Paralytic poliomyelitis in Romania, 1984-1992. Evidence for a high risk of vaccine-associated disease and reintroduction of wild-virus infection

Neutralization capacity of highly divergent type 2 vaccine-derived polioviruses from immunodeficient patients

The use of the oral poliovirus vaccine (OPV) in developing countries has reduced the incidence of poliomyelitis by >99% since 1988 and is the primary tool for global polio eradication. Spontaneous reversions of the vaccine virus to a neurovirulent form can impede this effort. In persons with primary B-cell immunodeficiencies, exposure to OPV can result in chronic infection, mutation, and excretion of immunodeficiency-associated vaccine-derived polioviruses, (iVDPVs). These iVDPVs may have the potential for transmission in a susceptible population and cause paralysis. The extent to which sera from OPV recipients are able to neutralize iVDPVs with varying degrees of antigenic site substitutions is investigated here. We tested sera from a population immunized with a combination vaccine schedule (both OPV and inactivated polio vaccine) against a panel of iVDPVs and found that increases in amino acid substitution in the P1 capsid protein resulted in a decrease in the neutralizing capacity of the sera. This study underscores the importance of maintaining high vaccine coverage in areas of OPV use as well as active surveillance of those known to be immunocompromised.

Lessons learnt to keep Europe polio-free: a review of outbreaks in the European Union, European Economic Area, and candidate countries, 1973 to 2013

Between 1973 and 2013, 12 outbreaks of paralytic poliomyelitis with a cumulative total of 660 cases were reported in the European Union, European Economic Area and candidate countries. Outbreaks lasted seven to 90 weeks (median: 24 weeks) and were identified through the diagnosis of cases of acute flaccid paralysis, for which infection with wild poliovirus was subsequently identified. In two countries, environmental surveillance was in place before the outbreaks, but did not detect any wild strain before the occurrence of clinical cases. This surveillance nonetheless provided useful information to monitor the outbreaks and their geographical spread. Outbreaks were predominantly caused by poliovirus type 1 and typically involved unvaccinated or inadequately vaccinated groups within highly immunised communities. Oral polio vaccine was primarily used to respond to the outbreaks with catch-up campaigns implemented either nationwide or in restricted geographical areas or age groups. The introduction of supplementary immunisation contained the outbreaks. In 2002, the European region of the World Health Organization was declared polio-free and it has maintained this status since. However, as long as there are non-vaccinated or under-vaccinated groups in European countries and poliomyelitis is not eradicated, countries remain continuously at risk of reintroduction and establishment of the virus. Continued efforts to reach these groups are needed in order to ensure a uniform and high vaccination coverage.

Vaccine-associated paralytic poliomyelitis: a review of the epidemiology and estimation of the global burden

BACKGROUND

Vaccine-associated paralytic poliomyelitis (VAPP) is a rare adverse event associated with oral poliovirus vaccine (OPV). This review summarizes the epidemiology and provides a global burden estimate.

METHODS

A literature review was conducted to abstract the epidemiology and calculate the risk of VAPP. A bootstrap method was applied to calculate global VAPP burden estimates.

RESULTS

Trends in VAPP epidemiology varied by country income level. In the low-income country, the majority of cases occurred in individuals who had received >3 doses of OPV (63%), whereas in middle and high-income countries, most cases occurred in recipients after their first OPV dose or unvaccinated contacts (81%). Using all risk estimates, VAPP risk was 4.7 cases per million births (range, 2.4-9.7), leading to a global annual burden estimate of 498 cases (range, 255-1018). If the analysis is limited to estimates from countries that currently use OPV, the VAPP risk is 3.8 cases per million births (range, 2.9-4.7) and a burden of 399 cases (range, 306-490).

CONCLUSIONS

Because many high-income countries have replaced OPV with inactivated poliovirus vaccine, the VAPP burden is concentrated in lower-income countries. The planned universal introduction of inactivated poliovirus vaccine is likely to substantially decrease the global VAPP burden by 80%-90%.

