Oral Poliovirus Vaccine Strains Research Articles

The Fight against Poliovirus Is Not Over.

Poliovirus (PV), the virus that causes both acute poliomyelitis and post-polio syndrome, is classified within the Enterovirus C species, and there are three wild PV serotypes: WPV1, WPV2 and WPV3. The launch of the Global Polio Eradication Initiative (GPEI) in 1988 eradicated two of the three serotypes of WPV (WPV2 and WPV3). However, the endemic transmission of WPV1 persists in Afghanistan and Pakistan in 2022. There are cases of paralytic polio due to the loss of viral attenuation in the oral poliovirus vaccine (OPV), known as vaccine-derived poliovirus (VDPV). Between January 2021 and May 2023, a total of 2141 circulating VDPV (cVDPV) cases were reported in 36 countries worldwide. Because of this risk, inactivated poliovirus (IPV) is being used more widely, and attenuated PV2 has been removed from OPV formulations to obtain bivalent OPV (containing only types 1 and 3). In order to avoid the reversion of attenuated OPV strains, the new OPV, which is more stable due to genome-wide modifications, as well as sabin IPV and virus-like particle (VLP) vaccines, is being developed and offers promising solutions for eradicating WP1 and VDPV.

Updated Characterization of Outbreak Response Strategies for 2019-2029: Impacts of Using a Novel Type 2 Oral Poliovirus Vaccine Strain.

Delays in achieving the global eradication of wild poliovirus transmission continue to postpone subsequent cessation of all oral poliovirus vaccine (OPV) use. Countries must stop OPV use to end all cases of poliomyelitis, including vaccine-associated paralytic polio (VAPP) and cases caused by vaccine-derived polioviruses (VDPVs). The Global Polio Eradication Initiative (GPEI) coordinated global cessation of all type 2 OPV (OPV2) use in routine immunization in 2016 but did not successfully end the transmission of type 2 VDPVs (VDPV2s), and consequently continues to use type 2 OPV (OPV2) for outbreak response activities. Using an updated global poliovirus transmission and OPV evolution model, we characterize outbreak response options for 2019-2029 related to responding to VDPV2 outbreaks with a genetically stabilized novel OPV (nOPV2) strain or with the currently licensed monovalent OPV2 (mOPV2). Given uncertainties about the properties of nOPV2, we model different assumptions that appear consistent with the evidence on nOPV2 to date. Using nOPV2 to respond to detected cases may reduce the expected VDPV and VAPP cases and the risk of needing to restart OPV2 use in routine immunization compared to mOPV2 use for outbreak response. The actual properties, availability, and use of nOPV2 will determine its effects on type 2 poliovirus transmission in populations. Even with optimal nOPV2 performance, countries and the GPEI would still likely need to restart OPV2 use in routine immunization in OPV-using countries if operational improvements in outbreak response to stop the transmission of cVDPV2s are not implemented effectively.

Development of a new oral poliovirus vaccine for the eradication end game using codon deoptimization

Enormous progress has been made in global efforts to eradicate poliovirus, using live-attenuated Sabin oral poliovirus vaccine (OPV). However, as the incidence of disease due to wild poliovirus has declined, vaccine-derived poliovirus (VDPV) has emerged in areas of low-vaccine coverage. Coordinated global cessation of routine, type 2 Sabin OPV (OPV2) use has not resulted in fewer VDPV outbreaks, and continued OPV use in outbreak-response campaigns has seeded new emergences in low-coverage areas. The limitations of existing vaccines and current eradication challenges warranted development of more genetically stable OPV strains, most urgently for OPV2. Here, we report using codon deoptimization to further attenuate Sabin OPV2 by changing preferred codons across the capsid to non-preferred, synonymous codons. Additional modifications to the 5′ untranslated region stabilized known virulence determinants. Testing of this codon-deoptimized new OPV2 candidate (nOPV2-CD) in cell and animal models demonstrated that nOPV2-CD is highly attenuated, grows sufficiently for vaccine manufacture, is antigenically indistinguishable from Sabin OPV2, induces neutralizing antibodies as effectively as Sabin OPV2, and unlike Sabin OPV2 is genetically stable and maintains an attenuation phenotype. In-human clinical trials of nOPV2-CD are ongoing, with potential for nOPV strains to serve as critical vaccine tools for achieving and maintaining polio eradication.

