Sabin Strains Research Articles

Remarkable cross-reaction of pan-pestivirus PCR primers with poliovirus genome

When several human vaccines were tested for pestivirus contamination using a one-tube closed nested RT-PCR method employing pan-pestivirus primers selected from the 5′-untranslated region (5′-UTR) of the pestivirus genome, a 224 bp DNA product was produced from a poliovirus vaccine. Although this amplicon was of the size expected for pestiviruses, its sequence showed a 100% similarity with the corresponding reverse complement of a nucleotide sequence from the VP2 gene of the poliovirus type 1 Sabin strain. It is recommended that all positive PCR products, especially those prepared using pan-pestivirus primers, obtained from screening biological substances for pestivirus contamination should be checked by use of a specific hybridization probe and preferably by sequencing.

Serial recombination during circulation of type 1 wild-vaccine recombinant polioviruses in China.

Type 1 wild-vaccine recombinant polioviruses sharing a 367-nucleotide (nt) block of Sabin 1-derived sequence spanning the VP1 and 2A genes circulated widely in China from 1991 to 1993. We surveyed the sequence relationships among 34 wild-vaccine recombinants by comparing six genomic intervals: the conserved 5'-untranslated region (5'-UTR) (nt 186 to 639), the hypervariable portion of the 5'-UTR (nt 640 to 742), the VP4 and partial VP2 genes (nt 743 to 1176), the VP1 gene (nt 2480 to 3385), the 2A gene (nt 3386 to 3832), and the partial 3D gene (nt 6011 to 6544). The 5'-UTR, capsid (VP4-VP2 and VP1), and 2A sequence intervals had similar phylogenies. By contrast, the partial 3D sequences could be distributed into five divergent genetic classes. Most (25 of 34) of the wild-vaccine recombinant isolates showed no evidence of additional recombination beyond the initial wild-Sabin recombination event. Eight isolates from 1992 to 1993, however, appear to be derived from three independent additional recombination events, and one 1993 isolate was derived from two consecutive events. Complete genomic sequences of a representative isolate for each 3D sequence class demonstrated that these exchanges had occurred in the 2B, 2C, and 3D genes. The 3D gene sequences were not closely related to those of the Sabin strains or 53 diverse contemporary wild poliovirus isolates from China, but all were related to the 3D genes of species C enteroviruses. The appearance within approximately 2.5 years of five recombinant classes derived from a single ancestral infection illustrates the rapid emergence of new recombinants among circulating wild polioviruses.

Paralytic poliomyelitis caused by a vaccine-derived polio virus in an antibody-deficient Argentinean child

We describe a case of poliomyelitis in a 3-year-old Argentinean boy with X-linked hypogammaglobulinemia. The child had no history of polio vaccination, but a poliovirus isolated from a stool sample had 97.2% genetic similarity to the Sabin 1 vaccine strain. According to the WHO definition, this is the first case reported of a vaccine-derived poliovirus infection recorded in continental Latin America.

Community-acquired poliovirus infection in children with primary immunodeficiencies in Tunisia.

The global polio eradication program recommends the use of massive vaccination campaigns with live vaccine through National Immunization Days (NIDs) to displace the wild virus from the community. Immunodeficient patients may be indirectly infected and become chronic excretors and potential reservoirs of polioviruses, a concern for the posteradication era. This prospective study aimed to assess the risk of community-acquired infection of immunodeficient patients following NIDs, the dynamics of viral excretion and the genetic variation of excreted viruses. Sixteen children with various primary immunodeficiencies, who did not receive the vaccine during the campaign, were investigated. Stool samples were collected weekly, shortly after the NIDs, during at least 3 months, and were processed for viral isolation. Isolates were characterized by three intratypic differentiation methods and partial sequencing of the VP1/2A region. Polioviruses were detected in 4 out of 16 patients (serotype 1 in 3 patients and serotype 3 in 1 patient). Sequencing revealed more than 99% homology with homotypic Sabin strains, suggesting recent infection. Duration of viral excretion ranged from 1 to 7 weeks. Nine out of eleven isolates from the three poliovirus serotype 1-infected patients disclosed a non-Sabin-like phenotype by enzyme-linked immunosorbent assay and had recurrent mutations within or close to the neutralizing antigenic sites. In summary, the risk of secondary infection in immunodeficient patients is within the range previously reported for the general population. Although none of the four infected patients developed prolonged viral excretion, particular viral variants were selected and may be of epidemiological significance.