History of polio vaccination

Poliomyelitis is an acute paralytic disease caused by three poliovirus (PV) serotypes. Less than 1% of PV infections result in acute flaccid paralysis. The disease was controlled using the formalin-inactivated Salk polio vaccine (IPV) and the Sabin oral polio vaccine (OPV). Global poliomyelitis eradication was proposed in 1988 by the World Health Organization to its member states. The strategic plan established the activities required for polio eradication, certification for regions, OPV cessation phase and post-OPV phase. OPV is the vaccine of choice for the poliomyelitis eradication program because it induces both a systemic and mucosal immune response. The major risks of OPV vaccination are the appearance of Vaccine-Associated Paralytic Poliomyelitis cases (VAPP) and the emergence of Vaccine Derived Polioviruses strains. The supplementary immunization with monovalent strains of OPV type 1 or type 3 or with a new bivalent oral polio vaccine bOPV (containing type 1 and type 3 PV) has been introduced in those regions where the virus has been difficult to control. Most countries have switched the schedule of vaccination by using IPV instead of OPV because it poses no risk of vaccine-related disease. Until 2008, poliomyelitis was controlled in Romania, an Eastern European country, predominantly using OPV. The alternative vaccination schedule (IPV/OPV) was implemented starting in September 2008, while beginning in 2009, the vaccination was IPV only. The risk of VAPP will disappear worldwide with the cessation of use of OPV. The immunization for polio must be maintained for at least 5 to 10 years using IPV.

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.

Reemergence of recombinant vaccine-derived poliovirus outbreak in Madagascar

BACKGROUND

After the 2001-2002 poliomyelitis outbreak due to recombinant vaccine-derived polioviruses (VDPVs) in the Toliara province of Madagascar, another outbreak reoccurred in the same province in 2005.

METHODS

We conducted epidemiological and virological investigations for each polio case patient and for their contacts.

RESULTS

From May to August 2005, a total of 5 cases of acute flaccid paralysis were reported among unvaccinated or partially vaccinated children 2-3 years old. Type-3 or type-2 VDPV was isolated from case patients and from healthy contacts. These strains were classified into 4 recombinant lineages that showed complex mosaic genomic structures originating from different vaccine strain serotypes and probably from human enterovirus C (HEV-C) species. Genetic relatedness could be observed among these 4 lineages. Vaccination coverage of the population was very low (<50%).

CONCLUSIONS

The broad distribution of VDPVs in the province and their close genetic relationship indicate intense and rapid cocirculation and coevolution of the vaccine strains and of their related HEV-C strains. The occurrence of an outbreak due to VDPV 3 years after a previous outbreak indicates that a short period with low vaccination coverage is enough to create favorable conditions for the emergence of VDPV in this setting.

Circulation of a type 1 recombinant vaccine-derived poliovirus strain in a limited area in Romania

After intensive immunisation campaigns with the oral polio vaccine (OPV) as part of the Global Polio Eradication Initiative, poliomyelitis due to wild viruses has disappeared from most parts of the world, including Europe. Here, we report the characterization of a serotype 1 vaccine-derived poliovirus (VDPV) isolated from one acute flaccid paralysis (AFP) case with tetraplegia and eight healthy contacts belonging to the same small socio-cultural group having a low vaccine coverage living in a small town in Romania. The genomes of the isolated strains appeared to be tripartite type 1/type 2/type 1 vaccine intertypic recombinant genomes derived from a common ancestor strain. The presence of 1.2% nucleotide substitutions in the VP1 capsid protein coding region of most of the strains indicated a circulation time of about 14 months. These VDPVs were thermoresistant and, in transgenic mice expressing the human poliovirus receptor, appeared to have lost the attenuated phenotype. These results suggest that small populations with low vaccine coverage living in globally well-vaccinated countries can be the origin of VDPV emergence and circulation. These results reaffirm the importance of active surveillance for acute flaccid paralysis and poliovirus in both polio-free and polio-endemic countries.

