Biologic and biophysical markers of a live varicella vaccine strain (Oka): identification of clinical isolates from vaccine recipients

Vaccine-type mutations identified in Varicella zoster virus passaged in cell culture

Varicella-zoster virus (VZV) is a causative agent for chickenpox and shingles. Comparative genomic sequence analysis of clinical and vaccine strains suggested potential sites responsible for attenuation. In this study, low and high passages of two VZV clinical strains cultured in human fibroblast cells were compared for genomic DNA sequences and growth characteristics. Mutations were detected at 187 and 162 sites in the strain YC01 and YC02, respectively. More than 86% of mutations were found in open reading frames, and ORF62 exhibited highest frequency of mutations. T to C and A to G transitions accounted for more 90% of all possible substitutions. Forty mutations were common to two strains, including 27 in ORF62. Mutations found in attenuated vaccine strains were also detected at 7 positions. Both high and low passage strains were infectious and grew similarly in human fibroblast cells. In guinea pig cells, however, high passage strain remained infectious while low passage strain lost infectivity. This study may provide new insight into the attenuating mutations associated with in vitro passaging of VZV.

Varicella-zoster virus vaccine: molecular genetics

The genetic differences that potentially account for the attenuation of the Oka vaccine VZV preparation are more clearly defined than for perhaps any other vaccine in current use. This is due in large part to the small number of differences between the vaccine and the parental strain from which it was derived, and to the high level of genomic conservation that characterizes VZV. This information has been used with great success to develop methods that discriminate vaccine from wild-type strains, to begin determining which specific vaccine markers contribute to the attenuated phenotype, to improve evaluations of vaccine efficacy and safety, and to observe the behavior of the live, attenuated preparation as it becomes more prevalent through widespread immunization.

Varicella-zoster virus infection induces autophagy in both cultured cells and human skin vesicles

When grown in cultured cells, varicella-zoster virus (VZV) forms many aberrant light particles and produces low titers. Various studies have explored the reasons for such a phenotype and have pointed to impaired expression of specific late genes and at lysosomal targeting of egressing virions as possible causes. In the studies presented here, we report that the autophagic degradation pathway was induced at late time points after VZV infection of cultured cells, as documented by immunoblot analysis of the cellular proteins LC3B and p62/SQSTM1, along with electron microscopy analysis, which demonstrated the presence of both early autophagosomes and late autophagic compartments. Autophagy was induced in infected cells even in the presence of phosphonoacetic acid, an inhibitor of viral late gene expression, thus suggesting that accumulation of immediate-early and early viral gene products might be the major stimulus for its induction. We also showed that the autophagic response was not dependent on a specific cell substrate, virus strain, or type of inoculum. Finally, using immunofluorescence imaging, we demonstrated autophagosome-specific staining in human zoster vesicles but not in normal skin. Thus, our results document that this innate immune response pathway is a component of the VZV infectious cycle in both cultured cells and the human skin vesicle, the final site of virion formation in the infected human host.

Molecular diagnosis of zoster post varicella vaccination

BACKGROUND

Herpes zoster can be caused by endogenous reactivation of both wild-type virus or vaccine varicella-zoster virus (VZV) becoming latent within sensory ganglia after natural primary infection or varicella vaccination.

OBJECTIVES

To demonstrate molecular biological methods for reliable discrimination between wild-type VZV and vaccine strain Oka by an example of zoster in a vaccinated girl.

STUDY DESIGN

VZV was isolated from zoster occurring 16 months after varicella vaccination in a 2-year-old infant. Including VZV wild-type and different Oka strains as controls, viral DNA fragments located in the open reading frames (ORF) 38, 54, 62 and the R5 variable repeat region were characterized by amplification and restriction fragment length polymorphisms (RFLP) analysis.

RESULTS

VZV vaccine strain Oka was definitely proven to be the causative virus in this case of zoster post vaccination.

CONCLUSIONS

Molecular procedures for characterization of ORF 38 allow reliable discrimination between Oka-like and wild-type VZV outside Japan and Japanese communities. To distinguish Oka vaccine virus from Oka-like wild strains, analysis of DNA fragments located in the ORF 62 should be included.