The Authors Respond to Hull et al.

Received for publication October 10, 2000, and accepted for publication October 25, 2000. Abbreviations: IPV, inactivated polio vaccine; OPV, oral polio vaccine; VDPV, vaccine-derived poliovirus. 1 Research Laboratory for Infectious Diseases, National Institute of Public Health and the Environment, Bilthoven, the Netherlands. 2 Department of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment, Bilthoven, the Netherlands. 3 Laboratory for Control of Biological Products, National Institute of Public Health and the Environment, Bilthoven, the Netherlands. 4 Department of Virology, Eijkman Winkler Institute, University Medical Center Utrecht, Utrecht, the Netherlands. Reprint requests to Dr. Marina A. E. Conyn-van Spaendonck, Department of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, the Netherlands (e-mail: mae.conyn@rivm.nl). Hull et al. (1) raise important issues regarding the eradication of poliovirus and the ending of polio immunization. Our study (2) provides generally reassuring data on the immunity of the Dutch population. Nonetheless, it also pinpoints “weak spots”: first, the community of people who refuse vaccination for religious reasons; second, the lower seroprevalence among those persons in the general population born before 1945. Approximately 8 percent of them were negative (titer < 1:8) for antibodies to poliovirus type 1 and approximately 15 percent for antibodies to poliovirus type 2 or 3. Approximately 30 percent lacked antibodies against one of the serotypes. These persons were not immunized during the routine vaccination campaigns and probably also have escaped natural infections. One of the patients in the last outbreak belonged to this group (3). We are currently investigating to what extent these persons are protected by memory immunity or are susceptible to poliovirus infection and are therefore at risk during an outbreak (the “Tiel study”). Although it is too early to draw firm conclusions, our first results indicate that a number of these persons excrete virus upon inoculation with oral polio vaccine (OPV). We agree with Hull et al. (1) that our data support the lack of widespread transmission of wild polioviruses and vaccine-derived polioviruses (VDPVs). Some cautionary remarks are needed, however. Our serologic method has an unknown sensitivity to detect poliovirus circulation in a population immunized with inactivated polio vaccine (IPV). Hence, we cannot completely rule out small episodes of silently circulating wild polioviruses and VDPVs. Detection of poliovirus-specific immunoglobulin A and of antibodies directed against nonstructural proteins of the virus might enhance the sensitivity of serology for this purpose. Preliminary data in which our poliovirus-specific immunoglobulin A assay was used support the lack of widespread poliovirus transmission (4), but additional work is required to define its sensitivity and specificity. Unfortunately, these serologic methods do not allow discrimination between infection with wild polioviruses and VDPVs and are therefore of no use in countries that use OPV. Furthermore, limited social interaction between the small Orthodox Reformed group and the general population might have limited virus transmission. Hull et al. (1) are concerned about the long-term supply of vaccine. This matter is indeed very serious; therefore, we would like to call on manufacturers and funding organizations for sustained support in this area, including the development of new approaches to producing vaccines. Because manufacturing IPV under maximum laboratory containment is very expensive, it may be wise to develop IPV from Sabin strains or from viral capsids produced by expression systems such as plants. Finally, Hull et al. (1) ask for an international consensus on the global strategy for stopping polio immunization. As discussed in Paris, France (1, 4), there are still a number of unresolved issues regarding the “endgame.” These issues include the moment and method of stopping immunization (at once or gradually), the risk of transmission of VDPVs, the genetic basis of transmissibility, the risk of recombinant viruses, the risk of long-term immunodeficient virus shedders, the sensitivity of surveillance, and the availability of rapid diagnostics. Countries also largely differ in their per capita annual expenditure on health care and hence in the quality of their public health infrastructure and vaccination coverage. Thus, it is unlikely that such an international consensus will be easy to reach. Vaccination should be stopped only when these questions have been answered satisfactorily and when a sufficient stock of emergency vaccine is available. We therefore recommend not to stop polio immunization too soon but to direct all efforts during the coming years toward bringing routine coverage of polio (and other childhood) immunizations as close to 100 percent as possible.

