Persistence of antibody titres three years after vaccination with Vi polysaccharide vaccine against typhoid fever

S. Typhi derived vaccines and a proposal for outer membrane vesicles (OMVs) as potential vaccine for typhoid fever

Typhoid fever is a serious systemic infection caused by Salmonella Typhi (S. Typhi), spread by the feco-oral route and closely associated with poor food hygiene and inadequate sanitation. Nearly 93% of S. Typhi strains have acquired antibiotic resistance against most antibiotics. Vaccination is the only promising way to prevent typhoid fever. This review covers the nature and composition of S. Typhi, pathogenecity and mode of infection, epidemiology, and nature of drug resistance. Several components (Vi-polysaccharides, O-antigens, flagellar antigens, full length OMPs, and short peptides from OMPs) of S. Typhi have been utilized for vaccine design for protection against typhoid fever. Vaccine delivery systems also contribute to efficacy of the vaccines. In this study, we propose to develop S. Typhi derived OMVs as vaccine for protection against typhoid fevers.

The need & the issues related to new-generation typhoid conjugate vaccines in India

The low- and middle-income countries bear the highest burden of typhoid fever in the world. India, along with other South Asian countries, has a significant incidence of typhoid fever among young children though there is a paucity of published data on community burden. In spite of the availability of Vi-polysaccharide (Vi-PS) and conjugated Vi-PS vaccines, these are not adequately utilized in India and in the neighbouring countries. To address many shortcomings of the unconjugated Vi-PS vaccines, typhoid conjugate vaccines (TCVs) are developed by conjugating Vi-PS with different carrier proteins. Three such vaccines using tetanus toxoid as a carrier protein are already licensed in India. Several other Vi-PS conjugates are currently in various stages of development. The current review provides an update on the existing and upcoming new TCVs along with a detailed discussion on the various issues involved with their clinical use and limitations.

Glycoconjugate vaccines: current approaches towards faster vaccine design

Introduction: Over the last decades, glycoconjugate vaccines have been proven to be a successful strategy to prevent infectious diseases. Many diseases remain to be controlled, especially in developing countries, and emerging antibiotic-resistant bacteria present an alarming public-health threat. The increasing complexity of future vaccines, and the need to accelerate development processes have triggered the development of faster approaches to glycoconjugate vaccines design. Areas covered: This review provides an overview of recent progress in glycoconjugation technologies toward faster vaccine design. Expert opinion: Among the different emerging approaches, glycoengineering has the potential to combine glycan assembly and conjugation to carrier systems (such as proteins or outer membrane vesicles) in one step, resulting in a simplified manufacturing process and fewer analytical controls. Chemical and enzymatic strategies, and their automation can facilitate glycoepitope identification for vaccine design. Other approaches, such as the liposomal encapsulation of polysaccharides, potentially enable fast and easy combination of numerous antigens in the same formulation. Additional progress is envisaged in the near future, and some of these systems still need to be further validated in humans. In parallel, new strategies are needed to accelerate the vaccine development process, including the associated clinical trials, up to vaccine release onto the market.

Effectiveness of the typhoid Vi vaccine in overseas travelers from England

BACKGROUND

Approximately 500 cases of enteric fever, caused by Salmonella enterica serovar Typhi and Paratyphi, are reported in the UK each year. The majority are associated with travel to the Indian subcontinent. The typhoid Vi vaccine protects against S. Typhi and is available to travelers from their general practice or private clinics. The effectiveness of this vaccine has been assessed previously in endemic regions of the world but not in travelers.

METHODS

Data from the enhanced surveillance scheme concerning persons in England aged ≥2 years who traveled from the UK and contracted culture-confirmed enteric fever were used to calculate the effectiveness of the vaccine in travelers. A "case-case" case-control design was used, in which patients with typhoid comprised the "cases" and those with paratyphoid acted as "controls."

