Laboratory-confirmed Pandemic Influenza Research Articles

PANDEMIC INFLUENZA A (H1N1) 2009 PRESENTING AS A MILD DISEASE IN CHILDREN IN A CROATIAN CLINICAL CENTRE.

SUMMARYPandemic influenza A virus (H1N1) 2009 causes a disease that is epidemiologically and clinically not significantly different from seasonal influenza, but there are differences. The aim of the study was to display and compare epidemiological and clinical characteristics of pandemic influenza in children. At Dr. Fran Mihaljević University Hospital for Infectious Diseases in Zagreb, in the first two seasons, the incidence of pandemic influenza virus A (H1N1) in particular was exhaustively analyzed only in patients with laboratory-confirmed pandemic influenza A virus (H1N1) 2009. In hospitalized children with documented influenza pandemic, moderate form of the disease predominated, which ultimately meant shorter hospital stay and fewer complications. Otitis media was the rarest complication in children in both seasons. In conclusion, children younger than 5 years, especially boys, were vulnerable groups for pandemic influenza, presenting as a mild disease with low mortality and few complications. Most of the affected children with influenza did not have important risk factors such as asthma and obesity, highlighted by other authors as significant risk factors.

Pandemic influenza and subsequent risk of type 1 diabetes: a nationwide cohort study

Aims/hypothesisCase reports have linked influenza infections to the development of type 1 diabetes. We investigated whether pandemic and seasonal influenza infections were associated with subsequent increased risk of type 1 diabetes.MethodsIn this population-based registry study, we linked individual-level data from national health registries for the entire Norwegian population under the age of 30 years for the years 2006–2014 (2.5 million individuals). Data were obtained from the National Registry (population data), the Norwegian Patient Registry (data on inpatient and outpatient specialist care), the Primary Care Database, the Norwegian Prescription Database and the Norwegian Surveillance System for Communicable Diseases. Pandemic influenza was defined as either a clinical influenza diagnosis during the main pandemic period or a laboratory-confirmed test. Seasonal influenza was defined by a clinical diagnosis of influenza between 2006 and 2014. We used Cox regression to estimate HRs for new-onset type 1 diabetes after an influenza infection, adjusted for year of birth, sex, place of birth and education.ResultsThe adjusted HR for type 1 diabetes after pandemic influenza infection was 1.19 (95% CI 0.97, 1.46). In the subgroup with laboratory-confirmed influenza A (H1N1), influenza was associated with a twofold higher risk of subsequent type 1 diabetes before age 30 years (adjusted HR: 2.26, 95% CI 1.51, 3.38).Conclusions/interpretationOverall, we could not demonstrate a clear association between clinically reported pandemic influenza infection and incident type 1 diabetes. However, we found a twofold excess of incident diabetes in the subgroup with laboratory-confirmed pandemic influenza A (H1N1).

Risk factors for severe outcome of cases with pandemic influenza A(H1N1)pdm09.

The aim of this study is to describe demographic, clinical and epidemiological characteristics of cases with laboratory-confirmed pandemic influenza virus A(H1N1)pdm09 reported in Slovakia from May 28, 2009 to December 30, 2009 and analyse the association between risk factors and severe outcome of these cases. In the spring of 2009, an outbreak of a pandemic influenza virus A(H1N1)pdm09, emerged in Mexico and spread globally. Until December 2009, 1,014 cases were notified in Slovakia. The data were collected within national influenza surveillance system. Odds ratios (95% CI) were calculated. Associations were found to be significantly associated with the worse outcome (p < 0.05) in the univariate analysis and were adjusted for possible effects of age and sex by using a logistic regression model. Out of the total number of 1,014 cases, 131 (12.9 %) cases were hospitalized, and 43 (4.2 %) of those were admitted to intensive care units. During the reporting period, 38 deaths were reported, representing a case fatality rate of 3.75 %. The median age of severe cases (35 years, IQR = 29 y) was significantly higher than the median age of mild cases (24 years, IQR = 19 y; p < 0.001). By using a logistic regression, we found out that chronic obstructive pulmonary disease (COPD) (aOR = 9.2; 95%CI: 1.42-59.98), cardiovascular diseases (aOR = 14.97; 95%CI: 5.49-40.79), malignity (aOR = 7.6; 95%CI: 1.95-29.37) and gravidity (aOR = 55.21; 95% CI: 14.40-211.58) were significantly associated with severe outcomes of the cases. The fact, that 35% of severely ill patients did not report any risk factor suggests the importance of vaccination as a prevention of influenza (Tab. 2, Fig. 1, Ref. 18). Text in PDF www.elis.sk.