Risks of paralytic disease due to wild or vaccine-derived poliovirus after eradication

After the global eradication of wild polioviruses, the risk of paralytic poliomyelitis from polioviruses will still exist and require active management. Possible reintroductions of poliovirus that can spread rapidly in unprotected populations present challenges to policymakers. For example, at least one outbreak will likely occur due to circulation of a neurovirulent vaccine-derived poliovirus after discontinuation of oral poliovirus vaccine and also could possibly result from the escape of poliovirus from a laboratory or vaccine production facility or from an intentional act. In addition, continued vaccination with oral poliovirus vaccines would result in the continued occurrence of vaccine-associated paralytic poliomyelitis. The likelihood and impacts of reintroductions in the form of poliomyelitis outbreaks depend on the policy decisions and on the size and characteristics of the vulnerable population, which change over time. A plan for managing these risks must begin with an attempt to characterize and quantify them as a function of time. This article attempts to comprehensively characterize the risks, synthesize the existing data available for modeling them, and present quantitative risk estimates that can provide a starting point for informing policy decisions.

Use of a multiple restriction fragment length polymorphism method for detecting vaccine-derived polioviruses in clinical samples

Since the announcement of the WHO program for the global eradication of poliomyelitis and the establishment of epidemiological and virological surveillance, the emergence and circulation of pathogenic vaccine-derived polioviruses (VDPV) presenting >1% nucleotide divergence from the sequence of the original vaccine strain have been demonstrated in certain regions. We developed and used a multiple restriction fragment length polymorphism (RFLP) method to investigate the frequency of these VDPV in a population with a high level of oral poliovirus vaccine coverage in northwestern Russia. Modified RFLP profiles were found to be strongly correlated with the presence of mutations and recombination events in vaccine strains. We found that a substantial proportion of vaccine strains had high percentages of nucleotide substitutions (>0.5%), including a type 3 VDPV with 1.4% nucleotide divergence. These findings indicate that VDPV or pre-VDPV strains are not rare in certain populations with high levels of vaccine coverage. The multiple RFLP method appears to be a simple and rapid tool for monitoring such strains, which could jeopardize the benefits of the eradication program.

Poliovirus sampling by using sodium dodecyl sulfate/EDTA-pretreated chromatography paper strips

To achieve the goal of poliovirus eradication, surveillance of endemic areas is a crucial step in the poliovirus eradication program. Currently, six countries still have endemic poliovirus. We have tested a novel method which uses SDS/EDTA-treated chromatography paper strips to collect and transport poliovirus-containing stool samples. The SDS/EDTA-treated paper strips were soaked with different dilutions of poliovirus-containing feces and stored at different temperatures. After storing the SDS/EDTA paper strips for 5 months at 37 degrees C, poliovirus RNA could be successfully amplified using RT-PCR. Infectivity of wild-type poliovirus type 1, 2, and 3 was lost upon contact with the SDS/EDTA-treated strips. This easy, inexpensive, and biosafe chromatography paper strip method for the collection and transportation of poliovirus samples can be of use in poliovirus surveillance and polio vaccination programs.

Sabin attenuated LSc/2ab strain of poliovirus spreads to the spinal cord from a peripheral nerve in bonnet monkeys (Macaca radiata)

Vaccine-associated paralytic poliomyelitis is a serious concern while using the live attenuated oral polio vaccine for the eradication of poliomyelitis. The bonnet monkey model of poliovirus central nervous system (CNS) infection following experimental inoculation into the ulnar nerve allows the comparative study of wild-type and attenuated poliovirus invasiveness. Dosages >/=10(4) TCID(50) of Mahoney strain of poliovirus type 1 [PV1(M)] result in paralysis. In contrast, even with 10(7) TCID(50) of Sabin attenuated strain of poliovirus type 1 (LSc/2ab), no paralysis occurs, but virus spreads into the CNS where viral RNA is found in spinal cord neurons. While wild-type PV1(M) viral RNA replicates in neurons (and possibly in glial cells) and in cells around vessel walls, which may be mononuclear or endothelial cells, attenuated viral RNA is detected only in neurons. Systemic viraemia and gastrointestinal virus shedding occurs only in PV1(M)-infected animals. While a systemic serologic response is detected in both groups of animals, cerebrospinal fluid antibodies are detected only in animals infected with PV1(M). Both the PV1(M) and LSc/2ab strains spread to the cervical spinal cord and then to the lumbar spinal cord following ulnar nerve inoculation. Neuronophagia and neuronal loss are only seen in PV1(M)-infected monkeys in whom clinical paralysis is observed. Infection with LSc/2ab does not result in neuronophagia, neuronal loss or clinical paralysis. Spread of attenuated poliovirus in spinal cord neurons without causing paralysis following inoculation into the ulnar nerve is an important finding.