RESULTS

The overall effectiveness of the vaccine, adjusted for age group, sex, ethnicity, birth in a typhoid-endemic country, and year (of receipt of specimen), was 65% (95% confidence interval 53%-73%). Effectiveness did not vary across subgroups of any of the factors in the model, but there was some evidence of waning effectiveness of the vaccine with increasing time since receipt (trend p = 0.05).

CONCLUSIONS

The vaccine has been demonstrated to have a similar effectiveness in travelers as that found in endemic populations. It appears to be protective in all ages, including in young children (aged 2-5 years), a finding not consistently replicated in other studies. However, good hygiene practices are necessary in addition to vaccination to prevent infection. The "case-case" case-control design provides a valuable method of calculating the effectiveness of this vaccine in travelers, given the availability of paratyphoid controls, a population with similar demographics and risk exposures.

Re-examination of immune response and estimation of anti-Vi IgG protective threshold against typhoid fever-based on the efficacy trial of Vi conjugate in young children

BACKGROUND

The capsular polysaccharide of Salmonella enterica serovar Typhi, Vi antigen, is an essential virulence factor and a protective antigen. Similar to other polysaccharide vaccines, the protective action of Vi, both to the polysaccharide alone or when presented as a conjugate, is mediated by serum IgG Vi antibodies. The evaluation of Vi capsular polysaccharide based vaccines to prevent typhoid fever would be significantly facilitated by the identification of a "protective level" of serum antibodies to Vi antigen.

METHODS

The protective level of anti-Vi IgG against typhoid fever was derived from the protective efficacy and immune response of a Vi-rEPA conjugate vaccine efficacy trial. The estimation was derived by two methods: correlation of the percent efficacy and the antibody distribution profile in the vaccine group at a given period of observation, and use of the relative ratio of anti-Vi IgG levels between the vaccine and placebo groups greater or equal to the Relative Risk of typhoid fever used in the efficacy determination.

RESULTS

Both methods predicted a similar range of a minimum protective level of anti-Vi IgG between 1.4 and 2.0μg/ml (short term threshold). When applying a protective threshold of 10μg/ml at 6 months post immunization, an IgG level in excess of 1.4μg/ml was achieved by 90% of children at 46 months post immunization, consistent with an 89% level of protection over the duration of the study. We thus suggest that the proportion of children with Vi IgG>10μg/ml (long term threshold) 6 months after immunization may reflect the proportion protected over at least a 4 year period.

CONCLUSION

The current assignment of an anti-Vi IgG protective level may be of value when evaluating vaccine performance of future Vi conjugate vaccines.

Immune responses to Vi capsular polysaccharide typhoid vaccine in children 2 to 16 years old in Karachi, Pakistan, and Kolkata, India

The geometric mean concentration (GMC) and the proportion maintaining a protective level (150 enzyme-linked immunosorbent assay (ELISA) units [ELU]/ml) 2 years following a single dose of 25 μg of injectable Vi capsular polysaccharide typhoid vaccine was measured against that of the control hepatitis A vaccine in children 2 to 16 years old in cluster randomized trials in Karachi and Kolkata. The GMC for the Vi group (1,428 ELU/ml) was statistically significantly different from the GMC of the control hepatitis A vaccine group (86 ELU/ml) after 6 weeks. A total of 117 children (95.1%) in the Vi group and 9 (7.5%) in the hepatitis A group showed a 4-fold rise in Vi IgG antibody concentrations at 6 weeks (P < 0.01). Protective antibody levels remained significantly different between the two groups at 2 years (38% in the Vi vaccine groups and 6% in the hepatitis A group [P < 0.01]). A very small proportion of younger children (2 to 5 years old) maintained protective Vi IgG antibody levels at 2 years, a result that was not statistically significantly different compared to that for the hepatitis A group (38.1% versus 10.5%). The GMCs of the Vi IgG antibody after 2 years were 133 ELU/ml for children 2 to <5 years old and 349 ELU/ml for children 5 to 16 years old. In conclusion, Vi capsular polysaccharide typhoid vaccine is immunogenic in children in settings of South Asia where typhoid is highly endemic. The antibody levels in children who received this vaccine remained higher than those in children who received the control vaccine but were significantly reduced at 2 years of follow-up.

