Specific and Cross-reactive Plasmablast Response in Humans after Primary and Secondary Immunization with Vi Capsular Polysaccharide Typhoid Vaccine.

Secondary immunization with polysaccharide vaccines may imply a risk of hyporesponsiveness. Despite the wide use of typhoid Vi capsular polysaccharide vaccine, its potential tendency to hyporesponsiveness has been inadequately addressed. While previous studies have explored serum antibody responses, we applied a more sensitive approach, a single-cell assay for circulating plasmablasts, to compare primary and secondary responses. Twelve subjects received primary and booster doses of the Vi vaccine (Typherix® ) at 30- to 37-month intervals. Plasmablasts specific to the Vi or typhoidal O antigens or cross-reactive with paratyphoid and non-typhoidal Salmonella strains were identified as antibody-secreting cells (ASC) with ELISPOT. Before vaccinations, none had plasmablasts specific to the antigens tested. Twelve of 12 subjects showed a Vi-specific response after primary, but only eight of 12 after booster vaccination. All responded to typhoidal O-9,12 antigen after both immunizations. The geometric mean of plasmablasts specific to the Vi antigen was 59 (95% CI 24-119) and 1 (0-54) IgA + IgG + IgM-ASC/106 peripheral blood mononuclear cell (PBMC) after primary and booster immunizations, respectively, and 20 (9-49) and 56 (29-103) to the O-9,12 antigen. We detected 1 (0-28) and 17 (6-36) ASC/106 PBMC cross-reactive with Salmonella Paratyphi A; 3 (0-30) and 22 (8-48) with S. Paratyphi B; 3 (0-29) and 18 (7-47) with S. Paratyphi C; 19 (10-34) and 51 (26-94) with Salmonella Enteritidis; and 1 (0-35) and 23 (9-52) with Salmonella Typhimurium, respectively. One-third of the vaccinees, although responding to the O-9,12 antigen, failed to respond to the Vi antigen after booster immunization, suggesting hyporesponsiveness in part of the vaccinees. The findings warrant further investigation.

Illness and injury of travellers abroad: Finnish nationwide data from 2010 to 2012, with incidences in various regions of the world.

The number of international tourist arrivals reached 1,000 million in 2012. Assessment of travellers' health problems has relied on proportionate morbidity data.Given the lack of data on number of visitors to each region, incidences have been impossible to calculate.This study, largest yet reporting travellers' health problems, is the first to present incidence of illness and injury. Data on Finnish travellers with health problems abroad during 2010 to 2012 were retrieved from the database of an assistance organisation,SOS International, covering 95% of those requiring aid abroad. The numbers were compared with those of Finnish travellers in the database of the Official Statistics of Finland. The SOS International database included 50,710 cases: infections constituted the most common health problem (60%), followed by injuries(14%), diseases of skin (5%), musculoskeletal system and connective tissue (5%), digestive tract (3%),and vascular system (2%). Gastroenteritis (23%) and respiratory infections (21%) proved the most frequent diagnoses. Overall incidence of illness or injury was high in Africa (97.9/100,000 travel days; 95% Bayesian credible interval (BCI): 53.1­145.5), southern Europe plus the eastern Mediterranean (92.3; 95% BCI: 75.4­110.1) and Asia (65.0; 95% BCI: 41.5­87.9). The data show significant differences between geographical regions, indicating the main risks and thus providing destination-specific tools for travelers' healthcare.

Cross-reactive immune response induced by the Vi capsular polysaccharide typhoid vaccine against Salmonella Paratyphi strains.

