Asymptomatic Norovirus Infection Research Articles

Asymptomatic norovirus infection during outbreaks in China: A systematic review and meta-analysis.

Acute gastroenteritis outbreaks may be caused by the excretion of norovirus (NoV) from asymptomatic individuals. Despite numerous studies involving asymptomatic NoV infection during outbreaks in China, a comprehensive assessment of its role has not been conducted, which is critical for emergency management. Our objective was to assess the prevalence of asymptomatic NoV infection during outbreaks in China. We conducted a comprehensive search of multiple databases, including PubMed, Web of Science, Cochrane Library, China National Knowledge Infrastructure, China Wanfang, and China Weipu, between January 1, 1997 and June 19, 2023. The retrieved articles and their references underwent screening, which utilized polymerase chain reaction-based assays for the detection of NoV in asymptomatic individuals during outbreaks that occurred in China. The primary summary data were the prevalence of asymptomatic NoV infection in outbreaks. We generated pooled estimates of asymptomatic prevalence in the population as a whole and in subgroups by using random-effect models. Of the 97 articles included, the pooled asymptomatic prevalence of NoV among 5117 individuals in outbreaks was 17.6% (95% confidence interval [CI]: 14.1-21.3). The asymptomatic prevalence of NoV GII (17.1%, 95% CI: 12.9-21.5) was similar to that of NoV GI (22.0%, 95% CI: 12.8-32.4). However, the proportion of asymptomatic individuals involved in NoV GII (57.44%) was significantly higher than that of NoV GI (5.12%), and NoV GII (75.26%) was reported much more frequently than NoV GI (14.43%) in the included articles. Meta-regression analysis of 11 possible influencing factors (geographic region, setting, season, sample type, genotype, transmission route, occupation, age, per capita income, study quality, and cases definition) showed that the source of heterogeneity might be related to the outbreak settings, per capita income, and study quality (p = 0.037, 0.058, and 0.026, respectively). Of particular note was the asymptomatic prevalence peaked in preschoolers (27.8%), afterward, it fell into trough in elementary and junior school children (10.5%), before the second peak located in adults (17.8%), and the elderly (25.2%). Prevalent genotypes reported include GII.4, followed by GII.17, GII.2, GII.3, GII.6, and so forth. The estimated asymptomatic prevalence of NoV during outbreaks in China was as high as 17.6%, with NoV GII dominating. In addition, genetic subtypes of NoV in outbreaks should be detected whenever possible. The role of asymptomatic individuals in NoV outbreaks cannot be ignored. This knowledge will help governments develop public health policies and emergency response strategies for outbreaks, assess the burden, and develop vaccines.

Global prevalence of asymptomatic norovirus infection in outbreaks: a systematic review and meta-analysis

BackgroundAlthough many studies on asymptomatic norovirus infection in outbreaks have been conducted globally, structured data (important for emergency management of outbreaks) on the prevalence of this epidemic are still not available. This study assessed the global prevalence of asymptomatic norovirus infection in outbreaks.MethodsWe identified publications on asymptomatic infections from norovirus outbreaks by searching the PubMed, Embase, Cochrane Library, Medline, and Web of Science databases and screening references from the articles reviewed. Prevalence of asymptomatic norovirus infection in outbreaks was employed as the primary summary data. The random-effects model of the meta-analysis was fitted to generate estimates of the prevalence in the overall and subgroup populations.ResultsIn total, 44 articles with a sample size of 8,115 asymptomatic individuals were included. The estimated pooled prevalence of asymptomatic norovirus infection in outbreaks was 21.8% (95%CI, 17.4–27.3). The asymptomatic prevalence of norovirus GII (20.1%) was similar to that of GI (19.8%); however, the proportion prevalence of asymptomatic individuals involved in the former (33.36%) was significantly higher than that of in the latter (0.92%) and the former (93.18%) was reported much more frequently than the latter (15.91%) in the included articles. These studies had significant heterogeneity (I2 = 92%, τ2 = 0.4021, P < 0.01). However, the source of heterogeneity could not be identified even after subgroup analysis of 10 possible influencing factors (geographical area, outbreak settings, outbreak seasons, sample types, norovirus genotypes, transmission routes, subjects’ occupations, subjects’ age, per capita national income, and clear case definition). Meta-regression analysis of these 10 factors demonstrated that the geographical area could be partly responsible for this heterogeneity (P = 0.012).ConclusionsThe overall pooled asymptomatic prevalence of norovirus in outbreaks was high, with genome II dominating. Asymptomatic individuals may play an important role in norovirus outbreaks. This knowledge could help in developing control strategies and public health policies for norovirus outbreaks.