Deep sequencing prompts the modification of a real-time RT-PCR for the serotype-specific detection of polioviruses

Polioviruses are members of the Enterovirus C species and asymptomatic fecal shedding allows for their transmission and persistence in a community, as well as the emergence of vaccine-derived polioviruses. Using three serotype-specific real-time RT-PCR (rRT-PCR) assays, the shedding and circulation of oral poliovirus vaccine (OPV) strains was previously investigated in a prospective cohort of Mexican children, their contacts, and nearby sewage. Subsequently, a deep sequencing approach targeting the P1 genomic region was applied to characterize OPV strains previously detected by rRT-PCR. Amplifiable RNA was obtained for sequencing from 40.3% (58/144) of stool samples and 71.4% (15/21) of sewage using nucleic acids extracted directly from primary rRT-PCR-positive specimens. Sequencing detected one or more OPV serotypes in 62.1% (36/58) of stool and 53.3% (8/15) of sewage samples. All stool and sewage samples in which poliovirus was not detected by deep sequencing contained at least one non-polio enterovirus C (NPEV-C) strain. To improve screening specificity, a modified, two-step, OPV serotype-specific multiplex rRT-PCR was evaluated. In stool specimens, the overall agreement between the original assays and the multiplex was 70.3%. By serotype, the overall agreement was 95.7% for OPV serotype-1 (S1), 65.6% for S2, and 96.1% for S3. Furthermore, most original rRT-PCR positive/multiplex rRT-PCR negative results were collected in the summer and fall months, consistent with NPEV-C circulation patterns. In conclusion, this deep sequencing approach allowed for the characterization of OPV sequences directly from clinical samples and facilitated the implementation of a more specific multiplex rRT-PCR for OPV detection and serotyping.

Update on Vaccine-Derived Polioviruses - Worldwide, January 2016-June 2017.

In 1988, the World Health Assembly launched the Global Polio Eradication Initiative (GPEI) (1). Among the three wild poliovirus (WPV) serotypes, only type 1 (WPV1) has been detected since 2012. Since 2014, detection of WPV1 has been limited to three countries, with 37 cases in 2016 and 11 cases in 2017 as of September 27. The >99.99% decline worldwide in polio cases since the launch of the GPEI is attributable to the extensive use of the live, attenuated oral poliovirus vaccine (OPV) in mass vaccination campaigns and comprehensive national routine immunization programs. Despite its well-established safety record, OPV use can be associated with rare emergence of genetically divergent vaccine-derived polioviruses (VDPVs) whose genetic drift from the parental OPV strains indicates prolonged replication or circulation (2). VDPVs can also emerge among persons with primary immunodeficiencies (PIDs). Immunodeficiency-associated VDPVs (iVDPVs) can replicate for years in some persons with PIDs. In addition, circulating vaccine-derived polioviruses (cVDPVs) can emerge very rarely among immunologically normal vaccine recipients and their contacts in areas with inadequate OPV coverage and can cause outbreaks of paralytic polio. This report updates previous summaries regarding VDPVs (3). During January 2016-June 2017, new cVDPV outbreaks were identified, including two in the Democratic Republic of the Congo (DRC) (eight cases), and another in Syria (35 cases), whereas the circulation of cVDPV type 2 (cVDPV2) in Nigeria resulted in cVDPV2 detection linked to a previous emergence. The last confirmed case from the 2015-2016 cVDPV type 1 (cVDPV1) outbreak in Laos occurred in January 2016. Fourteen newly identified persons in 10 countries were found to excrete iVDPVs, and three previously reported patients in the United Kingdom and Iran (3) were still excreting type 2 iVDPV (iVDPV2) during the reporting period. Ambiguous VDPVs (aVDPVs), isolates that cannot be classified definitively, were found among immunocompetent persons and environmental samples in 10 countries. Cessation of all OPV use after certification of polio eradication will eliminate the risk for new VDPV infections.

Update on Vaccine-Derived Polioviruses - Worldwide, January 2015-May 2016.