Assessment of poliovirus eradication in Japan: genomic analysis of polioviruses isolated from river water and sewage in toyama prefecture.

Seventy-eight poliovirus strains isolated from river water and sewage in Toyama Prefecture, Japan, during 1993 to 1995 were characterized by the PCR-restriction fragment length polymorphism (RFLP) method and by partially sequencing the VP3 and VP1 regions of the viral genome. Of these isolates, 36 were identified as Sabin vaccine strains, and 42 were identified as vaccine variant strains that had less than 1.4% nucleotide divergence from the Sabin strains, including 7 isolates with patterns different from those of Sabin strains as determined by PCR-RFLP analysis. These findings suggest that wild-type poliovirus was not circulating in Toyama Prefecture.

Molecular and antigenic characterization of a highly evolved derivative of the type 2 oral poliovaccine strain isolated from sewage in Israel.

An unusual, highly diverged derivative of the Sabin type 2 oral poliovaccine (OPV) strain was recovered from environmental samples during routine screening for wild polioviruses. Virus was cultivated in L20B cells and then passaged on BGM cells at 40 degrees C (RCT [reproductive capacity at supraoptimal temperature]-positive marker) to select against most OPV strains. All but 1 of 25 RCT-positive OPV-derived environmental isolates were antigenically and genetically (>99.5% VP1 sequence match) similar to the respective Sabin strains. However, isolate PV2/4568-1/ISR98 (referred to below as 4568-1) escaped neutralization with Sabin 2-specific monoclonal antibodies and cross-adsorbed sera, and had multiple nucleotide substitutions (220 of 2,646; 8.3%) in the P1 capsid region. Fourteen of the 44 associated amino acid substitutions in the capsid mapped to neutralizing antigenic sites. Neutralizing titers in the sera of 50 Israeli children 15 years old were significantly lower to 4568-1 (geometric mean titer [GMT], 47) than to Sabin 2 (GMT, 162) or to the prototype wild strain, PV2/MEF-1/EGY42 (GMT, 108). Two key attenuating sites had also reverted in 4568-1 (A(481) to G in the 5' untranslated region and the VP1 amino acid I(143) to T), and the isolate was highly neurovirulent for transgenic mice expressing the poliovirus receptor (PVR-Tg21 mice). The extensive genetic divergence of 4568-1 from the parental Sabin 2 strain suggested that the virus had replicated in one or more people for approximately 6 years. The presence in the environment of a highly evolved, neurovirulent OPV-derived poliovirus in the absence of polio cases has important implications for strategies for the cessation of immunization with OPV following global polio eradication.

Increased safety level of serotype 3 sabin oral poliomyelitis vaccine lots by improved seed virus, and tissue culture and virus infection conditions

The content of 472U to 472C revertant virus in serotype 3 oral poliomyelitis monovalent bulk vaccines can be quantified by MAPREC (Mutant Analysis by PCR and Restriction Enzyme Cleavage). Besides other wildtype reversions identified in propagated type 3 Sabin strain populations, the 472U to 472C reversion correlates most prominently with neurovirulence in the monkey neurovirulence test. Therefore, the results can be used for the discrimination of ‘good’ and ‘bad’ vaccines on the molecular level. In international collaborative studies it has been well established that vaccine lots containing revertant genomes below a critical threshold pass the in vivo monkey neurovirulence test (MNVT), while vaccine lots containing more revertants fail the MNVT. In this communication we show that the MAPREC test is a sensitive tool for quality control and the demonstration of consistency in large scale production. Furthermore, MAPREC offers a possibility to assess the effect of changed production conditions on the rate of reversion and to find conditions for consistent production with low reversion rates.