There are no vaccines in clinical use against paratyphoid fever, caused by Salmonella Paratyphi A and B or, rarely, C. Oral Salmonella Typhi Ty21a typhoid vaccine elicits a significant cross-reactive immune response against S. Paratyphi A and B, and some reports suggest cross-protective efficacy against the disease. These findings are ascribed to the O-12 antigen shared between the strains. The Vi capsular polysaccharide vaccine has been shown to elicit antibodies reactive with O-9,12. Twenty-five volunteers immunized with the parenteral Vi vaccine (Typherix(®) ) were explored for plasmablasts cross-reactive with paratyphoid strains; the responses were compared to those in 25 age- and gender-matched volunteers immunized with Ty21a (Vivotif(®) ). Before vaccination, 48/50 vaccinees had no plasmablasts reactive with the antigens. Seven days after vaccination, 15/25 and 22/25 Vi- and Ty21a-vaccinated volunteers had circulating plasmablasts producing antibodies cross-reactive with S. Paratyphi A, 18/25 and 23/25 with S. Paratyphi B and 16/25 and 9/25 with Paratyphi C, respectively. Compared to the Ty21a group, the Vi group showed significantly lower responses to S. Paratyphi A and B and higher to S. Paratyphi C. To conclude, the Vi vaccine elicited a cross-reactive plasmablast response to S. Paratyphi C (Vi antigen in common) and less marked responses to S. Paratyphi A and B than the Ty21a preparation. S. Paratyphi A and B both being Vi-negative, the result can be explained by trace amounts of bacterial cell wall O-12 antigen in the Vi preparation, despite purification. The clinical significance of this finding remains to be determined.

Double-blinded, randomised, placebo-controlled trial of convalescent plasma for COVID-19: analyses by neutralising antibodies homologous to recipients' variants.

INTRODUCTION: Convalescent plasma (CP) emerged as potential treatment for COVID-19 early in the pandemic. While efficacy in hospitalised patients has been lacklustre, CP may be beneficial at the first stages of disease. Despite multiple new variants emerging, no trials have involved analyses on variant-specific antibody titres of CP. METHODS: We recruited hospitalised COVID-19 patients within 10 days of symptom onset and, employing a double-blinded approach, randomised them to receive 200 ml convalescent plasma with high (HCP) or low (LCP) neutralising antibody (NAb) titre against the ancestral strain (Wuhan-like variant) or placebo in 1:1:1 ratio. Primary endpoints comprised intubation, corticosteroids for symptom aggravation, and safety assessed as serious adverse events. For a preplanned ad hoc analysis, the patients were regrouped by infused CP's NAb titers to variants infecting the recipients i.e. by titres of homologous HCP (hHCP) or LCP (hLCP). RESULTS: Of the 57 patients, 18 received HCP, 19 LCP and 20 placebo, all groups smaller than planned. No significant differences were found for primary endpoints. In ad hoc analysis, hHCPrecipients needed significantly less respiratory support, and appeared to be given corticosteroids less frequently (1/14; 7.1%) than those receiving hLCP (9/23; 39.1%) or placebo (8/20; 40%), (p = 0.077). DISCUSSION: Our double-blinded, placebo-controlled CP therapy trial remained underpowered and does not allow any firm conclusions for early-stage hospitalised COVID-19 patients. Interestingly, however, regrouping by homologous - recipients' variant-specific - CP titres suggested benefits for hHCP. We encourage similar re-analysis of ongoing/previous larger CP studies. TRIAL REGISTRATION: ClinTrials.gov identifier: NCT0473040.

Travellers returning with measles from Thailand to Finland, April 2012: infection control measures.

Countries with no autochthonous measles run the risk of the virus being imported by travellers and transmitted to unprotected citizens. In April 2012, two travellers from Finland and one from Estonia were diagnosed with measles after returning from Phuket, Thailand. They were contagious on their return flights and subsequently exposed several individuals, prompting extensive infection control measures. Two secondary cases were detected: one child who had received one vaccine dose and another who was fully vaccinated.

Identification of the five human Plasmodium species including P. knowlesi by real-time polymerase chain reaction.

Recently, Plasmodium knowlesi has been recognised as the fifth Plasmodium species causing malaria in humans. Hundreds of human cases infected with this originally simian Plasmodium species have been described in Asian countries and increasing numbers are reported in Europe from travellers. The growing impact of tourism and economic development in South and Southeast Asia are expected to subsequently lead to a further increase in cases both among locals and among travellers. P. knowlesi is easily misidentified in microscopy as P. malariae or P. falciparum. We developed new primers for the rapid and specific detection of this species by low-cost real-time polymerase chain reaction (PCR) and added this method to an already existing panel of primers used for the molecular identification of the other four species in one reaction. Reference laboratories should now be able to identify undisputably and rapidly P. knowlesi, as it is a potentially fatal pathogen.

Humoral immune response to keyhole limpet haemocyanin, the protein carrier in cancer vaccines.

