Paired, quantitative measurements of hepatitis B virus DNA in saliva, urine and serum of chronic hepatitis B patients

Seroprevalence and factors associated with Hepatitis B virus infection among students in two senior high schools in the Krachi Nchumuru district in Ghana-A cross-sectional study

BACKGROUND

There is paucity of hepatitis B virus (HBV) data among student populations although Ghana is HBV endemic. Screening and identification of factors associated with HBV transmission in schools will support the intervention in the elimination of the virus by 2030. This study assessed the seroprevalence and factors associated with HBV among students in two Senior High Schools in the Krachi Nchumuru District in Ghana.

METHODS

Through cross-sectional design and simple random sampling technique, 300 first-year students were enrolled from selected Senior High Schools. Structured questionnaires were used to collect data on demographic and exposure factors while rapid test kit was used to detect HBV infections. Chi-square/Fisher exact test and multivariable logistic regression were performed to determine the association between the variables at a 95% confidence interval and 0.05 significant level.

RESULTS

Seroprevalence of HBV was 14% (42/300) among the students. The prevalence was significantly (p = 0.001) higher in males 19.4% (34/175) than females 6.4% (8/125). 77.7% (233/300) were aware of HBV infection. Males who were circumcised were 4 times more likely to be infected with HBV (AOR = 4.09, 95%CI = 1.82-9.19) (p = 0.001) compared with those uncircumcised.

CONCLUSION

HBV screening and education on hygienic genital circumcision practices must be prioritized in endemic countries.

Horizontal transmission might be a common route of hepatitis B virus exposure in highly endemic areas

BACKGROUND AND AIMS

Hepatitis B virus (HBV) is a common viral pathogen that infects more than a third of the world's population; however, the transmission route remains to be further defined. The 18-year implementation of free HBV vaccine for children has greatly changed the prevalence of HBV infection in China, which presents a unique real-world model for assessing the pattern of HBV transmission.

METHODS

Cross-sectional data of HBV seromarkers between July 2019 and April 2020 were collected from 53,371 individuals aged 1-60 years in four areas of North to South in Eastern China. Longitudinal data of HBV seromarkers between 2007 and 2020 were collected from 177 adults in an area of South China. The regional- and age-specific changes in HBV seromarkers were analyzed.

RESULTS

Overall, positive rates of HBV surface antigen (HBsAg) (from 3.44% to 15.1%) and antibody against HBV core antigen (anti-HBc) (from 7.6% to 44.0%) significantly increased from North to South. Among persons aged ≤18 years, the positive rates of antibody against HBsAg (anti-HBs) and anti-HBc (+) remained at low levels in the North, while they were increasing among persons aged >12 years in the South, despite higher positive rates of anti-HBs (+). Among persons aged >18 years, the anti-HBs (+) rates remained relatively stable (~60%), while anti-HBc (+) rates increased significantly with age. Up to ~80% of the anti-HBs (+) adults in the South was anti-HBc (+) while it was 13.6% in the North. In the longitudinal cohort, the anti-HBc (+) rate among adults in the South increased by 14.2% during 10 years of follow-up.

CONCLUSION

Horizontal transmission might be a common route in highly endemic areas, and may help to explain the high HBV exposure worldwide. The risk of horizontal transmission among children without seroprotective anti-HBs should be notified in highly endemic areas. This article is protected by copyright. All rights reserved.

A Study of Prevalence of Occult Hepatitis B Virus Infection, Knowledge and Preventive Practices Against Hepatitis B Virus in Barbers Serving the Armed Forces

OBJECTIVE

Hepatitis B virus (HBV) infection is a major health problem in the world. Barbers deal with frequent abrasions/lacerations due to sharp equipment, making them a high-risk group. Determination of HBsAg positive status excludes most reservoirs of transmission in the population. However, Occult Hepatitis B continues to be a source of transmission. The aim of this study was to study the prevalence of occult HBV infection in barbers serving the armed forces clientele and evaluate their knowledge and preventive practices against HBV transmission.

METHODS

Seventy-nine HBsAg negative barbers were included in this study and interviewed for the status of immunisation and preventive practices. Anti-HBc total and HBV DNA levels were measured along with a complete haemogram, LFT, PT INR, ultrasound abdomen and Fibroscan of the liver.