Genetic variation of varicella-zoster virus: evidence for geographical separation of strains

Heteroduplex mobility assay was used to identify variants of varicella-zoster virus (VZV) circulating in the United Kingdom and elsewhere. Forty variable positions were identified. Sixteen substitutions were non-synonymous, resulting in an amino acid change, the majority of which were clustered within surface expressed proteins. Phylogenetic analysis distinguished at least three major clades (strains A, B, C) supported by significant bootstrap values. Apart from the United Kingdom and Brazil where all three strains were found, genotypes appeared to be closely associated with the geographical region in which they were sampled. Allelic co-segregation of widely spaced single nucleotide polymorphisms (SNPs) confirmed the genetic stability of the VZV. Recombination rates were difficult to calculate because of the low intra genotypic variation. However, one haplotype originating from Brazil is most parsimoniously explained as a recombinant between A and C strains, which co-occur in the region. Two further UK strains appeared to be recombinants between groups B and C.

Phylogenetic analysis of varicella-zoster virus: evidence of intercontinental spread of genotypes and recombination

A heteroduplex mobility assay was used to identify variants of varicella-zoster virus circulating in the United Kingdom and elsewhere. Within the United Kingdom, 58 segregating sites were found out of the 23,266 examined (0.25%), and nucleotide diversity was estimated to be 0.00063. These are an order of magnitude smaller than comparable estimates from herpes simplex virus type 1. Sixteen substitutions were nonsynonymous, the majority of which were clustered within surface-expressed proteins. Extensive genetic correlation between widely spaced sites indicated that recombination has been rare. Phylogenetic analysis of varicella-zoster viruses from four continents distinguished at least three major genetic clades. Most geographical regions contained only one of these three strains, apart from the United Kingdom and Brazil, where two or more strains were found. There was minimal genetic differentiation (one or fewer substitutions in 1,895 bases surveyed) between the samples collected from Africa (Guinea Bissau, Zambia) and the Indian subcontinent (Bangladesh, South India), suggesting recent rapid spread and/or low mutation rates. The geographic pattern of strain distribution would favor a major influence of the former. The genetic uniformity of most virus populations makes recombination difficult to detect. However, at least one probable recombinant between two of the major strains was found among the samples originating from Brazil, where mixtures of genotypes co-occur.

Vaccination to prevent varicella and shingles

Vaccination of healthy children against varicella using the live attenuated Oka vaccine has been available in Japan and south Korea for several years. In 1996, a programme of universal vaccination of children to prevent varicella was introduced in the USA and other countries, including Canada, Germany, and Sweden, have licensed the vaccine for use in healthy children. This article reviews the origin of the Oka vaccine and the evidence for vaccine safety and efficacy in children and adults. Universal vaccination of children and targeted vaccination of groups at risk of severe varicella are discussed. The possible use of the Oka vaccine to prevent zoster is reviewed, and initiatives to develop new varicella zoster virus vaccines are outlined.

Biologic and geographic differences between vaccine and clinical varicella-zoster virus isolates

Vaccine and wild-type strains of varicella-zoster virus differ both in their biologic characteristics and in the clinical manifestations of infection caused by each strain. The biologic differences described for the vaccine strain (temperature sensitivity and host cell preference) probably reflect the methods used to adapt the wild-type strain to the in vitro growth conditions imposed during the attenuation process in cell culture. In addition, restriction fragment polymorphisms have been described that reflect geographic strain variations between the parental virus used to develop the vaccine strain and other wild-type strains. These polymorphisms have been exploited as tools for the identification and differentiation of vaccine and wild-type strains in clinical studies. Infection with the wild-type strain results in the typical extensive rash of varicella, frequent transmission to other susceptible contacts, establishment of latency, and in some individuals, reactivation with the clinical picture of zoster. Infection with the vaccine strain results in the development of a protective immune response, minimal rash in a minority of individuals, rare transmission to other susceptible contacts, and a greatly reduced risk of zoster.