The ethics of sharing preliminary research findings during public health emergencies: a case study from the 2009 influenza pandemic.

During the 2009 A(H1N1) influenza pandemic, a suite of studies conducted in Canada showed an unexpected finding, that patients with medically attended laboratory-confirmed pandemic influenza were more likely to have received seasonal influenza vaccination than test-negative control patients. Different bodies, including scientific journals and government scientific advisory committees, reviewed the evidence simultaneously to determine its scientific validity and implications. Decision-making was complicated when the findings made their way into the media. The normal trajectory of non-urgent research includes peer-review publication after which decision-makers can process the information taking into account other evidence and logistic considerations. In the situation that arose, however, the congruence of an unexpected finding and the simultaneous review of the evidence both within and outside the traditional peer-review sphere raised several interesting issues about how to deal with emerging evidence during a public health emergency. These events are used in this article to aid discussion of the complex interrelationship between researchers, public health decision-makers and scientific journals, the trade-offs between sharing information early and maintaining the peer-review quality assurance process, and to emphasise the need for critical reflection on the practical and ethical norms that govern the way in which research is evaluated, published and communicated in public health emergencies.

Commentary

Commentary

Comparison of five influenza surveillance systems during the 2009 pandemic and their association with media attention

BackgroundDuring the 2009 influenza pandemic period, routine surveillance of influenza-like-illness (ILI) was conducted in The Netherlands by a network of sentinel general practitioners (GPs). In addition during the pandemic period, four other ILI/influenza surveillance systems existed. For pandemic preparedness, we evaluated the performance of the sentinel system and the others to assess which of the four could be useful additions in the future. We also assessed whether performance of the five systems was influenced by media reports during the pandemic period.MethodsThe trends in ILI consultation rates reported by sentinel GPs from 20 April 2009 through 3 January 2010 were compared with trends in data from the other systems: ILI cases self-reported through the web-based Great Influenza Survey (GIS); influenza-related web searches through Google Flu Trends (GFT); patients admitted to hospital with laboratory-confirmed pandemic influenza, and detections of influenza virus by laboratories. In addition, correlations were determined between ILI consultation rates of the sentinel GPs and data from the four other systems. We also compared the trends of the five surveillance systems with trends in pandemic-related newspaper and television coverage and determined correlation coefficients with and without time lags.ResultsThe four other systems showed similar trends and had strong correlations with the ILI consultation rates reported by sentinel GPs. The number of influenza virus detections was the only system to register a summer peak. Increases in the number of newspaper articles and television broadcasts did not precede increases in activity among the five surveillance systems.ConclusionsThe sentinel general practice network should remain the basis of influenza surveillance, as it integrates epidemiological and virological information and was able to maintain stability and continuity under pandemic pressure. Hospital and virological data are important during a pandemic, tracking the severity, molecular and phenotypic characterization of the viruses and confirming whether ILI incidence is truly related to influenza virus infections. GIS showed that web-based, self-reported ILI can be a useful addition, especially if virological self-sampling is added and an epidemic threshold could be determined. GFT showed negligible added value.

Effectiveness of Nonadjuvanted Monovalent Influenza A(H1N1)pdm09 Vaccines for Preventing Reverse Transcription Polymerase Chain Reaction–Confirmed Pandemic Influenza Hospitalizations: Case-Control Study of Children and Adults at 10 US Influenza Surveillance Network Sites

During 2009–2010, we examined 217 patients hospitalized with laboratory-confirmed pandemic influenza in 9 Influenza Hospitalization Surveillance Network sites and 413 age- and community-matched controls and found that a single dose of monovalent nonadjuvanted influenza A(H1N1)pdm09 vaccine was 50% (95% confidence interval, 13%–71%) effective in preventing hospitalization associated with A(H1N1)pdm09 virus infection.

Different prognosis in hospitalized patients with influenza one season after the pandemic H1N1 influenza of 2009-2010 in Spain.