Reevaluation of nucleotide sequences of wild-type and attenuated polioviruses of type 3

Published sequences of wild-type and attenuated Sabin strains of type 3 poliovirus (Leon/37 and Leon 12a 1b) were derived from cDNA clones. Recent direct sequencing of Sabin 3 RNA showed that it differed from the published sequence in at least two sites. Here results of direct sequencing of genomes of three independently re-derived sub-strains of attenuated Sabin 3 poliovirus used for oral poliovirus vaccine (OPV) production in addition to the most widely used Pfizer sub-strain are reported. The results showed that all four sub-strains of attenuated type 3 poliovirus contain unique patterns of mutations. Two stocks of the wild-type progenitor Leon/37 strain were also sequenced. Analysis of the two samples of Leon/37 virus showed that one of them is much closer to the Sabin 3 strain, and is an intermediate product of the attenuation process. In addition, we created genetically engineered constructs which contained some of the mutations suspected for their possible role in neurovirulence, and tested them in monkeys and in transgenic mice sensitive to poliovirus. The results suggested that none of them increased neurovirulence of the virus, but some may improve virus replication. Therefore the only mutation occurring in Sabin 3 under vaccine production conditions that appears to affect neurovirulence of the virus is the well known U→C reversion at nucleotide 472.

A method to detect low levels of enteric viruses in contaminated oysters.

Direct isolation and identification of pathogenic viruses from oysters implicated in gastroenteritis outbreaks are hampered by inefficient methods for recovering viruses, naturally occurring PCR inhibitors, and low levels of virus contamination. In this study we focused on developing rapid and efficient oyster-processing procedures that can be used for sensitive PCR detection of viruses in raw oysters. Poliovirus type 3 (PV3) Sabin strain was used to evaluate the efficacy of virus recovery and the removal of PCR inhibitors during oyster-processing procedures. These procedures included elution, polyethylene glycol precipitation, solvent extraction, and RNA extraction. Acid adsorption-elution in which glycine buffer (pH 7.5) was used was found to retain fewer inhibitors than direct elution in which glycine buffer (pH 9.5) was used. RNA extraction in which a silica gel membrane was used was more effective than single-step RNA precipitation for removing additional nonspecific PCR inhibitors. The final 10-microl volume of RNA concentrates obtained from 2 g of oyster tissue (concentration factor, 200-fold) was satisfactory for efficient reverse transcription-PCR detection of virus. The overall detection sensitivity of our method was 1 PFU/g of oyster tissue initially seeded with PV3. The method was utilized to investigate a 1998 gastroenteritis outbreak in California in which contaminated oysters were the suspected disease transmission vehicle. A genogroup II Norwalk-like virus was found in two of three recalled oyster samples linked by tags to the harvest dates and areas associated with the majority of cases. The method described here improves the response to outbreaks and can be used for rapid and sensitive detection of viral agents in outbreak-implicated oysters.

Poliovirus induces apoptosis in the mouse central nervous system.

Poliovirus (PV) is the etiological agent of human paralytic poliomyelitis. Paralysis results from the destruction of motoneurons, a consequence of PV replication. However, the PV-induced process leading to the death of motoneurons is not well known. We investigated whether PV-induced central nervous system (CNS) injury is associated with apoptosis by using mice as animal models. Transgenic mice expressing the human PV receptor were infected intracerebrally with either the neurovirulent PV-1 Mahoney strain or a paralytogenic dose of the attenuated PV-1 Sabin strain. Nontransgenic mice were infected with a mouse-adapted PV-1 Mahoney mutant. DNA fragmentation was demonstrated in CNS tissue from paralyzed mice by visualization of DNA oligonucleosomal laddering and by enzyme-linked immunosorbent assay. Viral antigens and DNA fragmentation detected by the in situ terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end-labeling technique were colocalized in neurons of spinal cords from paralyzed mice. In addition, morphological changes characteristic of cells undergoing apoptosis were observed in motoneurons by electron microscopy. Thus, we show that PV multiplication and CNS injury during paralytic poliomyelitis are associated with apoptosis.