Use of varicella vaccines to prevent herpes zoster in older individuals

It is likely that the frequency and severity of herpes zoster in older people is the result of an age-related decline in varicella-zoster virus-specific T-cell mediated immunity. Numerous trials of vaccines to boost these responses have demonstrated their safety and immunogenicity. Both live attenuated and inactivated vaccines have been studied. Persistence of booster responses is dose-related, and the half-life of some boosted measures of T-cell mediated immunity exceeds 5 years. Although these trials have been hampered by uncertainty about the critical immune responses to evaluate, the stage is set for a double-blind, placebo-controlled trial of sufficient size to determine efficacy. Such a trial is now underway.

Live-attenuated varicella vaccine

This article reviews the history and development of live attenuated varicella vaccine from its early days in Japan to its widespread use throughout the world. The vaccine has proven extremely safe after immunization of as many as 10 million healthy children and adults in the United States alone. The vaccine is also highly immunogenic and offers close to 100% protection from severe chickenpox and 90% protection from illness. It is expected to have a major impact on the epidemiology of varicella and zoster in countries with high vaccine uptake.

Comparison between of the attenuated BR-Oka and the wild type strain of Varicella Zoster Virus (VZV) on the DNA level

Oka strain VR-795 (Varicella Zoster Virus, VZV) of American Type Culture Collection (ATCC) has been used for chickenpox vaccine production. In order to use this strain for vaccine production, the strain must be identified and its stability must be confirmed. The identification of the Oka strain has been confirmed using Restriction Fragment Length Polymorphism (RFLP) and DNA sequence analysis of glycoprotein-II (gp-II). The amino acid sequences of Oka deduced from the DNA sequence of gp-II have changed at three amino acids against Ellen and at one amino acid against Webster. To prove the stability of the Oka strain during the passage, RFLP and DNA sequence analyses were also used with 11, 15 and 23 times of virus passage. We found that the Oka strain was stable at passages of up to 23 times, based on the RFLP and DNA sequence analyses. The confirmed Oka strain was renamed as BR-Oka for the purposes of chickenpox vaccine production.

Varicella vaccine

Varicella vaccine is safe, effective, and cost-effective in healthy children, adolescents, and adults. Breakthrough cases of MVLS are significantly milder than wild-type varicella infection. No severe adverse events have been reported following vaccination, and the incidence of herpes zoster is less in vaccinees than in individuals who have had natural varicella infections. To date, there is no evidence waning immunity following vaccination. "New and improved" varicella vaccines that may be more effective than the current vaccine and can be stored at refrigerator temperatures may soon become available in the United States.

Reactogenicity to a live attenuated varicella vaccine in Canadian children

OBJECTIVE

To assess the reactogenicity and safety of a thermostable, high titre, varicella vaccine in healthy infants and children.

DESIGN

Open study of 505 children monitored for 42 days after vaccination.

SETTING

Three urban Canadian centres (Halifax, Ottawa and Vancouver).

PARTICIPANTS

505 healthy children one to 12 years of age were enrolled and 504 completed the study. All were susceptible to varicella by history.

INTERVENTIONS

All participants received one dose of live attenuated varicella vaccine (1x10(4.5) plaque forming units/dose) subcutaneously.

MAIN OUTCOME MEASURES

The children were monitored from the day of vaccine administration (day 0) until day 42. All local and general symptoms and signs were recorded on diary cards by the patients' parents, who were encouraged to fill in the cards on days 2 to 3 and 18 to 24 via telephone calls from study personnel.

RESULTS

Most of the symptoms noted after vaccine administration were mild and transient, and all resolved within the respective follow-up periods. Injection site symptoms included pain (17.5%, 13.9% and 30.4% in centres 1, 2 and 3 respectively), redness (21.1%, 32.1% and 48.8%) and swelling (7%, 10.3% and 29.2%). The general symptoms reported were fever 37.5 degrees C or higher (3.5%, 4.8% and 3.0%) and varicella-like rashes (6.4%, 2.4% and 0%). Two subjects had severe symptoms (one with cervical lymphadenopathy, and one with a fever higher than 39 degrees C) probably related to vaccine administration. No serious adverse events were reported during the entire study.

CONCLUSION

The vaccine was well tolerated.

Variable R1 region in varicella zoster virus in fulminant type of acute retinal necrosis syndrome

BACKGROUND/AIMS

Varicella zoster virus (VZV) is a causative agent in acute retinal necrosis (ARN) syndrome. However, in spite of aggressive antiviral therapy, clinical characteristics among patients have varied. Different viral strains were examined to determine their respective role in producing clinical characteristics. The viral strains were also compared with those of previously reported ones.