The present report compares prognosis in hospitalized cases with the H1N1 pandemic virus in two seasons. Two series of hospitalized patients with laboratory-confirmed H1N1 pandemic influenza have been compared: 813 in the season 2009-2010 and 707 in the season 2010-2011. A detailed history of variables preceding hospital admission and during hospitalization was obtained by interview and clinical charts. A combined endpoint of death admission to intensive care was used as outcome due to the low number of deaths. Logistic regression was applied in the analysis for adverse outcome. Patients of the second season had different characteristics than in the first one (older, more underlying conditions, more malfunctioning organs and more symptoms). Patients with H1N1 pandemic virus when hospitalized were more frequently directly admitted to ICU during the 2010-2011 season than in the previous season (RR=2·10; 95% confidence intervals CI, 1·55-2·85), as a consequence of a higher presence of sepsis and respiratory distress. These patients also showed during hospitalization a higher risk of ICU admission or death (RR=3·22, 95% CI, 2·15-4·83). After adjusting for the differences in risk factors of adverse outcome, patients in the second season showed a higher risk of ICU admission and/or in-hospital death odds ratio (OR=3·77, 95% CI, 2·30-6·18). Hospitalized patients with H1N1 pandemic influenza during the second season were more severely affected at hospital admission and showed a worse prognosis than in previous season, independently of the differences found at hospital admission.

Laboratory-Confirmed Pandemic H1N1 Influenza in Hospitalized Adults: Findings from the Canadian Nosocomial Infections Surveillance Program, 2009-2010

Surveillance for pandemic H1N1 influenza was conducted between June 1, 2009, and May 31, 2010, among adults at 40 participating hospitals in the Canadian Nosocomial Infection Surveillance Program. The first wave was characterized by a higher proportion of Aboriginals and pregnant women as well as severe outcomes, compared to the second wave.

Influenza‐associated bacterial pathogens in patients with 2009 influenza A (H1N1) infection: impact of community‐associated methicillin‐resistant Staphylococcus aureus in Queensland, Australia

Secondary bacterial pneumonia due to community onset methicillin-resistant Staphylococcus aureus (MRSA) has become a highly publicised cause of influenza-associated death. There is a risk that case reports of fatal outcomes with post-influenza MRSA pneumonia may unduly influence antibiotic prescribing. The aim of this study was to demonstrate the incidence of community-onset MRSA pneumonia in 2009 H1N1 influenza patients. The microbiology records of patients positive for influenza A (H1N1) in 2009 were reviewed for positive blood or respiratory tract cultures and urinary pneumococcal antigen results within a Queensland database. Patients with such positive results within 48 h of hospital admission and a positive H1N1 influenza result in the prior 6 weeks were included. In 2009, 4491 laboratory-confirmed pandemic influenza A (H1N1) infections were detected. Fifty patients (1.1% of the H1N1 cohort) who were hospitalised with H1N1 and who had a bacterial respiratory tract pathogen were identified. Streptococcus pneumoniae (16 patients; 32%), Staphylococcus aureus (13 patients; 26%) and Haemophilus influenzae (9 patients; 18%) were the most commonly cultured organisms. Of the cohort of 4491 patients, MRSA was detected in only two patients, both of whom were admitted to intensive care units and survived after prolonged admissions. Influenza-associated community-onset MRSA pneumonia was infrequently identified in the 2009 H1N1 season in Queensland, despite community-onset MRSA skin and soft tissue infections being very common. Although post-influenza MRSA pneumonia is of great concern, its influence on empiric-prescribing guidelines should take into account its incidence relative to other secondary bacterial pathogens.