RESULTS

The prevalence of occult Hepatitis B status was 3.79%. Among barbers who were anti-HBc total positive, 100% were found to have replicative HBV DNA status. All barbers (100%) were unaware of the existence and modes of HBV transmission and were never screened for HBV; 98.73% of barbers followed improper disinfection practices and were never immunised.

CONCLUSION

The prevalence of occult HBV infection in barbers, absence of immunisation, unawareness and improper disinfection practices are significantly at risk for transmission to the unaware clients. It is important to educate barbers, establish a universal disinfection procedure and implement a system of compulsory Hepatitis B vaccination before the commencement of their trade work.

Efficacy of hepatitis B vaccination in children with rheumatic diseases

BACKGROUND

Vaccination to prevent hepatitis B (HB) virus infection is important for children undergoing immunosuppressive treatment. Information on the efficacy of HB vaccination in children with rheumatic diseases undergoing immunosuppressive therapy is scarce.

METHODS

Children with rheumatic diseases administered HB vaccine during immunosuppressive treatment between May 2013 and September 2016 were enrolled. Patients were vaccinated three times (primary series). Those who remained seronegative after the primary series received a secondary series of vaccinations. Patient baseline characteristics and treatment details from the medical records were retrospectively investigated. The proportion of patients that was seropositive for HB virus antibody after primary-and secondary series of vaccinations was calculated. Associations between immunosuppressants and serostatus were evaluated.

RESULTS

Fifteen of 26 patients (58%) produced anti-hepatitis B surface antibody (anti-HBs) after the primary vaccinations. Eight of 10 patients (80%) taking methotrexate and 3 of 11 (27%) taking mycophenolate mofetil (MMF) were seropositive. Multivariate analysis adjusted for dosage of prednisolone per body weight. Multivariate analysis showed MMF was a factor impeding seroconversion (odds ratio 0.093, 95% confidence interval 0.014-0.615). In six of seven patients (86%) who received a secondary series of vaccinations, anti-HBs were produced.

CONCLUSIONS

MMF may impede seroconversion after a primary series of HB vaccinations, thus requiring secondary series of vaccinations in pediatric patients with a rheumatic disease undergoing immunosuppressive therapy.

Molecular Mechanisms during Hepatitis B Infection and the Effects of the Virus Variability

The immunopathogenesis and molecular mechanisms involved during a hepatitis B virus (HBV) infection have made the approaches for research complex, especially concerning the patients’ responses in the course of the early acute stage. The study of molecular bases involved in the viral clearance or persistence of the infection is complicated due to the difficulty to detect patients at the most adequate points of the disease, especially in the time lapse between the onset of the infection and the viral emergence. Despite this, there is valuable data obtained from animal and in vitro models, which have helped to clarify some aspects of the early immune response against HBV infection. The diversity of the HBV (genotypes and variants) has been proven to be associated not only with the development and outcome of the disease but also with the response to treatments. That is why factors involved in the virus evolution need to be considered while studying hepatitis B infection. This review brings together some of the published data to try to explain the immunological and molecular mechanisms involved in the different stages of the infection, clinical outcomes, viral persistence, and the impact of the variants of HBV in these processes.

Molecular Detection of Occult Hepatitis B virus in plasma and urine of renal transplant patients in Khartoum state Sudan

OBJECTIVE

The aim of this study was to determine the OBI in plasma and urine samples from renal transplant patients using Multiplex Nested PCR.

METHOD

A total of 100 samples (plasma and urine) were collected from renal transplant patients admitted to the renal transplant center in Khartoum north, Sudan in 2019. For each sample, HBsAg, HBeAg and anti HBcAg were detected using Enzyme linked Immune sorbent assay (ELISA). The viral DNA was then extracted using viral DNA extraction kit and were then tested for HBV DNA by using multiplex nested PCR. Statistical analysis was done using statistical package of social science (IBM SPSS version 20.0) considering a P value ≤ 0.05 as a level of significance.

RESULTS

HBsAg were not detected in al patient but, HBeAg were 14 (14%) and anti HBcAg were 36 (36%)were detect by using ELISA. A total 18 (18%) and 3 out of 100 were found positive in plasma and urine samples, respectively. Regarding the virus genotypes, D, E and mixed D/E genotypes were detected in all positive samples. Females were significantly (P value=0.013) higher detectable with HBV than males in plasma samples CONCLUSION: OBI incidence in renal transplant patients is high in Sudan. The multiplex nested PCR had identified OBI with a high rate supporting the efficiency of using molecular techniques in detecting of HBV. This will lead to an appropriate diagnosis and minimizing the risk to be infected by HBV.