METHODS

To differentiate VZV strains R1 and R5, variable regions of VZV were amplified by nested polymerase chain reaction (PCR) in 11 eyes of 10 patients. Sequence analysis was also performed.

RESULTS

Four cases had strains diverted only at the tip of the 3' end of the R1 variable region, similar to that of the H-N3 strain, which was previously reported. Conversely, other cases were diverted to other regions. Interestingly, some of the latter cases showed multiple PCR products in the R1 region that were generated by the truncation of either the 5' or 3' R1 region. Final visual acuities of these patients were less than 0.2. The former cases showed final visual acuities more than 0.4. Only two variants were from the R5 region. No patient had the same viral strain as the European Dumas type.

CONCLUSION

These results showed that variable VZV strains participated in ARN. Using PCR of the R1 variable region, it was estimated that patients with a more fulminant type of ARN may have diverse viruses with extensive replication in the affected eyes.

Infection of human T lymphocytes with varicella-zoster virus: an analysis with viral mutants and clinical isolates

Varicella-zoster virus (VZV) disseminates in the body in peripheral blood mononuclear cells during chickenpox. Up to 1 in 10,000 mononuclear cells are infected during the viremic phase of the disease. We developed an in vitro system to infect human mononuclear cells with VZV by using umbilical cord blood. In this system, 3 to 4% of T cells were infected with VZV. VZV mutants unable to express certain genes, such as open reading frame 47 (ORF47) or ORF66, were impaired for growth in T cells, while other mutants showed little difference from parental virus. VZV unable to express ORF47 was even more impaired for spread from umbilical cord blood cells to melanoma cells in vitro. Early-passage clinical isolates of VZV infected T cells at a similar rate to the Oka vaccine strain; however, the clinical isolates were more efficient in spreading from infected T cells to melanoma cells. This in vitro system for infecting human T cells with VZV should be useful for identifying cellular and viral proteins that are important for virus replication in T cells and for the spread of virus from T cells to other cells.

Application of long PCR method of identification of variations in nucleotide sequences among varicella-zoster virus isolates

Restriction fragment length polymorphism (RFLP) analysis of whole viral DNA of varicella-zoster virus (VZV) requires the time-consuming and laborious preparation of a large amount of purified viral DNA. RFLP analysis of small DNA fragments amplified by PCR was developed as an alternative method. However, its use has been limited because of the small number of variations in VZV. To overcome these drawbacks and to identify variations in VZV, we developed an RFLP analysis method combined with the long PCR method which has recently been developed for the amplification of DNA fragments between 5 and 35 kb in length. We amplified three DNA regions ranging from 6.8 to 11.4 kb and demonstrated that RFLP analyses of these regions allowed for the classification of 40 VZV isolates in Japan into 17 groups. One-fourth of the isolates contained a nucleotide difference of C versus T, which abolished the StyI site at position 76530; this alteration was linked to the reported PstI site polymorphism at position 69349 (nucleotide positions are based on those of strain Dumas). Nucleotide sequence variation in the examined regions among VZV isolates in Japan was estimated at roughly less than 0.05%, confirming the previously proposed idea that VZV is genetically stable and not highly diversified. Our method will be useful for studies of the molecular epidemiology of VZV.

Live attenuated varicella vaccine

Varicella-zoster virus (VZV) is a ubiquitous human pathogen that causes varicella, commonly called chicken pox; establishes latency; and reactivates as herpes zoster, referred to as shingles. A live attenuated varicella vaccine, derived from the Oka strain of VZV has clinical efficacy for the prevention of varicella. The vaccine induces persistent immunity to VZV in healthy children and adults. Immunization against VZV also has the potential to lower the risk of reactivation of latent virus. The varicella vaccine may eventually reduce or eliminate herpes zoster, which is a serious problem for elderly and immunocompromised individuals.

The varicella vaccine. Clinical trials in immunocompromised individuals

A review of the use of live attenuated varicella vaccine in immunocompromised children, particularly those with underlying leukemia in remission, is presented. Data concerning safety, immunogenicity, and efficacy of this vaccine in high-risk children are reviewed. The unique contributions toward our understanding of varicella vaccine, including spread of vaccine-type virus, incidence of zoster, and immune correlates provided by studies of immunocompromised patients are discussed. The importance of protecting high-risk children against severe varicella by the use of varicella vaccine is apparent.