Clinical characteristics of pediatric hospitalizations associated with 2009 pandemic influenza A (H1N1) in Northern Bavaria, Germany

BackgroundThe 2009 pandemic influenza A (H1N1) (PIA) virus infected large parts of the pediatric population with a wide clinical spectrum and an initially unknown complication rate. The aims of our study were to define clinical characteristics and outcome of pandemic influenza A (H1N1) 2009-associated hospitalizations (PIAH) in children <18 years of age. All hospitalized cases of children <18 years of age with laboratory-confirmed pandemic influenza A (H1N1) 2009 in the region of Wuerzburg (Northern Bavaria, Germany) between July 2009 and March 2010 were identified. For these children a medical chart review was performed to determine their clinical characteristics and complications.ResultsBetween July 2009 and March 2010, 94 PIAH (62% males) occurred in children <18 years of age, with a median age of 7 years (IQR: 3–12 years). Underlying diseases and predisposing factors were documented in 40 (43%) children; obesity (n = 12, 30%), asthma (n = 10, 25%) and neurologic disorders (n = 8, 20%) were most frequently reported. Sixteen (17%) children received oxygen supplementation; three (3%) children required mechanical ventilation. Six (6%) children were admitted to an intensive care unit, four of them with underlying chronic diseases.ConclusionsMost PIAH demonstrated a benign course of disease. However, six children (6%) needed treatment at an intensive care unit for severe complications.

Usefulness of health registries when estimating vaccine effectiveness during the influenza A(H1N1)pdm09 pandemic in Norway

BackgroundDuring the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm09, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). 2.2 million persons (45% of the population) were vaccinated with an AS03-adjuvanted monovalent vaccine during the pandemic. Most of them were registered in the Norwegian Immunisation Registry (SYSVAK). Based on these registries, we aimed at estimating the vaccine effectiveness (VE) and describing vaccine failures during the pandemic in Norway, in order to evaluate the role of the vaccine as a preventive measure during the pandemic.MethodsWe conducted a population-based retrospective cohort study, linking MSIS and SYSVAK with pandemic influenza vaccination as exposure and laboratory-confirmed pandemic influenza as outcome. We measured VE by week and defined two thresholds for immunity; eight and 15 days after vaccination.ResultsThe weekly VE ranged from 77% to 96% when considering 15 days or more after vaccination as the threshold of immunity and from 73% to 94% when considering eight days or more. Overall, 157 individuals contracted pandemic influenza eight or more days after vaccination (8.4/100,000 vaccinated), of these 58 had onset 15 days or more after vaccination (3.0/100,000 vaccinated). Most of the vaccine failures occurred during the first weeks of the vaccination campaign. More than 30% of the vaccine failures were found in people below 10 years of age.ConclusionsHaving available health registries with data regarding cases of specific disease and vaccination makes it feasible to estimate VE in a simple and rapid way. VE was high regardless the immunity threshold chosen. We encourage public health authorities in other countries to set up such registries. It is also important to consider including information on underlying diseases in registries already existing, in order to make it feasible to conduct more complete VE estimations.

Systematic review of clinical and epidemiological features of the pandemic influenza A (H1N1) 2009

Please cite this paper as: Khandaker et al. (2011) Systematic review of clinical and epidemiological features of the pandemic influenza A (H1N1) 2009. Influenza and Other Respiratory Viruses DOI: 10.1111/j.1750‐2659.2011.00199.x.The aim of this systematic review was to summarise the clinical and epidemiological features of the pandemic influenza A (H1N1) 2009. We did a systematic search of published literature reporting clinical features of laboratory‐confirmed pandemic influenza A (H1N1) 2009 from 1 April 2009 to 31 January 2010. Forty‐four articles met our inclusion criteria for the review. The calculated weighted mean age of confirmed cases was 18·1 years, with the median ranging from 12 to 44 years. Cough (84·9%), fever (84·7%), headache (66·5%), runny nose (60·1%) and muscle pain (58·1%) were the most common symptoms of confirmed cases. One or more pre‐existing chronic medical conditions were found in 18·4% of cases. Almost two‐thirds (64%) of cases were aged between 10 and 29 years, 5·1% were aged over 50 years and only 1·1% were aged over 60 years. The confirmed case fatality ratio was 2·9% (95% CI 0·0–6·7%), an extracted average from 12 of 42 studies reporting fatal cases (937 fatal cases among 31 980 confirmed cases), which gives an overall estimated infected case fatality ratio of 0·02%. Early in the pandemic, disease occurred overwhelmingly in children and younger adults, with cough and fever as the most prevalent clinical symptoms of the confirmed cases. A high infection rate in children and young adults, with sparing of the elderly population, has implications for pandemic influenza management and control policies.