Vaccines for typhoid fever and other salmonelloses

PURPOSE OF REVIEW

This review summarizes the recent advances in vaccination against Salmonella enterica serovar Typhi and highlights the data supporting the development of next generation vaccines to address paratyphoid fever and invasive nontyphoidal Salmonella (iNTS) disease.

RECENT FINDINGS

There has been increasing awareness of the disease burden caused by S. Typhi particularly in Africa and greater recognition of S. Paratyphi A's contribution to enteric fever episodes throughout Asia. Groups have been working to improve the existing typhoid vaccines and provide comprehensive data on the feasibility of their implementation in endemic settings. These data have resulted in modifications to the recommendations for typhoid vaccination in traveller markets and endemic settings, and has also led to the development of S. Paratyphi A vaccine components that can be combined with existing typhoid vaccines to generate bivalent formulations against enteric fever. The epidemiology of iNTS serovars as cause of appreciable morbidity and mortality in Africa, and the need for vaccines, has also become more widely appreciated.

SUMMARY

Current typhoid vaccines, although moderately effective for short periods of time, cannot be used in all age groups and only target one of the clinically relevant Salmonella serovars. Greater effort must be placed on the development and implementation of improved vaccines for the disease burden resulting from Typhi, Paratyphi A or iNTS infections.

The immunogenic characteristics associated with multivalent display of Vi polysaccharide antigen using biodegradable polymer particles

Polysaccharides in their great majority are thymus-independent (TI) antigens. Anti-polysaccharide antibody responses are generally weak and characterized by lack of memory, isotype restriction and delayed ontogeny. We report here the generation of protective memory antibody response by multivalent display of polysaccharide antigens on biodegradable polymeric particles. Single dose immunization using polylactide (PLA) polymer particles entrapping Vi capsular polysaccharide antigen from Salmonella typhi promoted isotype switching and induced polysaccharide-specific memory antibody response in experimental animals. PLA nanoparticles as well as microparticles entrapping Vi polysaccharides elicited high IgG titer in comparison to soluble Vi immunization. Immunizations with particles co-entrapping both Vi polysaccharide and tetanus toxoid did not improve the anti-polysaccharide antibody responses. Lower antibody response from co-entrapped formulation was mostly due to inhibition of particle phagocytosis by the macrophages. Immunization using polylactide particles entrapping only Vi polysaccharide with higher density on surface elicited highest secondary antibody response as well as promoted isotype switching. The vaccination potential of particle based immunizations was further confirmed by the generation of quick memory antibody responses while challenging the immunized animals with live S. typhi. This approach provides a multivalent display of polysaccharide antigen using polymer particles and elicits protective memory antibody response without conjugation to a carrier protein.

Positive antibody response to vaccination in adolescence predicts lower C-reactive protein concentration in young adulthood in the Philippines

OBJECTIVES

Inflammation has been associated with a wide range of chronic degenerative diseases, but the developmental factors contributing to the regulation of inflammation are poorly understood. This study investigates the within-individual association between antibody response to vaccination in adolescence and C-reactive protein (CRP) concentration in young adulthood.

METHODS

In 1998-99, at age 14-15 years, a subset of participants (N = 96) in the Cebu Longitudinal Health and Nutrition Survey were administered a typhoid vaccine, and baseline and follow-up blood samples were drawn to assess the strength of the antibody response to vaccination. In 2005, at age 20-21 years, blood samples were drawn from the full cohort for measurement of CRP. N = 74 individuals had complete data at both time points. Bivariate associations and multivariate logistic regression models were evaluated to test the hypothesis that vaccine responsiveness in adolescence was significantly associated with CRP level in young adulthood.