Varicella-zoster virus. The virus

Varicella-zoster virus (VZV) causes chickenpox and herpes zoster. After acute infection the virus becomes latent in dorsal root and trigeminal ganglia for the lifetime of the individual. The viral genome encodes about 70 proteins, at least three of which are thought to be expressed during latency in humans. VZV grows in cell culture, but is very cell-associated; it is relatively difficult to obtain high titers of cell-free virus.

The varicella vaccine. Prevention of herpes zoster

The live attenuated varicella vaccine offers some hope that the frequency or severity of herpes zoster might be reduced. Universal immunization with this vaccine should result in less latent varicella-zoster virus in dorsal root and cranial nerve ganglia than that which occurs following varicella. Moreover, the vaccine virus is not well adapted for growth in human cells at normal body temperature. Thus, reduced virus for reactivation, and less robust replication, may lessen the problem of herpes zoster in vaccinees. For those individuals who have already had varicella, the risk of herpes zoster is closely related to the loss of varicella-zoster virus cell-mediated immune responses, which decline with aging (or immune suppression). In aging individuals these immune responses can be enhanced by booster immunization with the varicella vaccine, suggesting that a vaccine to prevent herpes zoster is feasible.

The varicella vaccine. Vaccine development

In this article, rationales and method of development of attenuated live varicella (Oka) vaccine are described, with biologic and biophysical characteristics of the vaccine virus. The results of early clinical trials in Japan are also described, along with the results of detection of viremia in vaccinees and a follow-up of incidence of zoster in acute leukemic children, which indicate possible immunopathogenesis of varicella and zoster.

Identification of varicella-zoster virus strains by PCR analysis of three repeat elements and a PstI-site-less region

We established a method of identifying varicella-zoster virus (VZV) strains, especially those of the Oka vaccine, in patients with clinical VZV infections. The DNAs of 30 clinically isolated strains and 4 laboratory strains including the Oka vaccine strain and its parent VZV strain, were analyzed by PCR with four sets of primers for the four variable regions, R2, R4, R5, and a region without a PstI site (PS). R4 was unstable in four laboratory VZV strains and was excluded from the study. The other regions were stable in several passages in cell culture. The number of copies in R2 and R5 were distributed from 2 to 13 and from 1 to 3, respectively, in the strains analyzed. The vaccine strain had seven copies in R2 and two copies in R5, and it was PS negative. Among 30 clinical isolates, 3, 23, and 11 strains had the same characteristics as the vaccine strain in R2, R5, and PS, respectively. Therefore, by this method, 97.2% of the isolates were distinguished from the Oka vaccine strain. This strategy will be useful in diagnosing VZV infections induced by the vaccine strain.

Infection of human fetal dorsal root neurons with wild type varicella virus and the Oka strain varicella vaccine

The relative ability of a varicella-zoster virus (VZV) clinical isolate and a live attenuated VZV vaccine strain (Oka) to infect human neurons was determined in vitro. VZV infection of neurons prepared in culture from dorsal root ganglia of fetuses was assessed using an infectious center assay. Cultures were infected with 50-5,000 pfu of either VZV and assayed at either 24 or 48 hours post-VZV infection. Cultures infected with the clinical VZV isolate had seven-fold more infected neurons than cultures infected with the vaccine strain VZV.

Endonuclease analyses of DNA of human herpesvirus-6 isolated from blood before and after bone marrow transplantation

Three strains of human herpesvirus-6 (HHV-6) were isolated from peripheral blood mononuclear cells of a leukemic child with the antibody of HHV-6 before and after bone marrow transplantation (BMT); two strains were obtained before BMT and one after BMT. The DNA extracted from the three isolates was analyzed by six different restriction endonucleases. The cleavage profiles of two strains obtained before BMT were different, but the third strain isolated after BMT was identical with one of the two, which suggest reactivation of HHV-6 from the recipient's own body after BMT and possible mutation or super-infection of the virus in an immunocompromised patient.