In 1988, the World Health Assembly resolved to eradicate poliomyelitis worldwide (1). One of the main tools used in polio eradication efforts has been the live, attenuated, oral poliovirus vaccine (OPV) (2), an inexpensive vaccine easily administered by trained volunteers. OPV might require several doses to induce immunity, but provides long-term protection against paralytic disease. Through effective use of OPV, the Global Polio Eradication Initiative (GPEI) has brought wild polioviruses to the threshold of eradication (1). However, OPV use, particularly in areas with low routine vaccination coverage, is associated with the emergence of genetically divergent vaccine-derived polioviruses (VDPVs) whose genetic drift from the parental OPV strains indicates prolonged replication or circulation (3). VDPVs can emerge among immunologically normal vaccine recipients and their contacts as well as among persons with primary immunodeficiencies (PIDs). Immunodeficiency-associated VDPVs (iVDPVs) can replicate for years in some persons with PIDs. In addition, circulating vaccine-derived polioviruses (cVDPVs) (3) can emerge in areas with low OPV coverage and can cause outbreaks of paralytic polio. This report updates previous summaries regarding VDPVs (4).

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.

Multiple Independent Emergences of Type 2 Vaccine-Derived Polioviruses during a Large Outbreak in Northern Nigeria

Since 2005, a large poliomyelitis outbreak associated with type 2 circulating vaccine-derived poliovirus (cVDPV2) has occurred in northern Nigeria, where immunization coverage with trivalent oral poliovirus vaccine (tOPV) has been low. Phylogenetic analysis of P1/capsid region sequences of isolates from each of the 403 cases reported in 2005 to 2011 resolved the outbreak into 23 independent type 2 vaccine-derived poliovirus (VDPV2) emergences, at least 7 of which established circulating lineage groups. Virus from one emergence (lineage group 2005-8; 361 isolates) was estimated to have circulated for over 6 years. The population of the major cVDPV2 lineage group expanded rapidly in early 2009, fell sharply after two tOPV rounds in mid-2009, and gradually expanded again through 2011. The two major determinants of attenuation of the Sabin 2 oral poliovirus vaccine strain (A481 in the 5'-untranslated region [5'-UTR] and VP1-Ile143) had been replaced in all VDPV2 isolates; most A481 5'-UTR replacements occurred by recombination with other enteroviruses. cVDPV2 isolates representing different lineage groups had biological properties indistinguishable from those of wild polioviruses, including efficient growth in neuron-derived HEK293 cells, the capacity to cause paralytic disease in both humans and PVR-Tg21 transgenic mice, loss of the temperature-sensitive phenotype, and the capacity for sustained person-to-person transmission. We estimate from the poliomyelitis case count and the paralytic case-to-infection ratio for type 2 wild poliovirus infections that ∼700,000 cVDPV2 infections have occurred during the outbreak. The detection of multiple concurrent cVDPV2 outbreaks in northern Nigeria highlights the risks of cVDPV emergence accompanying tOPV use at low rates of coverage in developing countries.

Retrospective molecular and phenotypic analysis of poliovirus vaccine strains isolated in Greece

The live oral poliovirus vaccine (OPV) strains are genetically unstable, causing, in rare cases, vaccine-associated paralytic poliomyelitis. Reversions of the known attenuating mutations in OPV strains and intertypic recombination have been identified as the underlying causes of the increased neurovirulence of poliovirus isolates. In this study, three OPV isolates (one non-recombinant and two recombinants) were tested in order to correlate phenotypic traits such as temperature sensitivity (Rct test) and growth kinetics (one-step growth curve test) with mutations and recombination events of the viral genome. Moreover, the immunity level of the western Greek population aged 1–40 years was evaluated against OPV isolates and Sabin vaccine strains, with a microneutralization assay. Members of the 1–40-year age group (both pooled and individual sera) showed no significant differences in neutralization test (NT) titres against OPV isolates in comparison with the Sabin vaccine strains. However, all three OPV isolates showed reverted phenotypic traits in Rct or one-step growth curve assays. The results of our study revealed a significant decrease in immunity level from the 1–10-year age group to the 21–30-year age group (pooled sera) for both poliovirus types 1 and 3. For both poliovirus types, the highest NT titres were observed in the 1–10-year age group, and the lowest NT titre was observed in the 21–30-year age group, towards poliovirus type 3. Our study underlines the need for immunological studies in all age groups, in order to allow reconsideration of the current vaccination policies and to avoid epidemics caused by the circulation of highly evolved OPV derivatives.