Asymptomatic Norovirus Infection Among Children in Kindergartens and Primary Schools - Beijing Municipality, China, 2021.

Children in kindergartens and primary schools are the high-incidence groups of norovirus acute gastroenteritis. However, asymptomatic norovirus infection among them is seldom reported. The norovirus positive rate was 3.48% among asymptomatic children in kindergartens and primary schools in Beijing Municipality in June 2021, the most common genotype was GII.4 Sydney, and no acute gastroenteritis outbreak was reported over the study period. The asymptomatic norovirus infection was relatively low among kindergarten children and primary school students in summer. Norovirus genotypes in asymptomatic children were similar to those circulating in the symptomatic cases. Asymptomatic norovirus infection may play a limited role in causing acute gastroenteritis outbreaks.

Follow-up evaluation for norovirus asymptomatic infection among healthy adults: a prospective matched cohort study

Objectives No study has evaluated the follow-up of asymptomatic norovirus infection among healthy adults. This study compared norovirus detection rates between previously known norovirus-positive and norovirus-negative adults without diarrheal symptoms, and evaluated reinfection risk among the previously-norovirus-positive group, and new asymptomatic norovirus infection risk among the previously-norovirus-negative group after 600 days. Methods This prospective age- and sex-matched cohort study, conducted in 2019 at a Japan teaching hospital, included apparently healthy asymptomatic adults with a positive norovirus result and those with a negative result (ratio 1:3) during the 2016–2017 screening. The primary outcome was real-time reverse-transcription polymerase chain reaction-confirmed norovirus in stool specimens. We evaluated descriptive statistics and associated factors, including demographics, social habits, and clinical parameters. Results Of 288 participants [mean age, 59.9 (standard deviation: 12.6) years; male, 143 (49.7%)], 73 [genogroup (G) I 35; GII 37; both included 1 each] were positive for norovirus previously, while 215 were negative. After a median of 599 (interquartile range 515–799) days between baseline screening and follow-up, 14 (4.9%; GI 0; GII 14) tested positive for norovirus (2.7% and 5.6% among positive- and negative-norovirus groups at baseline, respectively). Among previously norovirus-negative participants, being older, having elevated blood pressure and haemoglobin A1c level, and drinking Japanese sake at baseline were associated with positive results at follow-up. Conclusions Genogroup homotypic protective effect may exist for subsequent asymptomatic infection. There may be higher risks of future asymptomatic norovirus infection in previously no-norovirus asymptomatic infection people with specific lifestyles or medical histories. SUMMARY The detection follow-up rates of norovirus were 2.7% and 5.6% among asymptomatic adults with positive- and negative-norovirus status at baseline, respectively. Specific lifestyles or medical histories may confer higher risk of norovirus detection.

Epidemiology of Norovirus in the First 2 Years of Life in an Australian Community-based Birth Cohort.

Noroviruses are a leading cause of acute gastroenteritis across all age groups in Australia. We explored the epidemiology of symptomatic and asymptomatic norovirus infection and assessed risk factors and the related healthcare burden in Australian children during their first 2 years of life. Participants in the Observational Research in Childhood Infectious Diseases birth cohort provided weekly stool swabs, daily gastrointestinal symptoms (vomiting and loose stools) observations and healthcare data. Swabs were batch-tested for norovirus genogroups (GI and GII) using real-time polymerase chain reaction assays. Overall, 158 children returned 11,124 swabs. There were 221 infection episodes, of which 183 (82.8%) were GII. The incidence rate was 0.90 infections per child-year [95% confidence interval (CI): 0.74-1.09]. The symptomatic infection incidence rate was 0.39 per child-year (95% CI: 0.31-0.48), peaking between ages 6 and 11 months [0.58 (95% CI: 0.41-0.81)]. Incidence increased significantly with age and childcare attendance. Of 209 episodes with symptom diary data, 82 (39.2%) were symptomatic; of these 70 (85.4%) were associated with vomiting and 29 (35.4%) with diarrhea. Forty-one percent of symptomatic episodes required healthcare, including 4 emergency department presentations and 1 hospitalization. Children with initial infections had almost twice the risk of seeking primary healthcare compared to subsequent infections (adjusted risk ratio 1.92; 95% CI: 1.01-3.65). Norovirus infections, particularly GII, are common in Australian children 6-23 months of age. Estimates of norovirus incidence, including symptomatic infections and healthcare utilization in community settings in young children, are crucial for planning norovirus vaccine programs and determining vaccine effectiveness.