Antigenic and immunogenic properties of inactivated polio vaccine made from Sabin strains

Inactivated polio vaccine (IPV) was prepared with the Sabin strains, normally used for the attenuated live vaccine. The vaccine was characterized with respect to its antigenicity as determined by ELISA and biosensor analysis and its immunogenicity in rats. Compared with the vaccine prepared with virulent strains (Mahoney, MEF and Saukett) some distinct differences were found with regard to the interaction with glass vials (types 1 and 2) and antigenicity (type 3). The most profound difference however, was the immunogenicity of types 1 and 2. Standardized on the amount of virus, type 1 Sabin-IPV was about 3 times more immunogenic as compared to Mahoney-IPV. The immunogenicity of type 2 Sabin-IPV on the other hand was reduced approximately 10-fold, compared to MEF-IPV. Type 3 Sabin and Saukett IPV were comparable in immunogenicity but differences in antigenicity were evident. The lack of correlation between antigenicity and immunogenicity demonstrate that current IPV standards are not suitable to quantify IPV made from Sabin strains. The results indicate that future Sabin-IPV vaccines may have to contain virus amounts that are very different as compared to current IPV vaccines.

RNA structure adjacent to the attenuation determinant in the 5'-non-coding region influences poliovirus viability.

In attenuated Sabin strains, point mutations within stem-loop V of the 5'-non-coding region (NCR) reduce neurovirulence and cell-specific cap-independent translation. The stem-loop V attenuation determinants lie within the highly structured internal ribosome entry site. Although stem-loop V Sabin mutations have been proposed to alter RNA secondary structure, efforts to identify such conformational changes have been unsuccessful. A previously described linker-scanning mutation (X472) modified five nucleotides adjacent to the attenuation determinant at nt 480 [for poliovirus (PV) type 1]. Transfection of X472 RNA generated only pseudo-revertants in HeLa (cervical carcinoma) or SK-N-SH (neuroblastoma) cells. Pseudo-revertants from both cell types contained nucleotide changes within the X472 linker. In addition, some neuroblastoma-isolated revertants revealed second site mutations within the pyrimidine-rich region located approximately 100 nt distal to the original lesion. Enzymatic RNA structure probing determined that the X472 linker substitution did not disrupt the overall conformation of stem-loop V but abolished base pairing adjacent to the attenuation determinant. Our analyses correlated increased base pairing proximal to the stem-loop V attenuation determinant with growth of X472 revertant RNAs (measured by northern blot analysis). Potential roles of second site mutations in the pyrimidine-rich region are discussed. In addition, our enzymatic structure probing results are shown on a consensus secondary structure model for stem-loop V of the PV 5'-NCR.

Genetic stability of oral polio vaccine prepared on primary monkey kidney cells or Vero cells — effects of passage in cell culture and the human gastrointestinal tract

The genetic stability of the three Sabin oral poliovaccine (OPV) strains produced on either primary monkey kidney (VK) or Vero cell substrates was compared in vivo and in vitro by measuring the rate at which the bases most strongly associated with attenuation and reversion to neurovirulence (positions 480, 481, and 472 in the 5′ non-coding region of Sabin 1, 2 and 3 respectively, and 2034 in VP3 of Sabin 3) reverted during passage of the vaccine strains in the gastrointestinal tract of primary vaccinees and in cell culture. For the in vivo study, the poliovirus excretion patterns of 21 infants receiving OPV produced on either VK or Vero cells were followed for 21 days. No significant differences in either the frequency of excretion or the rate of reversion were observed between the two vaccine groups. The rate of accumulation of revertants during passage in vitro was compared for the three Sabin strains passaged 10 times in either VK or Vero cells. For types 1 and 3, revertants accumulated faster upon passage through VK cells compared with passage through Vero cells. Type 2 appeared to be stable as no revertants were detected in either cell type. Results of this study suggest that the use of Vero as opposed to VK cells as substrate for the manufacture of OPV does not negatively influence the genetic stability of the three Sabin OPV strains in vivo or in vitro.