Growth kinetic analysis of bi-recombinant poliovirus vaccine strains

Attenuated strains of Sabin poliovirus vaccine replicate in the human gut and in rare cases may cause vaccine-associated paralytic poliomyelitis (VAPP). Mutations at specific sites of the genome and recombination between Sabin strains may result in the loss of the attenuated phenotype of OPV (Oral Poliovirus Vaccine) strains and the acquisition of traits characteristic of wild polioviruses, such as increased neurovirulence and loss of temperature sensitivity. In this study, we determined the phenotypic traits such as temperature sensitivity and growth kinetics of eight OPV isolates (six bi-recombinant and two non-recombinant). The growth phenotype of each isolate as well as of Sabin vaccine strains in Hep2 cell line at two different temperatures (37 and 40 degrees C) was evaluated using two different assays, RCT test (Reproductive Capacity at different Temperatures) and one-step growth curve analysis. Moreover, the nucleotide and amino acid positions in the genomes of the isolates that have been identified as being involved in the attenuated and thermo sensitive phenotype of Sabin vaccine strains were investigated. Mutations that result in loss of the attenuated and thermo sensitive phenotype of Sabin vaccine strains were identified in the genomes of all isolates. Both mutations and recombination events correlated well with the reverted phenotypic traits of OPV-derivatives. In the post-eradication era of wild polioviruses, the identification and the characterization (genomic and phenotypic) of vaccine-derived polioviruses become increasingly important in order to prevent cases or even outbreaks of paralytic poliomyelitis caused by neurovirulent strains.

Investigations of clinical isolations of oral poliovirus vaccine strains between 2000 and 2005 in southern Taiwan

Background In Taiwan, trivalent oral poliovirus vaccine (tOPV) is in the routine immunization schedule, but its association with illnesses had not been examined. Objectives To investigate clinical presentations and viral characteristics of patients with poliovirus isolates. Study design Clinical data, vaccination records and viral sequences were retrospectively analyzed for patients from whom polioviruses were isolated during 2000–2005. Results OPV-like strains were the only pathogen identified in 208 children who were diagnosed with lower respiratory tract infection (24.5%), acute gastroenteritis (16.8%) or upper respiratory tract infection (10.6%). Timing of poliovirus isolation relative to the tOPV vaccination was unusual in 59 patients, including 6 before any dose and 53 more than 10 weeks after the 3rd or later dose of tOPV. Sequence analyses of the VP1, 2C and 3C/D regions for 19 poliovirus isolates revealed that 4 had previously reported neurovirulence reversions, 1 had intertypic recombination, and 6 had both. No patient had neurological complications, but 3 died of myocarditis, including 2 with recombinant strains and 1 who never received OPV. Conclusion This study describes the isolation of OPV-like strains from patients with a variety of illnesses, raising concerns about their pathogenic potential in an area where tOPV is routinely administered. The detection of genetic variations among OPV-like strains warrants continuing surveillance for these variants in patients with severe illnesses besides neurological complications.

Oral poliovirus vaccine type 3 from a patient with transverse myelitis is neurovirulent in a transgenic mouse model

Background It is accepted that oral poliovirus vaccine (OPV) can cause vaccine-associated paralytic poliomyelitis (VAPP) and that wild poliovirus infection can rarely present as transverse myelitis. It is therefore possible that OPV could cause transverse myelitis. We previously reported a case of transverse myelitis that developed in a 6-month-old boy, 7 days after receiving his second dose of OPV. Objectives Our aim was to test the virus from this patient with transverse myelitis for neurovirulence in a mouse model. Study design The TgPVR21 transgenic mouse line, which expresses the human poliovirus receptor CD155, was used to assess neurovirulence of the viruses tested. Neurovirulence was expressed as the PD 50, the dose of virus causing paralysis in 50% of the mice. Four type 3 polioviruses were tested: a prototype wild strain, a fully attenuated polio vaccine virus, a virus from a patient with VAPP and the virus from the patient with transverse myelitis. Results The PD 50 for the wild poliovirus strain was 3.83 and for the fully attenuated vaccine strain, 7.63. The PD 50 for the two clinical isolates were between these values, ≧4.96 for the poliovirus known to have caused VAPP and ≧4.81 for the virus from the patient with transverse myelitis. Conclusions The report of an OPV strain from a transverse myelitis case being neurovirulent in an in vivo mouse model provides further evidence for a causal association between OPV and transverse myelitis.