Current status and prospects of live varicella vaccine

Since its development in 1974 the Oka strain live attenuated varicella vaccine has been tested in healthy and immunocompromised adults and children. Its safety and efficacy have been established and it is now licensed for general use in Japan and Korea, and for immunocompromised patients in several other countries. Possibilities for the future include its use to prevent zoster in the elderly, its incorporation in a multivalent vaccine and its use as a vehicle to express foreign genes in recombinant vaccines.

Differentiation of oka varicella vaccine strain from wild varicella-zoster virus strains isolated from vaccinees and household contact

The Oka varicella vaccine strain can be differentiated from wild-type strains by its unique restriction endonuclease fingerprinting (REFP: HpaI-K and EcoRI-P) pattern of the gpV-coding region of the varicella-zoster virus (VZV) genome. VZV-DNAs from patients with complicated clinical courses related to vaccination were examined to determine whether they were vaccine-derived or wild-type. A virus was isolated from a one year-old boy with acute lymphocytic leukemia (ALL) who developed typical varicella 28 days after vaccination (case A). Another virus was isolated from a four-year-old boy without clinical symptoms following household contact with varicella patients at the age of two months, and he developed zoster 14 months after vaccination (case B). Also, two strains (OK1 and OK2) were isolated from household contacts (mother and sister) with a vaccine with ALL in Oklahoma who developed varicella 18 days after vaccination (case C). In case C, BgII-REFP did not determine conclusively whether the two strains (OK1 and OK2) were vaccine-derived or wild-type because the patterns obtained were different from both the Oka varicella vaccine strain and American wild-type strains [Gelb et al., Journal of Infectious Diseases, 155:633-640, 1987]. All VZV strains examined in the present study were identified as wild-type by our method using HpaI-K and EcoRI-P fragments as marker fragments. Thus it is becoming evident that REFP using HpaI and EcoRI endonucleases is a convenient and reliable means of distinguishing between the Oka vaccine virus strain and wild-type viruses isolated from individuals developing vesicular rashes shortly and long after varicella vaccination.

A rapid and simplified micromethod for subtyping varicella-zoster virus

A simplified and rapid micromethod based on restriction endonuclease analysis of radiolabelled varicella-zoster virus (VZV) DNA is described and applied to strains for comparison. This procedure is cheaper and less time consuming than that requiring viral nucleocapsid purification (macromethod). The micromethod is suitable for routine DNA analysis of VZV isolates, allows differentiation of vaccine strain from wild strains, and provides evidence for variability of wild strains.

Strain variation of R5 direct repeats in the right-hand portion of the long unique segment of varicella-zoster virus DNA

We located a region of interstrain size variability in a short segment in an area at the right-hand end of the long unique sequence of the varicella-zoster viral genome. Varicella-zoster virus strains isolated in a district of Japan were classified into three groups on the basis of the size of this segment. Sequence comparison of the variable segment among strains from different groups revealed that the tandem direct repeat, R5, in the segment was variable among strains. R5, which was first discovered in a European strain (Dumas), contained a direct duplication of 88-base-pair (bp) elements separated by a 24-bp element (A.J. Davison and J.E. Scott, J. Gen. Virol. 67:1759-1816, 1986). We found that one 88-bp element and one 24-bp element constitute a repeating unit whose copy number varied from one to three among strains. The simplest R5 we detected was similar to that of Dumas, but there were a few base mismatches between these two R5 structures.

The risk of zoster after varicella vaccination in children with leukemia

We examined the incidence of zoster in 346 children with underlying acute lymphoblastic leukemia who were immunized with live attenuated varicella vaccine while in remission. We also compared a subset of 84 of these children with a matched group of 84 children with leukemia who had had natural infection with varicella. Of the 346 vaccinated children, 5 (1.45 percent) became infected with zoster after 10,878 months of observation, for an incidence of 0.552 case per 100 person-years. Among the matched pairs of subjects, zoster occurred in 3 (3.6 percent) of the 84 vaccinated subjects during 2936 months of observation--an incidence of 1.23 cases per 100 person-years--and in 11 (13.1 percent) of the subjects with natural infection during 4245 months--an incidence of 3.11 cases of zoster per 100 person-years. Although the incidence of zoster was more than twice as high in the control children as in the vaccinated children (3.11 vs. 1.23 cases per 100 person-years), a Kaplan-Meier product-limit analysis revealed no significant differences in incidence between the two groups. Children from both groups in whom leukemia recurred were more likely to contract zoster than those who did not have a recurrence (7 of 35 vs. 7 of 133, P less than 0.025). Zoster was not a marker for impending relapse. No case of zoster was severe or disseminated. We conclude that the incidence of zoster following immunization with live attenuated varicella vaccine is no greater than that following natural varicella infection.