The health of the Roma people: a review of the published literature

BACKGROUND

The Roma people originated in northern India and have been known in Europe for nearly a thousand years. For much of that time they have been the subjects of discrimination and oppression, culminating in the extermination of half a million Roma in the Nazi death camps. While it is widely believed that the health of Roma people is often poorer than the majority population, these inequalities remain largely unresearched.

METHODS

Published literature on the health of the Roma people was identified using Medline. Opinion pieces were excluded, as were papers relating to anthropometry and to genetic markers. The resultant papers were analysed by country of study and by disease type or care group.

RESULTS

Some 70% of papers identified related to just three countries; Spain and the Czech and Slovak Republics. Much literature concentrates upon communicable disease or reproductive health. The limited evidence suggests increased morbidity from non-communicable disease, but there is little published on this topic. Evidence on health care, though fragmentary, suggests poorer access to health services and uptake of preventative care.

DISCUSSION

Published research on the health needs of the Roma population is sparse. The topics that have received attention suggest a focus on concepts of contagion or social Darwinism, indicating a greater concern with the health needs of the majority populations with which they live. There is a need for both further research into the health of Roma people; with particular emphasis on non-communicable disease; and also for interventions that improve Roma health. Such research must, however, be handled with sensitivity, recognising the social and political context of the society concerned.

Natural genetic exchanges between vaccine and wild poliovirus strains in humans

In a previous study of poliovirus vaccine-derived strains isolated from patients with vaccine-associated paralytic poliomyelitis (VAPP) (9, 11), we reported that a high proportion (over 50%) of viruses had a recombinant genome. Most were intertypic vaccine/vaccine recombinants. However, some had restriction fragment length polymorphism (RFLP) profiles different from those of poliovirus vaccine strains. We demonstrate here that five such recombinants, of 88 VAPP strains examined, carried sequences of wild (nonvaccine) origin. To identify the parental wild donor of these sequences, we used RFLP profiles and nucleotide sequencing to look for similarity in the 3D polymerase-coding region of 61 wild, cocirculating poliovirus isolates (43 type 1, 16 type 2, and 2 type 3 isolates). In only one case was the donor identified, and it was a wild type 1 poliovirus. For the other four vaccine/wild recombinants, the wild parent could not be identified. The possibility that the wild sequences were of a non-poliovirus-enterovirus origin could not be excluded. Another vaccine/wild recombinant, isolated in Belarus from a VAPP case, indicated that the poliovirus vaccine/wild recombination is not an isolated phenomenon. We also found wild polioviruses (2 of 15) carrying vaccine-derived sequences in the 3' moiety of their genome. All these results suggest that genetic exchanges with wild poliovirus and perhaps with nonpoliovirus enteroviruses, are also a natural means of evolution for poliovirus vaccine strains.

Vaccine-associated paralytic poliomyelitis: a retrospective cohort study of acute flaccid paralyses in Brazil

BACKGROUND

At the present time, in Brazil and other countries in the Americas, the only cases of paralytic poliomyelitis due to poliovirus are caused by vaccine strains. The recognition of possible determinants of vaccine-associated paralytic poliomyelitis (VAPP) by public health surveillance and immunization programmes is relevant to inform the debate on criteria for case definition and vaccination strategies.