Isolation of recombinant type 2 vaccine-derived poliovirus (VDPV) from a Nigerian child

A type 2 vaccine-derived poliovirus (VDPV), differing from Sabin 2 at 2.5% (22/903) of VP1 nucleotide (nt) positions, was isolated from an incompletely immunized 21-month-old Nigerian child who developed acute flaccid paralysis in 2002. Sequences upstream of nt position 620 (within the 5′-untranslated region [5′-UTR]) and downstream of nt position 5840 (in the 3C pro region) were derived from species C enteroviruses unrelated to the oral poliovirus vaccine (OPV) strains. The two substitutions associated with the attenuated phenotype had either recombined out (A 481 → G in the 5′-UTR) or reverted (Ile 143 → Thr in VP1). The VDPV isolate had lost the temperature sensitive phenotype of Sabin 2 and it was antigenically distinct from the parental OPV strain, having amino acid substitutions in or near neutralizing antigenic sites 1 and 3. The date of the initiating OPV dose, calculated from the number of synonymous substitutions in the capsid region, was estimated to be ∼16 to 18 months before onset of paralysis, a finding inconsistent with the most recent mass OPV campaign (conducted 12 days before onset of paralysis) as being the source of infection. Although no related type 2 VDPVs were detected in Nigeria or elsewhere, the VDPV was found in an area where conditions favor VDPV emergence and spread.

Poliovirus vaccine strains in sewage and river water in South Africa

Since the initiation of the global poliomyelitis eradication program in 1988, the number of wild-type polio cases decreased from 350,000 to fewer than 500, and the number of polio endemic countries declined from more than 125 to 10. The last case of polio in South Africa caused by a wild-type poliovirus (PV) occurred in 1989. The live attenuated oral poliovirus vaccine (OPV) has been effectively used in the reduction and control of poliomyelitis. However, as OPV strains are excreted in stools after vaccination, this vaccine could become a source of dissemination of PVs in the environment and the potential cause of poliomyelitis. Therefore, the aim of the study was to determine the occurrence of OPV strains in selected sewage and river water samples. During the period between 2001 and 2003, 138 samples of river water and 213 samples of settled sewage were collected from selected areas of South Africa. A total of 860 plaques were analysed, which consisted of 703 plaques from the sewage and 157 plaques from the river water samples. Using a reverse transcriptase (RT)-multiplex PCR, 49 PVs were successfully distinguished from 176 non-polio enteroviruses (NPEVs). The 176 NPEVs consisted of 50 coxsackie B2 viruses (CBV2), followed by 39 echoviruses 11 (ECV11), 25 CBV5, 21 CBV3, 15 CBV4, 14 coxsackie A6 viruses (CAV6), 7 CBV6, 2 CAV5, 2 CBV1, and 1 ECV19, which was in agreement with the prevalence of these EVs in other parts of the world. The Sabin-specific RT-triplex PCR revealed the presence of 29 Sabin PV type 1, 8 Sabin PV type 2, and 12 Sabin PV type 3 isolates. Buffalo green monkey kidney and primary liver carcinoma cell cultures allowed the amplification of a broad spectrum of EVs, whereas human epidermoid carcinoma cells were more selective for PVs. This study addressed some of the issues regarding the prevalence of OPV strains in the environment. The identification of 49 viable OPV isolates confirmed the presence and circulation of PV vaccine strains in sewage and river water. The extent of the potential health risk constituted by these OPV isolates remains to be investigated.

Prevalence of vaccine-derived polioviruses in stools of immunodeficient children in South Africa