Variation of R1 repeated sequence present in open reading frame 11 of varicella-zoster virus strains

We molecularly cloned the tandem direct reiteration (R1) present in open reading frame (ORF) 11 from three independent strains of varicella-zoster virus. Comparison of the R1 sequences among varicella-zoster virus strains revealed that, although the portion of R1 near the 5' terminus of ORF 11 was conserved among strains, the 3'-terminal portion varied remarkably. This variation was due to the different arrangement of two elements (A and B) and a segment produced by fusion of A and B and to a single-base change in the A element. Since the difference in the size of R1 among strains was a multiple of 3 base pairs, the variation in R1 caused no frame shift in ORF 11.

Recombination in tissue culture between varicella-zoster virus strains

Several clinical varicella-zoster virus isolates obtained during testing of a live varicella vaccine had DNA restriction fragment patterns resembling neither vaccine nor wild-type virus [Gelb et al., J Infect. Dis. 155, 633-640, 1987]. One explanation for these isolates was recombination in vivo. To determine if such recombination is likely, two strains of varicella-zoster virus, distinguishable by restriction endonuclease fragment size differences (wild-type strain EF and the OKA vaccine strain), were grown together in tissue culture. After three passages, the mixed infection virus was plaque-purified. DNA from about 13% of the plaque-purified isolates had one or more BglI fragments found in neither parental virus. Hybridization studies showed that isolates containing one of the new BglI fragments were recombinants of the two parental strains. The BglI restriction fragment pattern of these recombinants resembled those of the unusual varicella isolates from individuals either vaccinated with the live attenuated OKA varicella vaccine and later exposed to natural varicella, or simultaneously exposed to both a recent recipient of the vaccine and natural varicella.

A simplified plaque assay for varicella vaccine

A simple and accurate plaque assay is described for potency testing of attenuated varicella vaccine. Assays were performed on the African green monkey kidney continuous cell line CV-1, in multidish-plates, under a semi-solid carboxymethylcellulose overlay. The test is economical and yields accurate individual titre estimates, the reliability of which may be assessed by parallel titration of reference preparations.

Isomerization of the UL region of varicella-zoster virus DNA

Varicella-zoster virus (VZV) DNA exists principally as two isomers. Despite the presence of inverted repeats bounding the long sequence region, the unique long sequence, UL, is found in one (prototype) orientation in 95-98% of VZV DNA molecules and in the inverted orientation in only 2-5% of the molecules. In searching for an explanation for this disparity, we superinfected VZV-infected cells with herpes simplex virus type 1 or pseudorabies virus. Neither superinfecting virus produced a measurable change in the frequency of isomerization of the VZV DNA long sequence region.

Immunisation of nurses with a live varicella vaccine

34 nurses who had no previous history of chickenpox and were seronegative to varicella zoster virus (VZV) were immunised with a live attenuated varicella vaccine (OKA-RIT strain) and followed up for up to 36 months. No major vaccine reactions were observed. At 5 months and at 1 year, 94% of the nurses had seroconverted but at 3 years only 64% had detectable antibody. Lymphocyte transformation to VZV antigen tested in seven nurses in whom antibody was no longer detectable gave positive stimulation indices. 2 of 13 vaccinated nurses who looked after children with chickenpox became infected but 1 of them had not seroconverted after vaccination. Chickenpox developed in 6 out of 7 unvaccinated seronegative nurses who looked after children with chickenpox. This study shows that the OKA-RIT strain VZV vaccine is safe and immunogenic in adults and may therefore be considered for use in susceptible health care workers. The investigations also show that both cell-mediated and antibody tests are needed for long-term assessment of immunity to chickenpox after vaccination.