RESULTS

There was a strong and statistically significant association between antibody response to vaccination in adolescence and CRP in young adulthood. Median CRP was more than four times higher among nonresponders than responders, and nonresponders were 2.3 to 3.6 times more likely to have CRP in the top tertile of the sample distribution.

CONCLUSIONS

This study provides evidence for a prospective, within-individual link between more effective antibody-mediated immune defenses and lower levels of inflammation. In the context of prior research in this population, these results suggest that early environments are important determinants of multiple aspects of an individual's immuno-phenotype.

Revaccination with locally-produced vi typhoid polysaccharide vaccine among chinese school-aged children: safety and immunogenicity findings

OBJECTIVE

To evaluate the safety and immunogenicity of revaccination with locally-produced Vi polysaccharide vaccine 3 years after the first dose in Chinese children aged 9 to 14 years.

METHODS

A randomized, placebo-controlled trial was conducted in Suzhou, Jiangsu, China. Six hundred and sixty-seven eligible children who had previously received a primary dose of Vi vaccine were randomly assigned to receive 1 dose of 30 mug Vi vaccine or placebo. In addition, 331 eligible children received 1 dose of Vi polysaccharide vaccine as a primary vaccination. Adverse events were followed for 28 days after vaccination. Serum samples were collected from a subgroup of participants on day 0 and day 28, and Vi antibodies were analyzed using a passive hemagglutination method.

RESULTS

Revaccination was found to be safe and immunogenic. No severe adverse events were observed. A significant increase in antibody titers after vaccination was observed among children who had and had not been previously vaccinated. Twenty-eight days after injection, the seropositive rate was 79% in both revaccination and primary injection groups; the geometric mean antibody titer was 1:40 in the primary injection group and 1:29 in the revaccination group (P = 0.24). Although the difference of attained geometric mean titers in follow-up sera was not significantly different in these 2 groups, the fold-rise of these titers from baseline was significantly higher in the primary injection group than in the revaccination group (7.7 versus 3.1, P < 0.001).

CONCLUSION

We found that revaccination using the locally produced Vi polysaccharide vaccine among Chinese school-aged children was safe and increased antibody titers. Revaccination can be used to extend the duration of protection provided by Vi polysaccharide vaccine.

Outbreak of typhoid fever in vaccinated members of the French Armed Forces in the Ivory Coast

In 2001, an outbreak of typhoid fever occurred among the members of the French Armed Forces. All had received a typhoid vaccination as per the immunization schedule practiced in the Armed Forces (every 5 years). A retrospective cohort study was conducted in 94 personnel. The objectives were to confirm the diagnosis, determine the source of contamination and identify the factors associated with defective vaccinal efficacy. Twenty-four cases were clinically identified. A cucumber salad was identified as the contaminating dish (Risk Ratio = 3.6; 95%CI 1.5-8.9). Only one factor was related to defective vaccinal efficacy; the risk of typhoid fever was two-fold higher in people vaccinated more than 3 years previously (Risk Ratio = 2.2; 95%CI, 1.1-4.2). Compliance with food hygiene rules could have prevented 24 cases of typhoid fever. Nevertheless, repeat vaccination against typhoid fever is now conducted every 3 years in the French Forces, in compliance with the manufacturers' recommendations.

Evaluation of a novel Vi conjugate vaccine in a murine model of salmonellosis

Immunisation of BALB/c mice with a vaccine containing Vi polysaccharide conjugated to the Klebsiella pneumoniae outer membrane 40 kDa protein (rP40), in combination with Escherichia coli heat-labile toxin adjuvant (LT), elicited anti-Vi IgG antibodies after administration using different routes. Testing of the immune serum in opsonisation assays demonstrated the specific enhancement of Vi-positive bacterial uptake by cultured murine bone marrow derived macrophages. Intra-peritoneal challenge of mice immunised with the Vi-based vaccine elicited a degree of protection against virulent Vi+ Salmonella enterica serovar typhimurium (S. typhimurium). In contrast, Vi vaccination did not confer protection against oral challenge with virulent Vi-positive S. typhimurium or S. dublin.