Applicability of Oral Fluid and Dried Blood Spot for Hepatitis B Virus Diagnosis

Hepatitis B virus (HBV) is one of the major causes of chronic liver disease worldwide; however most of individuals are not aware about the infection. Oral fluid and dried blood spot (DBS) samples may be an alternative to serum to HBV diagnosis to increase the access to diagnosis in remote areas or high-risk groups. The main objective of this review is to give an insight about the usefulness of oral fluid and DBS for detecting HBV markers. Several groups have evaluated the detection of HBsAg, anti-HBc, and anti-HBs markers in oral fluid and DBS samples demonstrating 13 to 100% of sensitivity and specificity according different groups, sample collectors, and diagnosis assays. In the same way, HBV DNA detection using oral fluid and DBS samples demonstrate different values of sensitivity according type of collection, studied group, extraction, and detection methods. Thus, serological and molecular diagnostic tests demonstrated good performance for detecting HBV using oral fluid and DBS according some characteristics and could be useful to increase the access to the diagnosis of HBV.

Commentary: A review of risk of hepatitis B and C transmission through biting or spitting

A draft UK Parliamentary Bill sought to criminalize assaults on emergency workers through biting and spitting. This seemed to be based on a fear of bloodborne virus transmission. We undertook a literature search to clarify the risk of hepatitis infection from such exposures. We identified 245 possible papers and then reduced these to those relevant to HBV and HCV transmission through biting or spitting and the scientific plausibility. Nine papers were identified, reporting 16 possible cases of HBV (15 bites, 1 spitting) and 2 of HCV transmission (both bites). Only 3 HBV transmissions by bites and 1 by spitting and both HCV transmissions were felt to be plausible. Although both HBV DNA and HCV RNA can be found in the saliva of infected patients, it seems unlikely that there is enough to transmit infection unless there is blood contamination. In conclusion, the risk of acquiring HCV through spitting is negligible and is very low for HBV. The risk is also low for acquiring HBV and HCV through biting, especially if no blood is apparent in the saliva.

Viral Biomarkers in Chronic HBeAg Negative HBV Infection

Viral biomarkers are important tools for monitoring chronic hepatitis B virus (HBV) hepatitis B early antigen (HBeAg) negative infection, both in its natural course as well as during and after treatment. The biomarkers consist of antibodies against viral epitopes, viral proteins, and molecular surrogate markers of the quantity and transcriptional activity of the stable episomal HBV covalently closed circular DNA (cccDNA) which is located in the nuclei of the infected hepatocytes. HBV deoxyribonucleic acid (DNA) or else viral load measurement in plasma or serum is a marker of HBV replication of major clinical importance. HBV DNA is used for staging and treatment monitoring as described in international scientific guidelines. Quantification of HBV antigens, mainly hepatitis B surface antigen (HBsAg) as well as Hepatitis B core related antigen (HBcrAg), play an important yet secondary role, especially in cases of low or undetectable HBV DNA and has been evaluated for the classification of the inactive carrier state, as a predictor of subsequent HBsAg clearance, treatment outcome, and development of hepatocellular carcinoma (HCC). The measurement of the replicative intermediate HBV RNA in serum is currently evaluated and may also prove to be a significant biomarker particularly in patients treated with nucleot(s)ide analogs. This review focuses on the viral biomarkers mentioned above and their role in HBV, HBeAg negative, infection.

Characterization of the hepatitis B virus DNA detected in urine of chronic hepatitis B patients

Background

Detection of human hepatitis B virus (HBV) DNA in the urine of patients with chronic hepatitis B infection (CHB) has been reported previously, suggesting urine could provide a potential route of horizontal HBV transmission. However, it is not clear whether the HBV DNA detected in urine is indeed full-length, infectious viral DNA. The aim of this study is to assess the potential infectivity of urine from patients with CHB and to correlate HBV DNA detection in urine with clinical parameters, such as serum viral load and HBeAg status.