Surveillance To Prevent the Spread of Norovirus Outbreak from Asymptomatic Food Handlers during the PyeongChang 2018 Olympics

Human noroviruses are major causes of nonbacterial gastroenteritis and are transmitted by both food and water, as well as person-to-person. Asymptomatic norovirus infection of food handlers may play a role in transmission. The outbreak of norovirus infections was recognized in the PyeongChang Winter Olympics, starting with security staff on 3 February 2018. The Ministry of Food and Drug Safety in the Republic of Korea conducted norovirus surveillance from asymptomatic food handlers of food-catering facilities related to the Olympics to prevent the spread of noroviruses. Rectal swab samples (707) from food handlers were collected and examined for noroviruses by using real-time reverse transcription PCR and conventional reverse transcription PCR. Five of 707 samples were identified as noroviruses. Genotypes of the norovirus-positive samples were determined with sequencing analysis. Identified genotypes of norovirus in asymptomatic food handlers included GI.3, GII.4, and GII.17. The GII.17 strain was prevalent among the genotypes, accounting for three of five detections. Food handlers with noroviruses detected in rectal swabs were excluded from cooking, and all food handled by infected food handlers was discarded. Surveillance of norovirus infection for food handlers contributed to preventing norovirus spread.

Factors associated with the detection of norovirus among asymptomatic adults

ObjectivesLittle is known about asymptomatic norovirus infection and its risk factors in healthy adults. This study investigated detection of norovirus in stool and its associated factors among asymptomatic healthy adults in a high-income country. MethodsThis prospective cross-sectional study—conducted between February 2016 and January 2017 at a teaching hospital in Japan—included apparently healthy adults aged ≥18 years who underwent voluntary health check-ups. Our primary outcome was detection of norovirus in stool specimens confirmed by real-time RT-PCR. We evaluated descriptive statistics and associated factors, including demographics, social habits, and clinical parameters. ResultsAmong 15 532 participants, 4536 (29.2%, mean age 58.0 (standard deviation 11.8) years, male 44.6%) were enrolled, and 112 (2.5%, GI 57, GII 54, GI + GII 1) were norovirus-positive. Monthly prevalence rates of the GI norovirus were consistent throughout the year, while those of GII were high in May. Participants aged <40 and ≥ 80 years had higher rates of GII norovirus detection. Participants who occasionally consume alcohol, especially wine (odds ratio (OR) 0.17, 95% confidence interval (CI) 0.04–0.68), had lower norovirus detection rates than abstainers. Participants with untreated dyslipidaemia and a low high-density lipoprotein (HDL) cholesterol level had higher detection rates than those with treated dyslipidaemia (OR 1.48, 95%CI 1.07–2.05) and a normal HDL cholesterol level (OR 2.60, 95%CI 1.46–4.61). Some gastrointestinal and female genital diseases were associated with norovirus detection. ConclusionsThe norovirus detection rate in asymptomatic adults was 2.5%. Participants with specific lifestyles or medical histories may have higher risks of asymptomatic norovirus infection.

Virus Etiology, Diversity and Clinical Characteristics in South African Children Hospitalised with Gastroenteritis.

Background: Viral gastroenteritis remains a major cause of hospitalisation in young children. This study aimed to determine the distribution and diversity of enteric viruses in children ≤5 years, hospitalised with gastroenteritis at Kalafong Provincial Tertiary Hospital, Pretoria, South Africa, between July 2016 and December 2017. Methods: Stool specimens (n = 205) were screened for norovirus GI and GII, rotavirus, sapovirus, astrovirus and adenovirus by multiplex RT-PCR. HIV exposure and FUT2 secretor status were evaluated. Secretor status was determined by FUT2 genotyping. Results: At least one gastroenteritis virus was detected in 47% (96/205) of children. Rotavirus predominated (46/205), followed by norovirus (32/205), adenovirus (15/205), sapovirus (9/205) and astrovirus (3/205). Norovirus genotypes GI.3, GII.2, GII.3, GII.4, GII.7, GII.12, GII.21, and rotavirus strains G1P[8], G2P[4], G2P[6], G3P[4], G3P[8], G8P[4], G8P[6], G9P[6], G9P[8] and sapovirus genotypes GI.1, GI.2, GII.1, GII.4, GII.8 were detected; norovirus GII.4[P31] and rotavirus G3P[4] predominated. Asymptomatic norovirus infection (GI.3, GI.7, GII.4, GII.6, GII.13) was detected in 22% of 46 six-week follow up stools. HIV exposure (30%) was not associated with more frequent or severe viral gastroenteritis hospitalisations compared to unexposed children. Rotavirus preferentially infected secretor children (p = 0.143) and norovirus infected 78% secretors and 22% non-secretors. Conclusion: Rotavirus was still the leading cause of gastroenteritis hospitalisations, but norovirus caused more severe symptoms.