Capsular polysaccharide-protein conjugate vaccines and intravenous immunoglobulins

Capsular polysaccharides (CPs), present on the surface of most pathogenic bacteria, have been recognised as virulence factors. Antibodies specific to these polysaccharides can mediate the killing of these bacteria by phagocytes in the presence of complement. The conjugation of polysaccharides to carrier proteins enhances their immunogenicity and renders the immune response T-cell dependent. The currently licensed capsular polysaccharide vaccines and polysaccharide-protein conjugate vaccines under development for the prevention of bacterial infections will be discussed in this review. Use of these vaccines for active vaccination and for the vaccination of healthy plasma donors to produce hyperimmune iv. immunoglobulins for the passive immunisation of appropriate patient populations is also discussed.

Protective efficacy of IgA monoclonal antibodies to O and H antigens in a mouse model of intranasal challenge with Salmonella enterica serotype Enteritidis

Protective properties of immunoglobulin A (IgA) monoclonal antibodies (MAbs) directed against O and H antigens of Salmonella enterica serotype Enteritidis (S. enteritidis) were evaluated in a model of generalized infection after intranasal (i.n.) inoculation of BALB/c mice. Passive i.n. instillation of antibodies 1 h before i.n. challenge did not prevent infection, and mice developed rapid inflammatory response in the lower respiratory tract. The passive systemic immunization was partially protective and a single intravenous (i.v.) injection of both O and H antigen specific IgA antibodies prolonged survival period of the infected animals. Permanent secretion of O:9 specific IgA MAb 177E6 into the respiratory tract in a "backpack" tumor model protected 50% of animals infected i.n. with a high dose of virulent S. enteritidis strain. Thus, secretory IgA (S-IgA) directed against O:9 antigen alone can prevent bacterial invasion in the respiratory epithelium.

Travel vaccines and elderly persons: review of vaccines available in the United States

Aging is associated with alterations in immune responses and may lead to clinically significant changes in the safety, immunogenicity, and protective efficacy of certain vaccines. This review summarizes published data regarding the effects of age on responses after immunization with vaccines generally administered before travel. The specific vaccines discussed in detail include hepatitis A, typhoid, yellow fever, Japanese encephalitis, and rabies vaccines. There is some evidence of diminished serological responses to hepatitis A and rabies vaccines in older individuals. In addition, increased toxic effects following yellow fever vaccination in elderly recipients have recently been reported. However, many travel-related vaccines have never been studied specifically in elderly populations. Consideration of potential age-related differences in responses to travel vaccines is becoming increasingly important as elderly persons more frequently venture to exotic destinations.

Prenatal undernutrition, postnatal environments, and antibody response to vaccination in adolescence

BACKGROUND

Recently, researchers have considered the fetal and infant origins of several adult cardiovascular and metabolic diseases, but the implications of early events for immune function and infectious disease are unclear.

OBJECTIVE

We investigated the association between prenatal undernutrition and immunocompetence in adolescence and hypothesized that intrauterine growth retardation is associated with a lower likelihood of mounting an adequate antibody response later in life.

DESIGN

A subsample of one hundred three 14-15-y-olds was recruited from an ongoing longitudinal study in which data collection began while participants were in utero. A typhoid vaccine was given, and anti-typhoid antibodies were measured 2 wk and 3 mo later as a functional marker of immunocompetence. The likelihood of mounting an adequate antibody response was compared for adolescents who were small for gestational age or appropriate for gestational age at birth while controlling for a range of postnatal exposures.

RESULTS

The predicted probability of mounting a positive antibody response for adolescents who were prenatally and currently undernourished was 0.32, compared with probabilities of 0.49-0.70 for adequately nourished adolescents (P = 0.023). Diarrhea in the first year of life (P = 0.009) and fast weight gain during the first 6 mo (P = 0.003) were also associated with a higher probability of response.