Keyhole limpet haemocyanin (KLH) appears to be a promising protein carrier for tumor antigens in numerous cancer vaccine candidates. The humoral immune response to KLH was characterized at the single-cell level with ELISPOT combined with separations of cell populations according to their expression of homing receptors (HRs). The analysis of HR expressions is expected to reveal the targeting of the immune response in the body. Eight orally primed and four nonprimed volunteers received KLH-vaccine subcutaneously. Circulating KLH-specific plasmablasts were found in all volunteers, 60 KLH-specific plasmablasts/10(6) PBMC in the nonprimed and 136/10(6) in the primed group. The proportion of L-selectin(+) plasmablasts proved high and integrin α(4)ß(7) (+) low. KLH serving as protein carrier in several vaccines, the homing profile of KLH-specific response may be applicable to the cancer antigen parts in the same vaccines. The present data reflect a systemic homing profile, which appears advantageous for the targeting of immune response to cancer vaccines.

Revisiting travellers' diarrhoea justifying antibiotic treatment: prospective study.

BACKGROUND: As antimicrobials increase the risk of acquiring multidrug-resistant (MDR) bacteria, unnecessary antibiotics should be avoided for travellers' diarrhoea (TD). Antibiotics are recommended in TD accompanied by fever or incapacitation (TD justifying use of antibiotics, TDjuAB). Seeking tools for reducing antibiotic use, we explored factors predisposing to TDjuAB and scrutinized antibiotic treatment among those with TDjuAB [TDjuAB(+) subgroup] and those with diarrhoea not justifying antibiotics [TDjuAB(-) subgroup]. METHODS: We conducted a study among 370 prospectively recruited visitors to the tropics. Stool samples and questionnaires were collected before and after travel. Enteric pathogens were analysed by qPCR for enteropathogenic (EPEC), enteroaggregative (EAEC), enterotoxigenic (ETEC), enterohaemorrhagic (EHEC) and enteroinvasive (EIEC) E. coli/Shigella, Campylobacter, Salmonella, Yersinia and Vibrio cholerae, and for ETEC's toxins LT (heat-labile), STh (human heat-stable) and STp (porcine heat-stable). TD was defined by the WHO criteria and TDjuAB as diarrhoea accompanied by fever, and/or disrupting or preventing daily activities. Multivariable analysis was applied-separately for travel-related factors and pathogens-to identify risk factors for TDjuAB(+). RESULTS: Among the 370 travellers, TD was contracted by 253 (68%), categorized as TDjuAB(+) in 93/253 (37%) and TDjuAB(-) in 160/253 (63%) of the cases. Antibiotics were used for TD by 41% in TDjuAB(+) and by 7% in the TDjuAB(-) group. Relative risk ratios (RRR)s are presented for both the TDjuAB(+) and the TDjuAB(-) groups. TDjuAB(+) was associated with long travel duration and young age. Among the 298 subjects not having taken antibiotics, increased RRRs were found e.g. for findings of Campylobacter coli/jejuni and ETEC's STh toxin. CONCLUSIONS: The first to analyse risk factors for TDjuAB, our study presents RRRs for demographic and behavioural factors and for various pathogens. Only less than half of those in the TDjuAB(+) group took antibiotics, which demonstrates that most cases meeting the current criteria recover without antimicrobial treatment.

Decreased numbers of circulating plasmablasts and differences in IgA1-plasmablast homing to skin in coeliac disease and dermatitis herpetiformis.

The two clinical phenotypes of gluten enteropathy, coeliac disease (CD) and dermatitis herpetiformis (DH), were characterized for numbers and homing profiles of circulating final effector B cells, plasmablasts, identified as immunoglobulin (Ig)-secreting cells (ISC). In CD, the numbers of ISC were approximately 50% lower than in DH or controls. ISC expressed peripheral lymph node homing receptor (HR), L-selectin, less frequently in CD (54%) and DH (52%) patients than in controls (70%). The expression of gut mucosal HR, alpha(4)beta(7), was less frequent in CD (42%) than in DH (65%) or controls (60%). In DH, but not in CD or controls, a higher proportion of IgA1-ISC (40%) than IgA2-ISC (25%) expressed the skin HR, cutaneous lymphocyte-associated antigen. In gluten enteropathy circulating plasmablasts are more mature, but decreased in number, and have distorted homing profiles. Differential IgA1-plasmablast homing could be associated with the development of skin rash with IgA1-deposits in DH but not in CD.