Methods

Urine from 60 CHB patients with serum viral loads ranging from undetectable to 10⁸ IU/mL were analyzed for HBV DNA and serum immune markers. HBV DNA was detected from total urine DNA and size-fractionated urine DNA (separated into ≤1 kb and > 1 kb fractions) by PCR analysis of six regions of the HBV genome.

Results

Twenty-seven of 59 (45.7%) patients with HBV serum viral load (≥20 IU/mL) contained at least 20 copies per mL of fragmented HBV DNA in urine detected in at least 1 of the 6 PCR assay regions. Only one patient contained HBV DNA detected by all six regions, and was found to have evidence of blood in the urine. Sixteen of 25 urine samples with high viral load (> 10⁵ IU/mL) and 11 of 34 urine samples with low viral load (< 10⁵ IU/mL) contained detectable HBV DNA. Twelve of 27 (44.44%) patients with detectable HBV DNA in urine were HBeAg positive, and only 5 of these HBeAg positive patients were in the group of 33 (15.15%) patients with no detectable HBV DNA in urine. By Fishers’ exact test, HBV DNA in urine is significantly associated with high serum viral load (P = 0.0197) and HBeAg (P = 0.0203).

Conclusions

We conclude that urine from CHB patients with healthy kidney function should not contain full-length HBV DNA, and therefore should not be infectious.

Electronic supplementary material

The online version of this article (10.1186/s12876-018-0767-1) contains supplementary material, which is available to authorized users.

Detection of occult hepatitis B in serum and oral fluid samples

In occult hepatitis B infection (OBI), hepatitis B virus DNA (HBV DNA) can be detected in serum samples; however, oral fluid collection for detection of HBV DNA has not yet been explored, despite the availability of collection devices. Serum and oral fluid samples from 45 hepatitis B core antibody (anti-HBc)-positive patients were collected for the amplification of the HBV polymerase gene. HBV DNA was detected in five serum and four oral fluid samples (the detection limit for oral fluid was 1.656 log IU/mL in paired serum). In conclusion, simple methodologies of sample collection and in-house polymerase chain reaction (PCR) allowed detection of HBV DNA, and these could be used to improve the diagnosis of OBI, especially in locations with limited resources.

Update on hepatitis B and C virus diagnosis

Viral hepatitis B and C virus (HBV and HCV) are responsible for the most of chronic liver disease worldwide and are transmitted by parenteral route, sexual and vertical transmission. One important measure to reduce the burden of these infections is the diagnosis of acute and chronic cases of HBV and HCV. In order to provide an effective diagnosis and monitoring of antiviral treatment, it is important to choose sensitive, rapid, inexpensive, and robust analytical methods. Primary diagnosis of HBV and HCV infection is made by using serological tests for detecting antigens and antibodies against these viruses. In order to confirm primary diagnosis, to quantify viral load, to determine genotypes and resistance mutants for antiviral treatment, qualitative and quantitative molecular tests are used. In this manuscript, we review the current serological and molecular methods for the diagnosis of hepatitis B and C.

Lack of infectivity of HBV in feces from patients with chronic hepatitis B virus infection, and infection using chimeric mice

BACKGROUND

Body fluids such as saliva and tears from patients with hepatitis B virus (HBV) infection are known as infectious agents. The infectivity of feces from patients with HBV infection has not been established. The aim of this study was to determine whether feces from HBV carriers can be a source of HBV infection.

METHODS

Thirty-three children and 17 adults (ages 0-49 years, median age 13 years) who were chronically infected with HBV were enrolled. The levels of HBV DNA in the feces from these patients were quantified by real-time PCR, and the levels of fecal HBsAg were measured. Isolated human hepatocytes from chimeric mice with humanized livers were co-cultured with serum, tears and feces from the HBV carriers. Four chimeric mice were inoculated intravenously with sterilized feces from HBV carriers.

RESULTS

HBV DNA was detected in the feces of 37 (74%) of the 50 patients. The fecal HBV DNA levels ranged from 2.8 to 8.4 log copies/mL (mean ± SD  =  5.6 ± 1.2 log copies/mL). A significant correlation was observed in the levels of HBV DNA between serum and feces (r  =  0.54, p < 0.05). Of the 13 HBV carries, 7 (54%) were positive for fecal HBsAg. The fecal HBsAg levels ranged from 0.06 to 1.0 IU/mL (median 0.28 IU/mL). Immunogold electron microscopy showed Dane particles in feces. HBV DNA was detected in the human hepatocytes co-cultured with serum and tears, but not in those co-cultured with feces. HBV DNA was not detected in the serum of the chimeric mice after oral or intravenous inoculation with sterilized fecal samples, which contained 5 log copies/mL of HBV DNA levels.