Epidemiological characteristics of asymptomatic Norovirus infection in a population from oyster (Ostrea rivularis Gould) farms in southern China.

The following paper investigates the prevalence and characteristics of asymptomatic norovirus infection in the population living around oyster farm sites. Two consecutive surveys were conducted from January 2014 to December 2014 and 4549 stool samples were screened during the same time period. The total asymptomatic infection rate was 4.04% (184/4549). Norovirus infection rate was 5.20% in oyster farming population which was significantly higher compared with non-farming population where the infection rate was 3.65% (χ2 = 5.49, P < 0.05). A total of 184 NoV positive samples were identified by real time-quantitative polymerase chain reaction (RT-qPCR) and semi-nested RT-PCR and 136 sequences were obtained. The sequences were clustered into 14 genotypes. GI strains were clustered into six genotypes, including GI.2, GI.3, GI.5, GI.6, GI.8 and GI.9; while GII strains were clustered into GII.2, GII.3, GII.4, GII.5, GII.6, GII.8 and GII.13. GI.9 and GII.17 were the predominant and most prevalent genotypes, respectively. The GII.17 genotype replaced GII.4 becoming the dominant genotype in the oyster farming area in 2014. To sum up, long-term monitoring of asymptomatic infection is crucial for the detection of new variant strains and for identifying outbreaks during the early stage.

Global Prevalence of Asymptomatic Norovirus Infection: A Meta-analysis

BackgroundStudies about asymptomatic norovirus infections have been frequently reported. We aim to assess the global prevalence of asymptomatic infections. MethodWe identified publications that included the proportion of asymptomatic norovirus infections by searching in PubMed, Ovid, Scopus, and Web of Science and by screening references from the articles reviewed. The principal summary data were the prevalence of asymptomatic norovirus infection. Random-effect models for meta-analysis were fitted to generate estimates of overall and subgroup prevalence. FindingsOf 81 studies included, asymptomatic norovirus prevalence was estimated at 7% (95% CI: 6%–9%). Africa, Meso America and South America had higher prevalence (15%, 14%, 11%, respectively) while the prevalence in Europe and North America was lower (4%). Prevalence was similar between community and hospital (9%). Prevalence was higher in children (8%) than adults (4%). For food handlers, prevalence was estimated at 3%. In context of outbreaks, prevalence estimated from 15 studies was as high as 18% (95% CI: 10%–30%). InterpretationThis knowledge could have an impact on the development of transmission prevention strategies in the future. The high prevalence indicated asymptomatic individuals must not be overlooked. Outstanding questionsThe high prevalence indicated asymptomatic individuals must not be overlooked. Asymptomatic individuals may play an important role in norovirus transmission. This knowledge could have an impact on the development of transmission prevention strategies.

Asymptomatic norovirus infection associated with swimming at a tropical beach: A prospective cohort study.

BackgroundSwimming in fecally-contaminated waterbodies can result in gastrointestinal infections. However, the pathogenic microorganisms responsible are not well understood because sporadic cases of illness are not reported completely, exposure information is often not collected, and epidemiology studies rely on self-reported symptoms. Noroviruses are considered a likely cause because they are found in high densities in sewage, resistant to wastewater treatment and survive in the environment. In this study, saliva samples were collected from subjects at a beach in Puerto Rico and tested for evidence of norovirus-specific IgG responses as an indicator of incident norovirus infection.MethodsSaliva samples were collected from 1298 participants using an oral swab. Samples were collected on the day of the beach visit (S1); after 10–12 days (S2); and after three weeks (S3). Saliva was tested for IgG responses to GI.1 and GII.4 noroviruses using a microsphere based multiplex salivary immunoassay. Immunoconversion was defined as a four-fold increase in median fluorescence intensity (MFI) from S1 to S2 with the S3 sample at least three times above the S1 MFI.ResultsThirty-four subjects (2.6%) immunoconverted to GI.1 or GII.4 norovirus. Swimmers who immersed their head in water had a higher rate of immunoconversion (3.4%), compared to either non-swimmers (0.0%, p = 0.003) or waders and non-swimmers combined (0.4%, Odds Ratio: 5.07, 95% Confidence Interval:1.48–17.00). Immunoconversion was not associated with gastrointestinal symptoms.ConclusionsThis is the first study to demonstrate an association between swimming at a beach impacted by fecal contamination and asymptomatic norovirus infection. The findings implicate recreational water as potentially important transmission pathway for norovirus infection.

