolated from sewage in China, and these results underscore the value of environmental surveillance in the polio surveillance system even in countries with high rates of OPV coverage.

PCR assays for the identification of rare recombination types from VP1 to 3D genomic region of vaccine derived poliovirus strains

Poliomyelitis has been effectively controlled by the use of inactivated poliovirus vaccine (IPV) or trivalent live attenuated oral poliovirus vaccine (OPV). Since 1964, the use of OPV in mass vaccinations has resulted in drastic reductions of the number of poliomyelitis cases caused by wild-type polioviruses. However, the characterization of OPV derivatives with increased neurovirulence, constituted a real problem with respect to OPV safety. Mutations at attenuating sites of the genome and recombination events between Sabin strains of the trivalent OPV vaccine have been correlated with the loss of the attenuated phenotype of OPV strains and the acquisition of traits characteristic of wild polioviruses. In consequence, early detection and characterization of recombinant evolved derivatives of vaccine strains is highly important. In this report, ten PCR assays are described which allow for the identification of rare recombination events located in VP1, 2A, 2C, 3A, 3C and 3D genomic regions and predominant recombination events located in 2C and 3D genomic regions of OPV derivatives. These assays could be readily implemented in diagnostics laboratories lacking sequencing facilities as a first approach for the early detection and characterization of recombinant OPV derivatives.

Characteristics of an Environmentally Monitored Prolonged Type 2 Vaccine Derived Poliovirus Shedding Episode that Stopped without Intervention

Vaccine derived poliovirus (VDPV) type 2 strains strongly divergent from the corresponding vaccine strain, Sabin 2, were repeatedly isolated from sewage in Slovakia over a period of 22 months in 2003–2005. Cell cultures of stool specimens from known immune deficient patients and from an identified putative source population of 500 people failed to identify the potential excretor(s) of the virus. The occurrence of VDPV in sewage stopped without any intervention. No paralytic cases were reported in Slovakia during the episode. According to a GenBank search and similarity plotting-analysis, the closest known relative of the first isolate PV2/03/SVK/E783 through all main sections of the genome was the type 2 poliovirus Sabin strain, with nucleotide identities in 5′UTR, P1, P2, P3, and 3′UTR parts of the genome of 88.6, 85.9, 87.3, 88.5, and 94.0 percent, respectively. Phenotypic properties of selected Slovakian aVDPV strains resembled those of VDPV strains isolated from immune deficient individuals with prolonged PV infection (iVDPV), including antigenic changes and moderate neurovirulence in the transgenic mouse model. One hundred and two unique VP1 coding sequences were determined from VDPV strains isolated from 34 sewage specimens. Nucleotide differences from Sabin 2 in the VP1 coding region ranged from 12.5 to 15.6 percent, and reached a maximum of 9.6 percent between the VDPV strains under study. Most of the nucleotide substitutions were synonymous but as many as 93 amino acid positions out of 301 in VP1 showed substitutions. We conclude that (1) individuals with prolonged poliovirus infection are not as rare as suggested by the studies on immune deficient patients known to the health care systems and (2) genetic divergence of VDPV strains may remain extensive during years long replication in humans.

Evolution of Type 2 Vaccine Derived Poliovirus Lineages. Evidence for Codon-Specific Positive Selection at Three Distinct Locations on Capsid Wall

Partial sequences of 110 type 2 poliovirus strains isolated from sewage in Slovakia in 2003–2005, and most probably originating from a single dose of oral poliovirus vaccine, were subjected to a detailed genetic analysis. Evolutionary patterns of these vaccine derived poliovirus strains (SVK-aVDPV2) were compared to those of type 1 and type 3 wild poliovirus (WPV) lineages considered to have a single seed strain origin, respectively. The 102 unique SVK-aVDPV VP1 sequences were monophyletic differing from that of the most likely parental poliovirus type 2/Sabin (PV2 Sabin) by 12.5–15.6%. Judging from this difference and from the rate of accumulation of synonymous transversions during the 22 month observation period, the relevant oral poliovirus vaccine dose had been administered to an unknown recipient more than 12 years earlier. The patterns of nucleotide substitution during the observation period differed from those found in the studied lineages of WPV1 or 3, including a lower transition/transversion (Ts/Tv) bias and strikingly lower Ts/Tv rate ratios at the 2nd codon position for both purines and pyrimidines. A relatively low preference of transitions at the 2nd codon position was also found in the large set of VP1 sequences of Nigerian circulating (c)VDPV2, as well as in the smaller sets from the Hispaniola cVDPV1 and Egypt cVDPV2 outbreaks, and among aVDPV1and aVDPV2 strains recently isolated from sewage in Finland. Codon-wise analysis of synonymous versus non-synonymous substitution rates in the VP1 sequences suggested that in five codons, those coding for amino acids at sites 24, 144, 147, 221 and 222, there may have been positive selection during the observation period. We conclude that pattern of poliovirus VP1 evolution in prolonged infection may differ from that found in WPV epidemics. Further studies on sufficiently large independent datasets are needed to confirm this suggestion and to reveal its potential significance.