Import of community-associated, methicillin-resistant Staphylococcus aureus to Europe through skin and soft-tissue infection in intercontinental travellers, 2011-2016.

OBJECTIVES: Recently, following import by travel and migration, epidemic community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has caused nosocomial outbreaks in Europe, sometimes with a fatal outcome. We describe clinico-epidemiological characteristics of CA-MRSA detected by the European Network for the Surveillance of imported S. aureus (www.staphtrav.eu) from May 2011 to November 2016. METHODS: Sentinel surveillance at 13 travel clinics enrolling patients with travel-associated skin and soft-tissue infection (SSTI) and analysing lesion and nose swabs at one central laboratory. RESULTS: A total of 564 independent case-patients with SSTI were enrolled and had 374 (67%) S. aureus-positive lesions, of which 14% (51/374) were MRSA. The majority of CA-MRSA isolates from SSTI were Panton-Valentine leucocidin (PVL) -positive (43/51, 84%). The risk of methicillin-resistance in imported S. aureus varied by travel region (p <0.001) and was highest in Latin America (16/57, 28%, 95% CI 17.0-41.5) and lowest in Sub-Saharan Africa (4/121, 3%, 95% CI 0.9-8.3). Major epidemic clones (USA300 / USA300 Latin-American Variant, Bengal Bay, South Pacific) accounted for more than one-third (19/51, 37%) of CA-MRSA imports. CA-MRSA SSTI in returnees was complicated (31/51 multiple lesions, 61%; 22/50 recurrences, 44%), led to health-care contact (22/51 surgical drainage, 43%; 7/50 hospitalization, 14%), was transmissible (13/47 reported similar SSTI in non-travelling contacts, 28%), and associated with S. aureus nasal colonization (28 of 51 CA-MRSA cases, 55%; 24 of 28 colonized with identical spa-type in nose and lesion, 85%). CONCLUSIONS: Travel-associated CA-MRSA SSTI is a transmissible condition that leads to medical consultations and colonization of the infected host.

A prospective study of travellers' diarrhoea: analysis of pathogen findings by destination in various (sub)tropical regions.

OBJECTIVES: Eighty million travellers visiting (sub)tropical regions contract travellers' diarrhoea (TD) each year, yet prospective data comparing the prevalence of TD pathogens in various geographical regions are scarce. Our recent study using modern molecular methods found enteropathogenic (EPEC) and enteroaggregative (EAEC) Escherichia coli to be the most frequent pathogens, followed by enterotoxigenic E. coli (ETEC) and Campylobacter. We revisited our data to compare the findings by geographical region. METHODS: A total of 459 prospectively recruited travellers provided stool samples and completed questionnaires before and after visiting destinations in various geographical regions. A multiplex quantitative real-time PCR assay was used to analyse Salmonella, Yersinia, Campylobacter jejuni/Campylobacter coli, Shigella, Vibrio cholerae, EPEC, EAEC, ETEC, enterohaemorrhagic E. coli and enteroinvasive E. coli. RESULTS: TD was contracted by 69% (316/459) of the subjects; EPEC and EAEC outnumbered ETEC and Campylobacter in all regions. Multiple pathogens were detected in 42% (133/316) of the samples. The proportions of all pathogens varied by region. The greatest differences were seen for Campylobacter: while relatively frequent in South Asia (n = 11; 20% of the 55 with TD during travel) and Southeast Asia (15/84, 15%), it was less common in East and West Africa (5/71, 7% and 1/57, 2%) and absent in South America and the Caribbean (0/40). CONCLUSIONS: EPEC and EAEC outnumbered ETEC and Campylobacter everywhere, yet the proportions of pathogen findings varied by region, with ETEC and Campylobacter rates showing the greatest differences. The high frequency of multibacterial findings in many regions indicates a need for further investigation of the clinical role of each pathogen.

Patients hospitalized abroad as importers of multiresistant bacteria-a cross-sectional study.