Molecular Epidemiology of Norovirus in Asymptomatic Food Handlers in South Korea

Norovirus (NoV) is the most common cause of acute gastroenteritis in all age groups worldwide. In this study, prevalence of asymptomatic norovirus infection was investigated in food handler being employed at food catering facilities in South Korea. A total of 2,729 fecal specimens from asymptomatic food handlers were analyzed, and 1.06% of food handlers (29/2,729) had asymptomatic NoV infection. Of these, 17.2% (5/29) were positive for NoV GI and 82.7% (24/29) were positive for NoV GII. Especially, sequencing and phylogenetic analysis showed that GII-4 was the most prevalent genotype and a large number of asymptomatic food handlers were infested with norovirus GII-4 strains. The results of this study show that asymptomatic food handlers may be potential transmission sources for NoV infection. These results emphasize the need for training of food catering employees about norovirus prevention. Asymptomatic norovirus infection should receive more attention.

Occurrence of norovirus infection in an asymptomatic population in Indonesia

Norovirus (NoV) is a major cause of nonbacterial acute gastroenteritis worldwide in all age groups, and asymptomatic individuals may contribute to NoV transmission as a reservoir. Nonetheless, little information is available regarding asymptomatic NoV infection in Indonesia. We performed an epidemiological analysis of NoV infection among asymptomatic healthy volunteers in the city of Surabaya, Indonesia (population ~2.75 million). A total of 512 stool samples from 18 individuals (age range 20–42years) collected from July 2015 to June 2016 were examined. The detection of NoV and the genotype classification were carried out by a reverse transcription-polymerase chain reaction (RT-PCR) direct sequencing method. NoV was detected in 14 of the 512 stool samples (2.7%), with 7 individuals (38.9%) having at least 1 positive stool sample. All 14 of the NoV strains detected belonged to genogroup GII. The phylogenetic analysis indicated that 10 strains (71.4%) were grouped with GII.2, 2 (14.3%) were GII.17, 1 was GII.4 Sydney 2012, and 1 was GII.1. The circulation of GII.Pg/GII.1 and GII.Pe/GII.4 Sydney 2012 recombinant variants was detected among an asymptomatic population in Surabaya, Indonesia. Of the 7 positive individuals, 2 were repeatedly infected with the same strain and heterogenous strains. Taken together, our results suggest that the excretion of NoV from healthy individuals is one of the sources of NoV outbreak.

Norovirus prevention and the prevalence of asymptomatic norovirus infection in kindergartens and primary schools in Changzhou, China: Status of the knowledge, attitudes, behaviors, and requirements

We estimated the status of norovirus infection and prevention in kindergartens and primary schools and identified poor areas for which future education is needed. We evaluated the knowledge, attitude, behavior, requirements of outbreak prevention, and rate of asymptomatic carriers. Two self-administered questionnaires were conducted for data collection. Five hundred seventy-four teachers from kindergartens and primary schools and 3,114 students from primary schools in Changzhou city were sampled for questionnaire investigation. In addition, 511 rectal swabs from healthy children were taken for norovirus detection. Twenty-one of 511 children were asymptomatically infected with norovirus. The rate of asymptomatic norovirus infection children in kindergartens was 2.7% and 4.7% in primary schools. Nineteen of 21 positive samples were GII strains, the other 2 were GI strains. In addition, we found that primary schools were less prepared to prevent norovirus outbreaks than kindergartens. Settings in the suburban counties needed more education on norovirus prevention. The results of this study emphasize the need for training of primary schools and suburban schools about norovirus prevention. Asymptomatic norovirus infection should receive more attention.