Risks associated with the use of live-attenuated vaccine poliovirus strains and the strategies for control and eradication of paralytic poliomyelitis

The Global Polio Eradication Initiative was launched in 1988 with the aim to eliminate paralytic poliomyelitis. Two effective vaccines are available: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). Since 1964, OPV has been used instead of IPV in most countries due to several economic and biological advantages. However, in rare cases, the live-attenuated Sabin strains of OPV revert to neurovirulence and cause vaccine-associated paralytic poliomyelitis in vaccinees or lead to emergence of vaccine-derived poliovirus strains. Attenuating mutations and recombination events have been associated with the reversion of vaccine strains to neurovirulence. The substitution of OPV with an improved new-generation IPV and the availability of new specific drugs against polioviruses are considered as future strategies for outbreak control and the eradication of paralytic poliomyelitis worldwide.

A new RT-PCR assay for the identification of the predominant recombination types in 2C and 3D genomic regions of vaccine-derived poliovirus strains

In the post-eradication era of wild polioviruses, the only remaining sources of poliovirus infection worldwide would be the vaccine-derived polioviruses (VDPVs). As the preponderance of countries certified to be polio-free has switched from OPV (oral poliovirus vaccine) to IPV (inactivated poliovirus vaccine), importation of recombinant evolved derivatives of vaccinal strains would have serious implication for public health. To test the robustness of the proposed RT-PCR screening analysis, eleven recombinant vaccine-derived polioviruses that were characterized previously by sequencing by our group, in addition to three recently identified recombinant environmental isolates were assayed. Although the most definitive characterization of VDPVs is by genomic sequencing, in this study we describe a new, inexpensive and broadly applicable RT-PCR assay for the identification of the predominant recombination types S3/Sx in 2C and S2/Sx in 3D genomic regions respectively of VDPVs, that can be readily implemented in laboratories lacking sequencing facilities as a first approach for the early detection of vaccine-derived poliovirus (VDPVs).

Changes in Population Dynamics during Long-Term Evolution of Sabin Type 1 Poliovirus in an Immunodeficient Patient

The evolution of the Sabin strain of type 1 poliovirus in a hypogammaglobulinemia patient for a period of 649 days is described. Twelve poliovirus isolates from sequential stool samples encompassing days 21 to 649 after vaccination with Sabin 1 were characterized in terms of their antigenic properties, virulence in transgenic mice, sensitivity for growth at high temperatures, and differences in nucleotide sequence from the Sabin 1 strain. Poliovirus isolates from the immunodeficient patient evolved gradually toward non-temperature-sensitive and neurovirulent phenotypes, accumulating mutations at key nucleotide positions that correlated with the observed reversion to biological properties typical of wild polioviruses. Analysis of plaque-purified viruses from stool samples revealed complex genetic and evolutionary relationships between the poliovirus strains. The generation of various coevolving genetic lineages incorporating different mutations was observed at early stages of virus excretion. The main driving force for genetic diversity appeared to be the selection of mutations at attenuation sites, particularly in the 5' noncoding region and the VP1 BC loop. Recombination between virus strains from the two main lineages was observed between days 63 and 88. Genetic heterogeneity among plaque-purified viruses at each time point seemed to decrease with time, and only viruses belonging to a unique genotypic lineage were seen from day 105 after vaccination. The relevance of vaccine-derived poliovirus strains for disease surveillance and future polio immunization policies is discussed in the context of the Global Polio Eradication Initiative.