CONCLUSIONS

Although the positive rate of fecal HBV DNA was high, the fecal HBsAg levels were extremely low. The chimeric mice were not infected with HBV after oral or intravenous inoculation with sterilized fecal samples. Therefore, feces from HBV carriers seem not to serve as an infectious vehicle for the transmission of HBV.

Sequential occurrence of acute hepatitis B among members of a high school Sumo wrestling club

A 17-year-old male was admitted to our hospital and diagnosed with acute hepatitis B. Six weeks later, a 15-year-old male was admitted with acute hepatitis B as well. They were Sumo wrestling players in the same club. A detailed survey in the club revealed that a 28-year-old male coach was a hepatitis B surface antigen carrier with high-level viremia. The consistency of hepatitis B virus (HBV) DNA in the infected players was revealed by analyzing the complete HBV genome sequences. Sumo players are more likely to get injured, including cuts and bleeding, compared with players of other sports because of the characteristic wrestling style. Several past reports have suggested that highly viremic HBV carriers have high HBV DNA titers in both their blood and other body fluids such as sweat. In our cases, percutaneous HBV transmission through the bleeding wounds was the most probable infection route. We conclude that a universal HBV immunization program should be introduced urgently in Japan, similar to those implemented in other countries worldwide.

Tears from children with chronic hepatitis B virus (HBV) infection are infectious vehicles of HBV transmission: experimental transmission of HBV by tears, using mice with chimeric human livers

BACKGROUND

Body fluids such as saliva, urine, sweat, and tears from hepatitis B virus (HBV) carriers are potential sources of HBV transmission.

METHODS

Thirty-nine children and 8 adults who were chronically infected with HBV were enrolled. Real-time polymerase chain reaction was used for the quantification of HBV DNA.

RESULTS

HBV DNA was detected in 73.7% of urine samples (14 of 19), 86.8% of saliva samples (33 of 38), 100% of tear samples (11 of 11), and 100% of sweat samples (9 of 9). Mean HBV DNA levels (±SD) in urine, saliva, tears, and sweat were 4.3 ± 1.1 log copies/mL, 5.9 ± 1.2 log copies/mL, 6.2 ± 0.7 log copies/mL, and 5.2 ± 0.6 log copies/mL, respectively. A statistically significant correlation was observed between the HBV DNA level in serum specimens and HBV DNA levels in saliva and tear specimens (r = 0.88; P < .001). Tear specimens from a child were injected intravenously into 2 human hepatocyte-transplanted chimeric mice. One week after inoculation, both chimeric mice had serum positive for HBV DNA.

CONCLUSIONS

The levels of HBV DNA in tear specimens from young children were high. Tears were confirmed to be infectious, using chimeric mice. Strict precautions should be taken against direct contact with body fluids from HBV carriers with high-level viremia.

A comparison of molecular methods for hepatitis B virus (HBV) DNA detection from oral fluid samples

The objective of the present study was to evaluate four commercial DNA extraction methods and three PCR protocols for hepatitis B virus (HBV) detection in artificially contaminated oral fluid samples. The extraction protocols were selected based on ease of use and cost, and were also compared with respect to sensitivity and cost. Prior PCR optimization was conducted, in which the sample volume for DNA extraction and the concentrations of DNA and Taq DNA polymerase in the PCR were adjusted. One-round PCR, used to amplify the core region of the HBV genome, achieved high levels of sensitivity in comparison with nested and semi-nested PCR experiments that were designed for the amplification of HBV surface protein genes. Of the four extraction protocols evaluated, the RTP DNA/RNA Virus Mini kit and the QIAamp DNA Mini kit gave the highest recovery rates, presenting 20 copies of HBV DNA ml(-1) as the limit of detection. These results suggest that HBV DNA can be detected from oral fluid samples but that the optimization of the PCR assays and the choice of extraction methods must be determined by laboratories before the implementation of this method in routine diagnostics.