OBJECTIVES: The pandemic spread of multidrug-resistant (MDR) bacteria poses a threat to healthcare worldwide, with highest prevalence in indigent regions of the (sub)tropics. As hospitalization constitutes a major risk factor for colonization, infection control management in low-prevalence countries urgently needs background data on patients hospitalized abroad. METHODS: We collected data on 1122 patients who, after hospitalization abroad, were treated at the Helsinki University Hospital between 2010 and 2013. They were screened for methicillin-resistant Staphylococcus aureus (MRSA), extended-spectrum ß-lactamase-producing Enterobacteriaceae (ESBL-PE), vancomycin-resistant enterococci, carbapenemase-producing Enterobacteriaceae (CPE), multiresistant Pseudomonas aeruginosa and multiresistant Acinetobacter baumannii. Risk factors for colonization were explored by multivariate analysis. RESULTS: MDR colonization rates were higher for those hospitalized in the (sub)tropics (55%; 208/377) compared with temperate zones (17%; 125/745). For ESBL-PE the percentages were 50% (190/377) versus 12% (92/745), CPE 3.2% (12/377) versus 0.4% (3/745) and MRSA 6.6% (25/377) versus 2.4% (18/745). Colonization rates proved highest in those returning from South Asia (77.6%; 38/49), followed by those having visited Latin America (60%; 9/16), Africa (60%; 15/25) and East and Southeast Asia (52.5%; 94/179). Destination, interhospital transfer, short time interval to hospitalization, young age, surgical intervention, residence abroad, visiting friends and relatives, and antimicrobial use proved independent risk factors for colonization. CONCLUSIONS: Post-hospitalization colonization rates proved higher in the (sub)tropics than elsewhere; 11% (38/333) of carriers developed an MDR infection. We identified several independent risk factors for contracting MDR bacteria. The data provide a basis for infection control guidelines in low-prevalence countries.

Prospective study of pathogens in asymptomatic travellers and those with diarrhoea: aetiological agents revisited.

Travellers' diarrhoea (TD) remains the most frequent health problem encountered by visitors to the (sub)tropics. Traditional stool culture identifies the pathogen in only 15% of cases. Exploiting PCR-based methods, we investigated TD pathogens with a focus on asymptomatic travellers and severity of symptoms. Pre- and post-travel stools of 382 travellers with no history of antibiotic use during travel were analysed with a multiplex quantitative PCR for Salmonella, Yersinia, Campylobacter, Shigella, Vibrio cholerae and five diarrhoeagenic Escherichia coli: enteroaggregative (EAEC), enteropathogenic (EPEC), enterotoxigenic (ETEC), enterohaemorrhagic (EHEC) and enteroinvasive (EIEC). The participants were categorized by presence/absence of TD during travel and on return, and by severity of symptoms. A pathogen was indentified in 61% of the asymptomatic travellers, 83% of those with resolved TD, and 83% of those with ongoing TD; 25%, 43% and 53% had multiple pathogens, respectively. EPEC, EAEC, ETEC and Campylobacter associated especially with ongoing TD symptoms. EAEC and EPEC proved more common than ETEC. To conclude, modern methodology challenges our perception of stool pathogens: all pathogens were common both in asymptomatic and symptomatic travellers. TD has a multibacterial nature, but diarrhoeal symptoms mostly associate with EAEC, EPEC, ETEC and Campylobacter.

Should close contacts of returning travellers with typhoid fever be protected by vaccination?

Increasing international travel to areas endemic for typhoid fever correlates with increased risk for travellers to contract the disease. At home, the acutely ill/convalescent patients may pose some risk to their close contacts. In Finland an unofficial guideline suggests vaccination for close contacts of patients with acute typhoid fever; in other developed countries, routine typhoid vaccinations are only recommended to contacts of chronic carriers. This paper discusses the possibilities and limitations of prophylactic/post-exposure typhoid vaccination for contacts of patients with acute disease.

Predominance of dfrG as determinant of trimethoprim resistance in imported Staphylococcus aureus.

To investigate the global occurrence of trimethoprim-sulfamethoxazole resistance and the genetic mechanisms of trimethoprim resistance, we analysed Staphylococcus aureus from travel-associated skin and soft-tissue infections treated at 13 travel clinics in Europe. Thirty-eight per cent (75/196) were trimethoprim-resistant and 21% (41/196) were resistant to trimethoprim-sulfamethoxazole. Among methicillin-resistant S. aureus, these proportions were 30% (7/23) and 17% (4/23), respectively. DfrG explained 92% (69/75) of all trimethoprim resistance in S. aureus. Travel to South Asia was associated with the highest risk of acquiring trimethoprim-sulfamethoxazole-resistant S. aureus. We conclude that globally dfrG is the predominant determinant of trimethoprim resistance in human S. aureus infection.