The aim of the study was to determine the prevalence of vaccine-derived polioviruses (VDPVs) in stool specimens of immunodeficient patients such as HIV-positive children (including those with an AIDS indicator condition, according to the Centres for Disease Control and Prevention classification) by applying various molecular techniques. A total of 164 stool samples from HIV-positive children and 23 stool samples from healthy immunocompetent children (the control group) were analysed during 2003 and 2004. By applying a reverse transcription polymerase chain reaction (RT-PCR) in combination with a nested PCR, a total of 54 enteroviruses were detected in the stool specimens of the immunodeficient children. The use of restriction enzymes and a Sabin specific RT-triplex PCR confirmed the presence of 13 polioviruses (PVs), such as seven Sabin PV type 1, four Sabin PV type 3 and two Sabin PV type 2 isolates. The 5'untranslated region and the VP1 capsid-encoding protein of the 13 PVs and the three PVs from the stools of the immunocompetent children were partially sequenced and their genetic relatedness was deduced from the constructed phylogenetic trees. The majority of the PVs isolated from the stools of the immunodeficient children (10 of 13 isolates) were classified as 'oral poliovirus vaccine (OPV)-like viruses', as these isolates had close sequence relationships (>99% in VP1 nucleotide sequences) to the original Sabin PV vaccine strains. Three PVs showed < or =99% VP1 sequence identity to the Sabin PV vaccine strains and were classified as 'suspected' immunodeficient VDPVs (iVDPVs). All of the OPV-like isolates and the 'suspected' iVDPVs carried mutations at specific positions in their partially sequenced regions, which have been associated with reversion of the attenuated Sabin PV vaccine strains to increased neurovirulence. Thus, this study adds further evidence to the observation that immunodeficient individuals may excrete OPV strains with potential neurovirulent phenotypes. Prolonged excretion of PVs by immunodeficient individuals is of major concern, because continued replication of PVs in the human gut could result in the reversion of these viruses to greater neurovirulence. When exposed to OPV, immunodeficient patients may become chronically infected, spreading potentially neurovirulent VDPVs for many months or years to close contacts and children who are no longer being vaccinated after termination of OPV vaccination in the near future.

Modulation of Poliovirus Replicative Fitness in HeLa Cells by Deoptimization of Synonymous Codon Usage in the Capsid Region

We replaced degenerate codons for nine amino acids within the capsid region of the Sabin type 2 oral poliovirus vaccine strain with corresponding nonpreferred synonymous codons. Codon replacements were introduced into four contiguous intervals spanning 97% of the capsid region. In the capsid region of the most highly modified virus construct, the effective number of codons used (N(C)) fell from 56.2 to 29.8, the number of CG dinucleotides rose from 97 to 302, and the G+C content increased from 48.4% to 56.4%. Replicative fitness in HeLa cells, measured by plaque areas and virus yields in single-step growth experiments, decreased in proportion to the number of replacement codons. Plaque areas decreased over an approximately 10-fold range, and virus yields decreased over an approximately 65-fold range. Perhaps unexpectedly, the synthesis and processing of viral proteins appeared to be largely unaltered by the restriction in codon usage. In contrast, total yields of viral RNA in infected cells were reduced approximately 3-fold and specific infectivities of purified virions (measured by particle/PFU ratios) decreased approximately 18-fold in the most highly modified virus. The replicative fitness of both codon replacement viruses and unmodified viruses increased with the passage number in HeLa cells. After 25 serial passages (approximately 50 replication cycles), most codon replacements were retained, and the relative fitness of the modified viruses remained well below that of the unmodified virus. The increased replicative fitness of high-passage modified virus was associated with the elimination of several CG dinucleotides. Potential applications for the systematic modulation of poliovirus replicative fitness by deoptimization of codon usage are discussed.

Antigenic evolution of vaccine-derived polioviruses: changes in individual epitopes and relative stability of the overall immunological properties.

The Sabin oral poliovirus vaccine (OPV) readily undergoes changes in antigenic sites upon replication in humans. Here, a set of antigenically altered descendants of the three OPV serotypes (76 isolates) was characterized to determine the driving forces behind these changes and their biological implications. The amino acid residues of OPV derivatives that lie within or close to the known antigenic sites exhibited a marked tendency to be replaced by residues characteristic of homotypic wild polioviruses, and these changes may occur very early in OPV evolution. The specific amino acid alterations nicely correlated with serotype-specific changes in the reactivity of certain individual antigenic sites, as revealed by the recently devised monoclonal antibody-based enzyme-linked immunosorbent assay. In comparison to the original vaccine, small changes, if any, in the neutralizing capacity of human or rabbit sera were observed in highly diverged vaccine polioviruses of three serotypes, in spite of strong alterations of certain epitopes. We propose that the common antigenic alterations in evolving OPV strains largely reflect attempts to eliminate fitness-decreasing mutations acquired either during the original selection of the vaccine or already present in the parental strains. Variability of individual epitopes does not appear to be primarily caused by, or lead to, a significant immune evasion, enhancing only slightly, if at all, the capacity of OPV derivatives to overcome immunity in human populations. This study reveals some important patterns of poliovirus evolution and has obvious implications for the rational design of live viral vaccines.