CONCLUSIONS

These findings extend the concept of fetal and early infant programming of adult diseases to the immune system and suggest that early environments may have long-term implications for immunocompetence and infectious disease risk, particularly in developing countries.

Duration of efficacy of Ty21a, attenuated Salmonella typhi live oral vaccine

Currently, two different formulations of Ty21a live oral typhoid vaccine are commercialized. The enteric-coated capsule formulation was licensed based on results of three years of follow-up of a randomized, placebo-controlled, double-blind field trial in Area Occidente, Santiago, Chile, which demonstrated that three doses of this formulation, given on an every other day immunization schedule, conferred the best protection among several options evaluated. Subsequently, a liquid formulation (lyophilized vaccine organisms reconstituted with buffer and water into a vaccine cocktail) was commercialized after it was shown to provide superior protection than enteric-coated capsules over three years of follow-up in a randomized, placebo-controlled field trial in Area Sur Oriente and Area Norte, Santiago. Surveillance in the Area Occidente trial was continued for four additional years (i.e., total seven years of follow-up) and in the Area Sur Oriente/Area Norte trial for two additional years (i.e., a total of five years of follow-up). These additional surveillance data, which were analyzed to ascertain the longevity of protection conferred by these formulations of Ty21a, revealed that three doses of Ty21a in enteric-coated capsules (every other day schedule) conferred 67% protection over three years and 62% protection over seven years of follow-up, whereas three doses of liquid formulation (every other day schedule) elicited 77% protection over three years and 78% over five years of follow-up. Based on its excellent clinical acceptability, ease of oral administration, proven practicality in school-based mass immunization, and long-term efficacy enduring at least seven years, it is proposed that school-based immunization with Ty21a be utilized as a control measure in areas where the incidence of typhoid fever is high and Salmonella typhi are antibiotic-resistant.

Clinical immunogenicity and tolerance studies of liquid vaccines delivered by jet-injector and a new single-use cartridge (Imule): comparison with standard syringe injection. Imule Investigators Group

A new needleless jet-injector, Mini-Imojet, was developed that administers liquid vaccines from a single-use, pre-filled cartridge named Imule, which avoids the risk of cross-contamination. We conducted clinical trials in several settings in France and West Africa to compare the immunogenicity and tolerance of five vaccines (influenza vaccine, Vi capsular polysaccharide typhoid vaccine, tetanus toxoid vaccine, diphtheria-tetanus-whole cell pertussis vaccine, and inactivated hepatitis A vaccine) administered with the Imule system vs standard syringe technique. In each vaccine study, all subjects of either group were tested for serum antibody titres to calculate the geometrical mean titres and seroconversion rates after complete vaccination. Immediate local-reactions were noted after each injection, and local and general reactions were evaluated during a predetermined period of follow-up. When delivered by the Imule technique, all the administered vaccines were of equivalent or superior immunogenicity, compared to the syringe technique. The tolerance to vaccines injected by the Imule system was acceptable in all studies. The most frequently observed reactions were mild (e.g. minor bleeding, superficial papules, erythema and induration) and could be considered to be inherent to the injection technique. The technical and safety advantages of the Mini-Imojet/Imule system, compared to sterilizable, standard disposable or autodestruct syringes and to classical multi-dose vial jet-injectors, reinforces the interest of this new injection technique for collective immunizations.

Immunogenicity, efficacy and serological correlate of protection of Salmonella typhi Vi capsular polysaccharide vaccine three years after immunization

The protective efficacy and immunogenicity of Vi capsular polysaccharide vaccine against typhoid fever was measured 3 years after its administration in a double-blind randomized trial. Vaccine efficacy was not significantly different during each year of the trial and was 55% (95% CI: 30-71%) over the 3 year period. In a case-control study at 3 years after vaccination, recipients of Vi had higher levels of Vi antibodies than controls, as measured by radio-immunoassay (GMT 1.28 vs 0.76 microgram ml-1, P = 0.0004) and by passive haemagglutination assay (GMT 10.46 vs 3.52, P = 0.0001). The serological correlate of protection has been estimated using the relative risks of typhoid fever in the 2 groups and the relative ratio of antibody levels. The estimated protective level is 1 microgram ml-1 suggesting that at a mean age of 9 years, 64% of vaccinates and 40% of controls had protective antibody against typhoid fever in this endemic area.