Skin and soft tissue infections in intercontinental travellers and the import of multi-resistant Staphylococcus aureus to Europe.

Staphylococcus aureus is emerging globally. Treatment of infections is complicated by increasing antibiotic resistance. We collected clinical data and swabs of returnees with skin and soft tissue infections (SSTI) at 13 travel-clinics in Europe (www.staphtrav.eu). Sixty-two percent (196/318) SSTI patients had S. aureus-positive lesions, of which almost two-thirds (122/196) were Panton-Valentine leukocidin (PVL) positive. PVL was associated with disease severity, including hospitalization for SSTI (OR 5.2, 95% CI 1.5-18.2). In returnees with SSTI, longer travel and more intense population contact were risk factors for nasal colonization with PVL-positive S. aureus. Imported S. aureus frequently proved resistant to trimethoprim-sulfamethoxazole (21%), erythromycin (21%), tetracycline (20%), ciprofloxacin (13%), methicillin (12%) and clindamycin (8%). Place of exposure was significantly (p < 0.05) associated with predominant resistance phenotypes and spa genotypes: Latin America (methicillin; t008/CC24/304), Africa (tetracycline, trimethoprim-sulfamethoxazole; t084/CC84, t314/singleton, t355/CC355), South Asia (trimethoprim-sulfamethoxazole, ciprofloxacin; t021/CC21/318), South-East Asia (clindamycin; t159/CC272). USA300-like isolates accounted for 30% of all methicillin-resistant S. aureus imported to Europe and were predominantly (71%) acquired in Latin America. Multi-resistance to non-ß-lactams were present in 24% of imports and associated with travel to South Asia (ORcrude 5.3, 95% CI 2.4-11.8), even after adjusting for confounding by genotype (ORadjusted 3.8, 95% 1.5-9.5). Choosing randomly from compounds recommended for the empiric treatment of severe S. aureus SSTI, 15% of cases would have received ineffective antimicrobial therapy. These findings call for the development of regionally stratified guidance on the antibiotic management of severe imported S. aureus disease and put the infected and colonized traveller at the centre of interventions against the global spread of multi-resistant S. aureus.

Induction of specific immune responses in the genital tract of women after oral or rectal immunization and rectal boosting with Salmonella typhi Ty 21a vaccine.

The purpose of this study was to determine the efficacy of intestinal tract immunization in the induction of specific antibodies in human female genital tract secretions. Live attenuated typhoid vaccine Ty 21a was administered to three groups of healthy female volunteers, who were not using hormonal contraceptives. Group 1 included 15 women vaccinated orally. Group 2 included seven of the same women, who were vaccinated rectally 6 months later. Group 3 included 11 volunteers, who were vaccinated rectally. Salmonella-specific antibodies of IgG and IgA were measured in vaginal lavage and cervical mucus after oral or rectal primary vaccination. Salmonella-specific antibodies measured 1 month after rectal booster vaccination demonstrated significant increases in vaginal fluids and cervical mucus and were dominated by IgA. These results indicate that specific antibodies in the human female genital tract induced by primary vaccination can be enhanced by subsequent rectal administration of vaccines.

Peripheral blood antibody-secreting cells in the evaluation of the immune response to an oral vaccine.

Specific antibody-secreting cells (ASC) appear in the blood as a response to oral vaccination in humans. Based on information from animal experiments, these cells are believed to be migrating to the mucosa. This review summarizes a series of studies aimed at a detailed characterization of the ASC response to a prototype oral vaccine Salmonella typhi Ty21a, with respect to its kinetics, Ig-class distribution, antigen specificity, influence of the administration route and nature of the antigen, and the corresponding antibody responses in serum. Different vaccine formulations as well as dosage schedules are compared, and the response to booster immunization is described. The response manifested by ASC in blood is shown to be independent from serum antibody responses. Moreover, it is shown to parallel with the results obtained for protection in field trials. Finally, some data on the homing receptor expression of these cells are presented, giving further evidence for the mucosal homing of these cells. The ASC assay offers a practical means for assessing immune response to oral vaccines in humans. It can be used as a laboratory parameter correlated with protection conferred by an oral typhoid vaccine. It can even be applied to measure active mucosal immunity, i.e., protective immunity by showing the relative reduction of the ASC response to an oral dose of live vaccine.