METHODS

A retrospective cohort study based on the cases of acute flaccid paralysis (AFP) reported to the Ministry of Health (MoH) was designed, with the objective of studying cases of VAPP in Brazil between 1989 and 1995. Clinical, laboratory and epidemiological data from 3656 acute flaccid paralysis (AFP) cases, 30 of them diagnosed as VAPP, were analysed.

RESULTS

An 8.88 risk ratio of VAPP (95% CI : 4.37-18.03) was found when comparing individuals who received oral poliovirus vaccine (OPV) between 4 and 40 days before the onset of paralysis and individuals who did not receive the vaccine within this period. A risk of 1 case/2.39 million first doses and 1 case/13.03 million OPV doses administered was estimated for the general population.

CONCLUSIONS

Cases of AFP who received OPV between 4 and 40 days before the onset of paralysis and had fever, a prodrome of gastrointestinal symptoms, history of first dose of OPV, isolation of vaccine poliovirus type 2, and young age deserve careful investigation, since they are at increased risk for the condition studied.

High level of immunity against poliomyelitis in Albanian refugees in southern Italy

BACKGROUND

The Apulia region (Southern Italy) may be considered a "border region" which, due to its position, has to face daily arrivals of refugees, especially from Albania. When the exodus of Albanians took place in 1991, a seroepidemiologic study revealed a low seroimmunity to poliomyelitis. In 1996, a large outbreak of paralytic poliomyelitis occurred in Albania. The aim of the study was to evaluate the poliomyelitis immunization level in a sample of Albanian refugees who arrived in the Apulia region between April and May 1997.

METHODS

Blood samples were obtained, after informed consent and on a voluntary basis, from 667 subjects housed in seven refugee camps in the Apulia region. Titration of neutralizing antibodies to the three polioviruses was carried out.

RESULTS

The findings showed that Albanian refugees had adequate levels of immunity to all polioviruses (95% for poliovirus type 1, 98.6% for poliovirus type 2 and 91.4% for poliovirus type 3). Moreover, a high immunization rate was found in all age groups irrespective of the areas of origin of the refugees and their socioeconomic conditions.

CONCLUSION

Our findings show that Albanian refugees in Apulia region have adequate levels of immunity against polioviruses and confirm the effectiveness of mass vaccination campaigns with OPV conducted by WHO in Albania following an outbreak of poliomyelitis in 1996.

Genomic characterization of human and environmental polioviruses isolated in Albania

Between April and December 1996, a serious outbreak of poliomyelitis occurred in Albania; almost 140 subjects were involved, and the episode presented an unusually high mortality rate (12%). During the outbreak, water samples from the Lana River in Tirana, Albania, and stool samples from two cases of paralytic poliomyelitis were collected and analyzed for the presence of polioviruses. Six polioviruses were isolated from the environmental and human samples, according to standard methods. All the samples were characterized by partial genomic sequencing of 330 bases across the 5' untranslated region (5'-UTR) (nucleotide positions 200 to 530) and of 300 bases across the VP1 region (nucleotide positions 2474 to 2774). Comparison of these sequences with those present in data banks permitted the identification of environmental isolates Lana A and Lana B as, respectively, a Sabin-like type 2 poliovirus and an intertypic recombinant poliovirus (Sabin-like type 2/wild type 1), both bearing a G instead of an A at nucleotide position 481. The two other environmental polioviruses were similar to the isolates from the paralytic cases. They were characterized by a peculiar 5'-UTR and by a VP1 region showing 98% homology with the Albanian epidemic type 1 isolates reported by other authors. This study confirms the environmental circulation in Albania of recombinant poliovirus strains, likely sustained by a massive vaccination effort and by the presence in the environment of a type 1 poliovirus, as isolated from the Lana River in Tirana about 2 months before the first case of symptomatic acute flaccid paralysis was reported in this town.