Oral fluid and hepatitis A, B and C: a literature review

BACKGROUND AND AIMS

Viral hepatitis is a significant global health problem that, depending upon the virus, affects individuals of the developing and/or developed world. In recent years, there has been renewed interest in whether oral fluids can be considered as a source of viral hepatitis transmission and whether oral fluid, in particular, whole saliva, may be a useful source for viral detection as part of the diagnosis and monitoring of viral hepatitis. The aim of this article was to review current data concerning the possible carriage of the hepatitis A, B and C viruses within saliva and gingival crevicular fluid. Such knowledge will indicate if (i) oral fluid is a possible source of infection and (ii) whether oral fluid can be used for diagnosis and monitoring of viral hepatitis.

DATA AND SOURCES

A literature search was conducted using PubMed (Medline), EMBASE/Excerpta medica, the Cochrane database and Scopus. The results were limited to published material after 2000. Relevant material was evaluated and reviewed.

CONCLUSION

There is some evidence that hepatitis viruses A, B and C are present in oral fluids, particularly whole saliva and gingival crevicular fluid and may thus be possible sources of viral detection in clinical diagnosis and monitoring. However, the data are inconsistent and warrant the need for well-planned longitudinal studies to explore the precise frequency of oral carriage of such viruses and to determine the virological and host factors that may influence the oral presence of hepatitis A, B and C viruses.

Hepatitis B virus DNA in saliva from children with chronic hepatitis B infection: implications for saliva as a potential mode of horizontal transmission

To explore the mechanism of horizontal transmission of hepatitis B virus (HBV) among children, we investigated the quantitative relationship between HBV in saliva and blood from 46 children with chronic hepatitis B. We found high levels of HBV DNA in saliva of HBeAg (+) children, suggesting saliva as a vehicle for horizontal transmission of HBV among children.

Use of saliva as a lubricant in anal sexual practices among homosexual men

OBJECTIVES

Compared with other sexually active adults, men who have sex with men (MSM) are more frequently infected with several pathogens including cytomegalovirus, hepatitis B virus, and Kaposi sarcoma-associated herpesvirus. Because one common element between these organisms is their presence in saliva, we evaluated saliva exposure among MSM in a heretofore relatively unrecognized route-via use of saliva as a lubricant in anal sex.

METHODS

MSM in a San Francisco population-based cohort were interviewed regarding use of saliva by the insertive partner as a lubricant in various anal sexual practices.

RESULTS

Among 283 MSM, 87% used saliva as a lubricant in insertive or receptive penile-anal intercourse or fingering/fisting at some point during their lifetime; 31%-47% did so, depending upon the act, in the prior 6 months. Saliva use as a lubricant was more common among younger men and among HIV-infected men when with HIV-infected partners. Even among MSM following safe sex guidelines by avoiding unprotected penile-anal intercourse, 26% had anal exposure to saliva via use as a lubricant.

CONCLUSIONS

Among MSM, use of saliva as a lubricant is a common, but not ubiquitous, practice in anal sex. The findings provide the rationale for formal investigation of whether saliva use in this way contributes to transmission of saliva-borne pathogens in MSM.

High levels of hepatitis B virus DNA in body fluids from chronic carriers

Chronic infection with hepatitis B virus (HBV) is a major global health problem. Transmission is mainly blood-borne, although the route of infection during horizontal transmission in childhood is unclear. Nosocomial outbreaks of HBV have been widely reported, but have mainly focused on blood-borne transmission. There is limited knowledge of the viral load levels in other body fluids. In the present study, chronic HBV carriers were tested for the presence of HBV DNA in serum, saliva, nasopharyngeal fluid, urine and tears by means of qualitative and quantitative polymerase chain reaction (PCR) methods. Twenty-five patients who were positive for HBV DNA with both PCRs were included. Low titres in real-time PCR corresponded with weak bands in the qualitative assay. HBV DNA was found in two urine samples, 10 saliva samples, five nasopharyngeal swabs and in tear fluid from four patients. One highly viraemic HBeAg-positive carrier with serum HBV DNA levels of 7 x 10(9) genome copies had high copy numbers detected in both saliva and nasopharyngeal fluid. These results demonstrate that highly viraemic HBV carriers may have high titres of HBV DNA in other body fluids. This has particular importance for infection control programmes and regulations, underlining the importance of aiming towards regular HBV DNA testing and thus infectivity assessment of chronic carriers in order to prevent transmission.