Clinical and serological responses following primary and booster immunization with Salmonella typhi Vi capsular polysaccharide vaccines

Clinical and serum antibody responses following intramuscular injection of two formulations of Salmonella typhi Vi capsular polysaccharide (Vi) were assessed in a double-blind evaluation. Healthy adults were randomly assigned to receive a 25 micrograms dose of liquid (Vi-Liq; n = 182) or freeze-dried Vi vaccine (Vi-Lyoph; n = 55), or placebo (n = 86). Erythema and/or induration > or = 1 cm in diameter at the injection site developed in 13/182 (7%) of Vi-Liq and 3/55 (5%) of Vi-Lyoph recipients (not significant, n.s.). Fever (oral temperature > or = 100 degrees F (37.8 degrees C)) occurred in < 2% of vaccinees. The frequencies of rises of fourfold or greater and of maximal Vi antibody levels were similar in the two vaccine groups. Fourfold or greater rises in serum Vi antibody levels (RIA) developed in 53% of Vi-Lyoph and 60% of Vi-Liq recipients by 1 week (n.s.), and 98 and 93%, respectively, by 1 month (n.s.). The frequencies of adverse reactions and mean Vi antibody levels following booster immunization with Vi-Liq 27 to 34 months after primary immunization (n = 55) were similar to those observed following primary immunization, although subjects given a booster dose were more likely to develop local reactions > or = 1 cm in diameter than those given a first dose (10/55 versus 13/182, p = 0.013 by the chi 2 test). Primary and booster immunizations with the Vi vaccines are well tolerated in healthy adults; mean Vi antibody levels remain significantly elevated for up to 34 months after primary immunization.

Experience with Vi typhoid capsular polysaccharide vaccine in the U.K

Adult volunteers were immunised with a single dose of typhoid Vi capsular polysaccharide vaccine. After immunisation, 96% of the 103 subjects seroconverted and 94% had antibody levels above the protective threshold. Systemic reactions were uncommon, local reactions were mild and transient.

Typhoid Vi: a less reactogenic vaccine

A typhoid vaccine derived from the purified Vi capsular polysaccharide (CPS) antigen of Salmonella typhi was compared with a heat-killed whole-cell typhoid vaccine in 637 healthy male volunteers. The individuals were placed in three groups: group 1 received two doses of heat-killed whole-cell typhoid vaccine, at an interval of 28 days; group 2 received a single dose of typhoid Vi CPS vaccine followed after 28 days by water for injection; and group 3 received water for injection on the first occasion and a single dose of typhoid Vi CPS vaccine 28 days later. Local and systemic adverse reactions were recorded for 5 days following each injection. Subjects receiving the typhoid Vi CPS vaccine complained of fewer local adverse reactions on each of the first 3 days following immunization: on day 1, 18.6% of subjects given typhoid Vi CPS vaccine reported local reactions compared with 59.7% of those receiving heat-killed whole-cell vaccine (P less than 0.001). The percentage of subjects receiving the heat-killed whole-cell vaccine who complained of systemic reactions was more than twice that of subjects receiving the Vi CPS vaccine (7.9% and 3.4%, respectively, on day 1; P less than 0.01).

Safety and immunogenicity of combined rabies and typhoid fever immunization

The prevalence of rabies and typhoid fever in many developing countries poses a serious health hazard to travellers. The development of a combined immunization schedule would be advantageous. A study was performed on 104 adult volunteers using purified Vero cell rabies vaccine and Typhim Vi, a purified capsular polysaccharide, either separately or in combination. No significant difference was observed in immunogenicity or tolerance between the two groups. A 3-year follow-up study is planned.