Mutations in Sabin 2 strain of poliovirus and stability of attenuation phenotype

In this study, we attempted to identify the molecular determinants in the genome of the attenuated Sabin 2 vaccine strain of poliovirus that may change during vaccine production and result in an increase in monkey neurovirulence. An extensive search for suitable vaccine lots identified six batches that had failed the monkey neurovirulence test (MNVT). On repeated tests, these batches were found to have acceptable levels of monkey neurovirulence. One of the batches was additionally passaged six times under conditions used in vaccine production, and the resulting high-passage sample was screened for the presence of mutations and tested in monkeys. In addition to the previously described A --> G reversion at nucleotide 481, high-passage stock also contained a mutation in the VP1-coding region (3364 = G --> A) that consistently accumulated in the course of passaging. However, despite the presence of substantial amounts of these mutations, high-passage stock passed the MNVT. Replication of Sabin 2 poliovirus in the central nervous system of transgenic mice susceptible to poliovirus or in cultures of mouse cells, resulted in another mutation (3363 = A --> G). Even though its presence correlated with paralysis in mice, the introduction of 3363-G into the Sabin 2 genome did not increase neurovirulence of the virus. Previous studies identified the 481-G mutation as an important determinant of monkey neurovirulence. We prepared virus samples with varying amounts of genetically defined single mutants at this nucleotide and tested them in monkeys. The results demonstrated that even a 100% substitution at this site introduced into Sabin 2 strain did not increase monkey neurovirulence. The determination of the nucleotide sequence of an alternative strain used for the production of type 2 OPV (Chung 2) showed that it contained 100% of the wild-type 481-G but possessed an extremely low level of neurovirulence. These results demonstrate the remarkable stability of the attenuated phenotype of the Sabin 2 strain and show that (1) no batch of OPV 2 has ever repeatedly failed the MNVT, (2) growing the virus beyond the passage level allowed in vaccine production did not result in increased neurovirulence in monkeys, (3) a test for neurovirulence in transgenic mice may be more sensitive than the MNVT, and (4) determination of the mutational profile of vaccine batches detects inconsistencies in vaccine manufacturing processing that would not be detected by the MNVT.

Serum antibodies to polioviruses in Alexandria, Egypt

Random sera, in a total of 192, were collected in the Fever Hospital of Alexandria, Egypt, and analysed for the presence of antibodies against polioviruses. The results show good antibody levels, only three sera (1.5%) were negative for poliovirus type 1, 5 (2.6%) for poliovirus type 2 and 10 (5.2%) for poliovirus type 3; one subject was completely negative.

Antigenic and molecular characterization of wild type 1 poliovirus causing outbreaks of poliomyelitis in Albania and neighboring countries in 1996

Mass vaccination has led poliomyelitis to become a rare disease in a large part of the world, including Western Europe. However, in the past 20 years wild polioviruses imported from countries where polio is endemic have been responsible for outbreaks in otherwise polio-free European countries. We report on the characterization of poliovirus isolates from a large outbreak of poliomyelitis that occurred in Albania in 1996 and that also spread to the neighboring countries of Yugoslavia and Greece. The epidemics involved 145 subjects, mostly young adults, and caused persisting paralysis in 87 individuals and 16 deaths. The agent responsible for the outbreak was isolated from 74 patients and was identified as wild type 1 poliovirus by both immunological and molecular methods. Sequence analysis of the genome demonstrated the involvement of a single virus strain throughout the epidemics, and genotyping analysis showed 95% homology of the strain with a wild type 1 poliovirus strain isolated in Pakistan in 1995. Neutralization assays with both human sera and monoclonal antibodies were performed to analyze the antigenic structure of the epidemic strain, suggesting its peculiar antigenic characteristics. The presented data underline the current risks of outbreaks due to imported wild poliovirus and emphasize the need to improve vaccination efforts and also the need to implement surveillance in countries free of indigenous wild poliovirus.