Detection of low levels of enteric viruses in metropolitan and airplane sewage.

To detect less prevalent viruses, such as wild-type polioviruses in sewage from a highly immunized community, a method was developed to efficiently recover viruses and remove PCR inhibitors. The method consisted of initial separation of solids from liquid, followed by solvent extractions, polyethylene glycol precipitations, Sephadex G-200 chromatography, and guanidinium isothiocyanate (GIT) extraction. To elute viruses from the separated solids, 0.5 M threonine (pH 7.5) was as efficient as 3% beef extract but conferred no PCR inhibition. In samples that were concentrated approximately 1,000-fold, 21% of the initially seeded viruses were recovered. When poliovirus type 3 (PV3) Sabin strain at low levels and PV1 LSc strain at high levels were seeded in raw sewage, PV3 was specifically detected in the final sample concentrates at sensitivities of 14 PFU by direct PCR and 0.7 PFU by GIT extraction-PCR. While applying the method to international airplane sewage, which contains high levels of solids as well as commercial sanitizers, 44% (7 of 16) of the samples were found to harbor enteroviruses by both cell culture infectivity and pan-enterovirus PCR analyses. Nucleotide sequencing of the PCR products revealed that multiple enterovirus genotypes were amplified from each final sewage concentrate, whereas the fewer virus genotypes detected by cell culture infectivity were probably the better growing strains. By this method, we demonstrated that air travel may contribute to the intercontinental dissemination of enteric pathogens.

Potency of Wild-type or Sabin Trivalent Inactivated Poliovirus Vaccine, by Enzyme-Linked Immunosorbent Assay using Monoclonal Antibiotics Specific for Each Antigenic Site

Potency testing of inactivated poliovirus vaccine (IPV) is hampered by the absence of a standardizedin vitrotest, as well as the lack of a generally accepted quantitative animal test.In vitrotests must be able to measure selectively the content of the “D” antigen in the vaccine which includes virus neutralizing antibodies. We tested 12 poliovirus type 1, 12 type 2 and six type 3, D antigen-specific monoclonal mouse antibodies (mAb) for use in the enzyme-linked immunosorbent assay (ELISA). We characterized the site-specific reactivities of three mAbs, one for each poliovirus type. The reactivity of the complete mAb panel encompassed the important antigenic sites on the virus surface of each of the poliovirus serotypes. Some of the mAbs were cross-reactive between wild-type and Sabin strain IPV. At least one mAb of each poliovirus type that was D antigen-specific and reacted with both wild-type and Sabin IPV was directed against an antigenic site thought to be immunogenic in humans. These reagents may be useful for improved standardization of the ELISA for IPV.

Comparative sensitivities of Sabin and Mahoney poliovirus type 1 prototype strains and two recent isolates to low concentrations of glutaraldehyde.

Significant intratypic differences in the glutaraldehyde (GTA) sensitivity of echovirus isolates have been shown. While exploring ways to optimize the study of GTA sensitivity of enteroviruses, we also observed intratypic differences in poliovirus type 1 isolates collected in France. A suspension procedure was used for assessing the virucidal effect of GTA at low concentrations (< or = 0.10%) against purified viruses. Two recent isolates of poliovirus type 1 tested were first fully characterized by the PCR restriction fragment length polymorphism (RFLP) test. The RFLP pattern of clinical isolate 5617 was similar to that of poliovirus type 1 LS-c, 2ab (Sabin strain), confirming the vaccine origin of strain 5617. The RFLP pattern of strain 5915 recovered from sewage was different from that of the Mahoney strain, suggesting a genetic variation in this wild isolate. We then analyzed under the same controlled conditions the GTA sensitivities of both isolates and their respective prototype strains. The wild Mahoney and 5915 strains exhibited significantly lower sensitivities to GTA than did the vaccine Sabin and 5617 strains. The inactivation rates of clinical isolates 5617 and 5915 were very similar to those of their corresponding reference Sabin and Mahoney strains. Both the conformational structure of the capsid of each strain and the amino acid constitution of structural polypeptides could be involved in the variations observed. The relevance of our comparative sensitivity studies to standardization of virucidal tests is discussed.