Poliovirus vaccine strains detected in stool specimens of immunodeficient children in South Africa

After exposure to the oral poliovirus vaccine (OPV), immunocompetent persons excrete poliovirus (PV) vaccine strains for a limited period. In contrast, immunodeficient individuals remain sometimes chronically infected, and in some cases, PV excretion times as long as 10 years have been reported. During prolonged replication in the human intestine, the PV vaccine strain almost invariably reverts its attenuated character and acquires neurovirulent properties (vaccine-derived PVs, or VDPVs), which resemble wild-type PV strains. The aim of this study was to determine the occurrence of OPV strains in stools of immunodeficient children from a selected area in South Africa, as a first step toward future research on the prevalence and potential health impact of VDPVs. In a period of 1 year, a total of 164 stool samples of HIV-positive children aged 4 months to 8 years were studied for the excretion of OPV strains. In addition, 23 stool samples from healthy immunocompetent children were analyzed after receiving their OPV immunization. By applying a reverse transcription–polymerase chain reaction in combination with a nested PCR, a total of 54 enteroviruses (EVs) were detected in the stool specimens of the immunodeficient children. Using restriction enzyme analysis, 13 PVs were distinguished from 41 nonpolio EVs (NPEVs). A Sabin-specific RT-triplex PCR confirmed the presence of 7 Sabin PV type 1, 4 Sabin PV type 3, and 2 Sabin PV type 2 isolates. The majority of the NPEV group was made up of 7 coxsackievirus B3 (CBV3), 6 echovirus 11 (ECV11), 5 ECV9, and 3 coxsackievirus A6 (CAV6) isolates. According to the results, two of the immunodeficient patients (P023 and P140) who had received their last OPV immunization more than 15 months before (vaccinated at 14 weeks of age) tested positive for Sabin PVs types 3 and 1, respectively. A 5-year-old immunodeficient patient (P052) who had received her last OPV immunization more than 42 months before (vaccinated at 18 months of age) tested positive for Sabin PV type 1. These results suggested that immunodeficient patients vaccinated with OPV might excrete potentially pathogenic VDPVs for a prolonged period. These VDPVs may circulate in the community, resulting in possible infections in the unvaccinated population. Therefore, the information obtained in this study would be essential for strategies aimed at the protection of both immunodeficient as well as immunocompetent individuals against complications of vaccination with OPV.

Effect of Different Vaccination Schedules on Excretion of Oral Poliovirus Vaccine Strains

Inactivated poliovirus vaccine (IPV) is believed to induce significantly lower mucosal immunity than oral poliovirus vaccine (OPV). Most of the data supporting this were generated before enhanced IPV (eIPV) was introduced. Excretion of poliovirus by OPV recipients can be used to assess intestinal immunity. We studied polymerase chain reaction amplification of viral complementary DNA from the stool of children vaccinated with either OPV alone or eIPV. Of first-time OPV recipients, 92% excreted virus after 1 week, and 81% excreted virus after 3 weeks. Prior vaccination with OPV reduced the number to 22% and shortened the duration of virus excretion (to 5% after 3 weeks). Two doses of IPV reduced the number of poliovirus-positive 1-week samples (to 76%), the duration of shedding (to 37% at 3 weeks), and the quantity of excreted virus. This suggests that IPV-vaccinated communities are partially protected from the spread of poliovirus. Further enhancement of IPV potency may lead to even higher levels of mucosal immunity.

Persistence of oral polio vaccine virus after its removal from the immunisation schedule in New Zealand

On Feb 1, 2002, inactivated poliomyelitis vaccines replaced live-attenuated oral poliovirus vaccine (OPV) in New Zealand's immunisation schedule, allowing systematic monitoring of OPV virus circulation. Findings of paediatric-inpatient surveillance indicate that 7% of children excreted polioviruses before this switch, but none did so 1 month afterwards. Acute flaccid paralysis surveillance detected no poliovirus during and after the switch, whereas enterovirus surveillance detected poliovirus only once during the switch. Environmental surveillance identified polioviruses in sewage samples until May, 2002, after which they were detected infrequently. Intratypic differentiation and sequencing showed that all polioviruses were Sabin-like. Multiple surveillance methods hence showed that OPV strains did not persist for extended periods after a vaccine switch in a developed country with a temperate climate. Sequence homology with Sabin vaccine parent strains indicated that polioviruses detected more than 4 months after the switch were of recent origin, consistent with importation from OPV-using countries.