Bacterial shedding and serologic responses following an outbreak of Salmonella Typhi in an endemic cohort

Background: Salmonella enterica serovar Typhi (S. Typhi), the causative agent of Typhoid fever, is transmitted faecal-orally. Some typhoid sufferers shed S. Typhi beyond convalescence, but culturing stool following every case is impractical. Here we hypothesised that serology might direct testing and identify shedding after a typhoid outbreak. Methodology/Principle Findings: In 2016 there was a typhoid outbreak in a Nursing School in Malosa, Malawi. We collected serum three and six-months post-outbreak. We measured IgG antibody titres against Vi capsular polysaccharide (anti-Vi IgG) and IgM / IgG antibodies against H:d flagellin (anti-H:d). We screened faecal samples from participants with high and low anti-Vi IgG (measured at visit one) by culture and PCR. Participants reported whether they had persistent fever for ≥ three days (in keeping with World Health Organization definitions for typhoid) during the outbreak. We tested for environmental S. Typhi. 368 people provided serum at 3-months, of whom 320 provided serum at 6-months; 49 participants provided a faecal sample (25 from the highest and 24 from the lowest deciles for anti-Vi IgG titre). We did not grow S. Typhi from faeces, but one sample produced a positive PCR amplification for S. Typhi. Median anti-Vi IgG titre fell amongst participants with persistent fever (8.08 to 3.7 EU/ml, <0.000001, Wilcoxon signed rank). Median anti-H:d IgG titres fell in those with and without persistent fever (87.8 to 77.4 EU/, p = <0.000001 and 82.4 to 79.2 EU/ml, p = 0.0002, Wilcoxon signed rank, respectively). Anti-H:d IgM titres did not change significantly. Non-Typhoidal Salmonellae were identified in water sampled at source and a kitchen tap. Conclusions / Significance: We did not identify culture-confirmed shedding through sero-surveillance. Serologic trends signify a fall from an outbreak-associated peak. Despite effective vaccines, identifying ways to detect and treat shedding remain vital to break transmission and eliminate typhoid.

Rotavirus spike protein ΔVP8* as a novel carrier protein for conjugate vaccine platform with demonstrated antigenic potential for use as bivalent vaccine

Conjugate vaccine platform is a promising strategy to overcome the poor immunogenicity of bacterial polysaccharide antigens in infants and children. A carrier protein in conjugate vaccines works not only as an immune stimulator to polysaccharide, but also as an immunogen; with the latter generally not considered as a measured outcome in real world. Here, we probed the potential of a conjugate vaccine platform to induce enhanced immunogenicity of a truncated rotavirus spike protein ΔVP8*. ΔVP8* was covalently conjugated to Vi capsular polysaccharide (Vi) of Salmonella Typhi to develop a bivalent vaccine, termed Vi-ΔVP8*. Our results demonstrated that the Vi-ΔVP8* vaccine can induce specific immune responses against both antigens in immunized mice. The conjugate vaccine elicits high antibody titers and functional antibodies against S. Typhi and Rotavirus (RV) when compared to immunization with a single antigen. Together, these results indicate that Vi-ΔVP8* is a potent and immunogenic vaccine candidate, thus strengthening the potential of conjugate vaccine platform with enhanced immune responses to carrier protein, including ΔVP8*.

Evidence of Extended Thermo-Stability of Typhoid Polysaccharide Conjugate Vaccines

Typhoid conjugate vaccines (TCV) are effective in preventing enteric fever caused by Salmonella enterica serovar Typhi in Southeast Asia and Africa. To facilitate vaccination with the Vi capsular polysaccharide–tetanus toxoid conjugate vaccine, Typbar TCV, and allow it to be transported and stored outside a cold chain just prior to administration, an extended controlled-temperature conditions (ECTC) study was performed to confirm the quality of the vaccine at 40 °C for 3 days at the end of its shelf-life (36 months at 2–8 °C). Studies performed in parallel by the vaccine manufacturer, Bharat Biotech International Limited, and an independent national control laboratory (NIBSC) monitored its stability-indicating parameters: O-acetylation of the Vi polysaccharide, integrity of the polysaccharide–protein conjugate, and its molecular size and pH. ECTC samples stored at 40 °C and 45 °C in comparison with control samples stored at 4 °C and 55 or 56 °C, were shown to have stable O-acetylation and pH; only very slight increases in the percentage of free saccharide and corresponding decreases in molecular size were observed. The deoxycholate method for precipitating conjugated polysaccharide was very sensitive to small incremental increases in percentage of free saccharide, in line with storage temperature and duration. This extended ECTC study demonstrated minimal structural changes to the Vi polysaccharide and conjugate vaccine and a stable formulation following extended exposure to elevated temperatures for the desired durations. This outcome supports the manufacturer’s ECTC claim for the vaccine to be allowed to be taken outside the cold chain before its administration.

Different Types of Vaccines

Vaccines can be classified according to their nature into the following types: ● Inactivated vaccines: ■ Whole organism; ■ Acellular extracts. ● Live attenuated vaccines. ● Toxoid vaccines. ● Subunit vaccines. ● Conjugate vaccines. ● DNA vaccines. ● Recombinant vector vaccines. Inactivation of the whole organism is the most basic form of vaccine produced by killing the micro-organism causing the disease using heat, chemical or radiation and presents all the antigens in the inactivated organism as a vaccine to induce immunity in the recipient. Other methods to produce an inactivated vaccine is by extracting acellular components of the organism through filtration. Examples of inactivated bacterial vaccines currently in use include: ● Anthrax—sterile filtrate from cultures of the Sterne strain of B. anthracis. ● Cholera—oral inactivated vaccine with 1mg of recombinant cholera toxin B (rCTB) in a liquid suspension of four strains of killed V. cholerae O1, representing subtypes Inaba and Ogawa and biotypes El Tor and classical. ● Pertussis—acellular vaccine has replaced previously used whole cell vaccine. ● Typhoid—purified Vi capsular polysaccharide from S. typhi; NB: the injectable, killed, whole-cell typhoid vaccine which contains heat-inactivated, phenol-preserved S. typhi organisms is no longer in use in the UK. Examples of inactivated viral vaccines currently in use in the UK include: ● Hepatitis A virus. ● Hepatitis E virus. ● Influenza A and B viruses. ● Japanese encephalitis virus. ● Polio viruses 1, 2, and 3 (IPV). ● Rabies virus. ● Tick-borne encephalitis virus. ● Bacterial vaccines: Bacillus Calmette-Guerin (BCG) vaccine is a live attenuated vaccine against tuberculosis derived from a Mycobacterium bovis strain. The oral typhoid vaccine contains a live attenuated strain of S. typhi (Ty21a) in an enteric-coated capsule. ● Viral vaccines: The measles, mumps, and rubella (MMR) vaccine contain live attenuated strains of measles, mumps, and rubella viruses, which are cultured separately and mixed before being lyophilized. Oral polio vaccine (OPV) against polio viruses 1, 2, and 3—OPV contains live attenuated strains of poliomyelitis virus types 1, 2, and 3 grown in cell cultures.

Whole-Genome Sequences of Two Salmonella enterica Serovar Dublin Strains That Harbor the viaA, viaB, and ompB Loci of the Vi Antigen

Here, we report the genome sequences of two Salmonella enterica serovar Dublin strains, 03EB8736SAL and 03EB8994SAL, isolated from raw-milk cheese and milk filtrate, respectively. Analysis of the draft genomes of the two isolates reveals the presence of the viaA, viaB, and ompB loci of the Vi capsular polysaccharide antigen (Vi antigen).