Host and viral factors affecting the decreased immunogenicity of Sabin type 3 vaccine after administration of trivalent oral polio vaccine to rural Mayan children.

Factors affecting immunogenicity of the first 2 doses of oral poliovirus vaccine (OPV) among unimmunized Mayan infants were prospectively evaluated. The relative impact of multiple variables, including mass or routine vaccination, concurrent enteric bacterial (salmonella, shigella, and campylobacter) and viral (adenovirus 40/41, astrovirus, nonpolio enteroviruses, and rotavirus) infections, interference among Sabin vaccine viruses, and preexisting poliovirus antibodies were studied. Sera were available from 181 infants after 2 OPV doses. Seroresponses were 86% to Sabin type 1, 97% to Sabin type 2, and 61% to Sabin type 3 vaccines. Mass versus routine vaccination and preexisting poliovirus antibodies did not affect immunogenicity. By multiple logistic regression analysis, fecal shedding of homologous Sabin strains was associated with increased seroresponses to all Sabin types, especially to Sabin type 3. Decreased OPV immunogenicity was primarily attributable to interference of Sabin type 3 by Sabin type 2. OPV formulations with higher doses of Sabin type 3 could improve immunogenicity among infants in developing countries.

Succession of Mutations in the Sabin Strain of Type 3 Poliovirus Replicating in the Central Nervous System of Monkeys

Sabin strains of oral poliovirus vaccine (OPV) undergo limited genetic changes during replication in cell cultures, the gastrointestinal tract of vaccinees, and the central nervous system of monkeys. Some of these changes are associated with loss of attenuation markers. Here we report the dynamics of mutant accumulation in the Sabin strain of poliovirus type 3 inoculated intraspinally into monkeys. Thr → Ile reversion in amino acid 6 of VP1 (2493 C → U) occurred within the first few days postinoculation (p.i.), but decreased on later days and completely disappeared by Day 17 p.i. 472 U → C reversion in the 5′-untranslated region appeared to accumulate slower and by Day 17 completely substituted for the vaccine-type nucleotide at this site. These results indicate that experimental infection of the central nervous system of monkeys consists of early and late phases in which a different genetic constitution of the virus is favored. In several isolates one additional neurovirulent revertant was found: a Phe → Ser at amino acid 91 of VP3 (2034 U → C). Since this mutation was never detected in vaccine lots and is strongly selected against in cell cultures at temperatures below 38.5°, it does not threaten the safety of OPV.

Mass polio vaccination

Albert Sabin always emphasised that the global eradication of wild poliovirus was possible but that to achieve eradication in developing countries would require mass administration of oral polio vaccine. Experiences in Cuba and Czechoslovakia have proved the effectiveness of this approach, but it was only with its deployment in Brazil in the 1980s that its role in eradicating the virus from a broad geographical area was realised.1 After 40 years of mass administration of oral and inactive poliovirus vaccines a pattern of impact is emerging. Both the vaccines have proved records of safety and efficacy, and there is now no doubt that applying oral polio vaccine over short periods and on a mass scale can control poliomyelitis in any country, irrespective of its geographical location or level of sanitation. The World Health Organisation has played a vital part in the development and use of polio vaccines since their inception. Since 1973 it has been directly responsible for the custody and distribution of the Sabin strains of the oral polio vaccine and has supervised the production laboratories.2 In 1974 it started the expanded programme on immunisation covering six childhood diseases, including poliomyelitis, and progress has been such that in 1988 the organisation declared its commitment to the goal of global eradication of poliomyelitis by 2000.3 Eradication is defined as zero cases of paralytic poliomyelitis due to wild …