Evolution of the Sabin type 1 poliovirus in humans: characterization of strains isolated from patients with vaccine-associated paralytic poliomyelitis

Attenuated strains of the Sabin oral poliovirus vaccine replicate in the human gut and in rare cases cause vaccine-associated paralytic poliomyelitis (VAPP). Reversion of vaccine strains toward a pathogenic phenotype is probably one of the main causes of VAPP, a disease most frequently associated with type 3 and type 2 strains and more rarely with the type 1 (Sabin 1) strain. To identify the determinants and mechanisms of safety versus pathogenicity of the Sabin 1 strain, we characterized the genetic and phenotypic changes in six Sabin 1-derived viruses isolated from immunocompetent patients with VAPP. The genomes of these strains carried either few or numerous mutations from the original Sabin 1 genome. As assessed in transgenic mice carrying the human poliovirus receptor (PVR-Tg mice), all but one strain had lost the attenuated phenotype. Four strains presented only a moderate neurovirulent phenotype, probably due at least in part to reversions to the wild-type genotype, which were detected in the 5' noncoding region of the genome. The reversions found in most strains at nucleotide position 480, are known to be associated with an increase in neurovirulence. The construction and characterization of Sabin 1 mutants implicated a reversion at position 189, found in one strain, in the phenotypic change. The presence of 71 mutations in one neurovirulent strain suggests that a vaccine-derived strain can survive for a long time in humans. Surprisingly, none of the strains analyzed were as neurovirulent to PVR-Tg mice as was the wild-type parent of Sabin 1 (Mahoney) or a previously identified neurovirulent Sabin 1 mutant selected at a high temperature in cultured cells. Thus, in the human gut, the Sabin 1 strain does not necessarily evolve toward the genetic characteristics and high neuropathogenicity of its wild-type parent.

Intramuscular injections within 30 days of immunization with oral poliovirus vaccine--a risk factor for vaccine-associated paralytic poliomyelitis

BACKGROUND

In Romania the rate of vaccine-associated paralytic poliomyelitis is for unexplained reasons 5 to 17 times higher than in other countries. Long ago it was noted that intramuscular injections administered during the incubation period of wild-type poliovirus infection increased the risk of paralytic disease (a phenomenon known as "provocation" poliomyelitis). We conducted a case-control study to explore the association between intramuscular injections and vaccine-associated poliomyelitis in Romania.

METHODS

The patients were 31 young children in whom vaccine-associated paralytic poliomyelitis developed from 1988 through 1992. Eighteen were vaccine recipients, and 13 had acquired the disease by contact with vaccine recipients. Each of these children was matched with up to five controls according to health center, age, and in the case of vaccine recipients, history of receipt of the live attenuated oral poliovirus vaccine. Data were abstracted from medical records that documented the injections administered in the 30 days before the onset of paralysis.

RESULTS

Of the 31 children with vaccine-associated disease, 27 (87 percent) had received one or more intramuscular injections within 30 days before the onset of paralysis, as compared with 77 of the 151 controls (51 percent) (matched odds ratio, 31.2; 95 percent confidence interval, 4.0 to 244.2). Nearly all the intramuscular injections were of antibiotics, and the association was strongest for the patients who received 10 or more injections (matched odds ratio for > or = 10 injections as compared with no injections, 182.1; 95 percent confidence interval, 15.2 to 2186.4). The risk of paralytic disease was strongly associated with injections given after the oral polio virus vaccine, but not with injections given before or at the same time as the vaccine (matched odds ratio, 56.7; 95 percent confidence interval, 8.9 to infinity). The attributable risk in the population for intramuscular injections given in the 30 days before the onset of paralysis was 86 percent (95 percent confidence interval, 66 to 95 percent); that is, we estimate that 86 percent of the cases of vaccine-associated paralytic poliomyelitis in this population might have been prevented by the elimination of intramuscular injections within 30 days after exposure to oral poliovirus vaccine.

CONCLUSIONS

Provocation paralysis, previously described only for wild-type poliovirus infection, may rarely occur in a child who receives multiple intramuscular injections shortly after exposure to oral poliovirus vaccine, either as a vaccine recipient or through contact with a recent recipient. This phenomenon may explain the high rate of vaccine-associated paralytic poliomyelitis in Romania, where the use of intramuscular injections of antibiotics in infants with febrile illness is common.