Decoding Norovirus Infection in Crohnʼs Disease

Although a causing viral infectious agent remains untraceable in Crohn's disease, most recent genome-wide association studies have linked the FUT2 W143X mutation (resulting in asymptomatic norovirus infection) with the pathogenesis of Crohn's ileitis and with vitamin B12 deficiency (i.e., a known risk factor for Crohn's disease with ileal involvement). In line with these findings, host variations in additional genes involved in host response to norovirus infection (such as ATG16L1 and NOD2) predispose humans to Crohn's ileitis. One may therefore presume that asymptomatic norovirus infection may contribute to disruption of the stability of the gut microbiota leading to Crohn's ileitis. These paradigms highlight not only the need to revisit the potential transmissibility of Crohn's disease, but also potential safety issues of forthcoming clinical trials on human probiotic infusions in Crohn's ileitis by rigorous donors screening program.

Kingdom-Agnostic Metagenomics and the Importance of Complete Characterization of Enteric Microbial Communities

Advanced sequencing techniques have shown that bacteria are not the only complex and important microbes in the human intestine. Nonbacterial organisms, particularly the virome and the mycobiome, are important regulators of intestinal immunity and inflammation. The virome is mucosal and systemic; it can alter the host response to bacteria and interact with host genes and bacteria to contribute to disease pathogenesis. The human mycobiome is also complex and can contribute to intestinal inflammation. We review what has recently been learned about the nonbacterial and nonarchaeal microbes in the gastrointestinal tract, discussing their potential effects on health and disease and analytical approaches for their study. Studies of associations between the microbiome and intestinal pathology should incorporate kingdom-agnostic approaches if we are to fully understand intestinal health and disease.

Occurrence of Norovirus Infections in Asymptomatic Food Handlers in South Korea

Prevalence of asymptomatic norovirus infection was investigated in food handlers in South Korea. Among 6,441 subjects, 66 (1.02%) had norovirus infections confirmed by reverse transcription (RT)-PCR (real time and nested). GII-12 and GII-4 were the prevalent genotypes. Our data suggest that infection of asymptomatic food handlers is an important transmission source in norovirus outbreaks.

Risk factors for symptomatic and asymptomatic norovirus infection in the community

The objective of this study was to investigate risk factors for norovirus-associated infectious intestinal disease (IID) and asymptomatic norovirus infection. Individuals with IID and healthy controls were recruited in a community-based study in England (1993-1996). This is the first risk-factor study to use viral load measurements, generated by real-time RT-PCR, to identify cases of norovirus-associated IID and asymptomatic infections. Using multivariable logistic regression the main risk factor identified for norovirus-associated IID was contact with a person with IID symptoms. Infectious contacts accounted for 54% of norovirus cases in young children and 39% of norovirus cases in older children and adults. For young children, contacts outside the household presented the highest risk; for older children and adults, the highest risk was associated with child contacts inside the household. Foreign travel and consumption of shellfish increased the risk of norovirus-associated IID. Lifestyle and dietary factors were associated with a decreased risk of both norovirus-associated IID and asymptomatic infection. No risk factors were identified for asymptomatic norovirus infection.

Seasonal Prevalence of Asymptomatic Norovirus Infection in Korean Children

The prevalence of asymptomatic norovirus (NoV) infection was investigated in children registered for kindergarten in Korea during the winter and summer. Children with no gastrointestinal symptoms, including diarrhea and abdominal pain, during the 2 weeks before and following sample collection were included in this investigation. NoV presence and genetic identification were determined with real-time reverse transcriptase-polymerase chain reaction and conventional nested reverse transcriptase-polymerase chain reaction. The prevalence of NoV in asymptomatic children was 5.5% in the winter and 3.5% in the summer, respectively. GII.4 was the most prevalent NoV genotype, but GII.2 and GI.10 were also found during genetic analysis. This study demonstrates that asymptomatic NoV infection may be an important source of transmission in kindergarten children.

Prevalence and characteristics of asymptomatic norovirus infection in the community in England

Norovirus is a major cause of infectious intestinal disease, and a substantial prevalence of asymptomatic infection has been reported. We describe the prevalence, seasonality and characteristics of asymptomatic norovirus infection in England. Healthy individuals were recruited at random from the general population during the Study of Infectious Intestinal Disease (1993-1996). Norovirus was identified using real-time RT-PCR. The age-adjusted prevalence of asymptomatic norovirus infection was 12%; prevalence was highest in children aged <5 years and showed wintertime seasonality. More work is needed to understand whether asymptomatic infections are important for norovirus transmission leading to sporadic illness and outbreaks.