Effect of immunomodulatory therapies in patients with pandemic influenza A (H1N1) 2009 complicated by pneumonia

To determine the effect of immunomodulatory therapies on the development of severe disease in hospitalized adults with laboratory-confirmed pandemic influenza A (H1N1) 2009 complicated by pneumonia. Observational, prospective cohort study at thirteen tertiary hospitals in Spain. The use of corticosteroids, macrolides and statins was recorded. The outcome of interest was severe disease, defined as the composite of intensive care unit admission or death after the first day of hospitalization. Of the 197 patients with pandemic influenza A (H1N1) 2009 complicated by pneumonia, 68 (34.5%) received some anti-inflammatory therapy since hospital admission (corticosteroids in 37, macrolides in 31 and statins in 12). Severe disease occurred in 29 (14.7%) patients. After adjustment for confounding factors, immunomodulatory therapies as a group were not associated with a lower risk for developing severe disease (odds ratio [OR] 0.64; 95% confidence interval [CI] 0.22-1.86). In a further a priori analysis, corticosteroids, macrolides and statins were included in a multivariate model. None of these therapies was found to be associated with a lower risk for developing severe disease. Immunomodulatory therapies use since hospital admission did not prevent the development of severe disease in adults with pandemic influenza A (H1N1) 2009 complicated by pneumonia.

Influenza pandemic (H1N1) 2009 activity during summer 2009. Effectiveness of the 2008-9 trivalent vaccine against pandemic influenza in Spain

Introduction The Spanish influenza surveillance system (SISS) maintained its activity during the summer of 2009 to monitor the influenza pandemic. Objectives To describe pandemic influenza activity from May to September 2009 and to estimate the effectiveness of the 2008-9 seasonal influenza vaccine against laboratory-confirmed pandemic (H1N1) 2009 influenza. Methods Data from the SISS were used to identify the trend of pandemic (H1N1) 2009 influenza outside the influenza season. For the effectiveness study, we compared the vaccination status of notified cases [influenza-like illnesses (ILI) laboratory confirmed as pandemic influenza] with that of the test-negative controls. Results The first laboratory-confirmed case of the pandemic virus was notified in the system in week 20/2009. The ILI rate increased gradually in the study period, exceeding basic activity in week 38. The proportion of pandemic (H1N1) 2009 influenza viruses detected by the system represented 14% in week 20/2009 and rapidly increased to 90% in week 34. The adjusted vaccine effectiveness of the 2008-9 seasonal vaccine against laboratory-confirmed pandemic influenza was 12% (-30; 41). Conclusions The SISS became an essential tool for pandemic monitoring in Spain. The improved SISS will provide more accurate information on influenza activity in future seasonal or pandemic waves. Using surveillance data, we could not demonstrate the effectiveness of the seasonal 2008-9 vaccine against laboratory-confirmed pandemic influenza.

Epidemiological characteristics of influenza A(H1N1) 2009 pandemic in the Western Pacific Region

The first laboratory-confirmed cases of infection with pandemic influenza A(H1N1) 2009 in the Western Pacific Region were reported on 28 April 2009. By 11 June 2009, the day the pandemic was declared by the World Health Organization, nine Western Pacific Region countries and areas had reported laboratory confirmed pandemic influenza A(H1N1) 2009 cases. From April 2009 to July 2010, more than 250000 cases and 1800 deaths from laboratory-confirmed pandemic influenza A(H1N1) 2009 were reported from 34 countries and areas in the Region. By age group region-wide, 8.6%, 41.9%, 48.3%, and 1.2% of cases were in the <5years, 5-14 years, 15-64 years, and 65+ years age groups, respectively; the overall crude case fatality ratio in the Western Pacific Region was 0.5%. The pandemic demonstrated that region-wide disease reporting was possible. Countries and areas of the Western Pacific Region should take this opportunity to strengthen the systems established during the pandemic to develop routine disease reporting.

Hospitalization in two waves of pandemic influenza A(H1N1) in England

Uncertainties exist regarding the population risks of hospitalization due to pandemic influenza A(H1N1). Understanding these risks is important for patients, clinicians and policy makers. This study aimed to clarify these uncertainties. A national surveillance system was established for patients hospitalized with laboratory-confirmed pandemic influenza A(H1N1) in England. Information was captured on demographics, pre-existing conditions, treatment and outcomes. The relative risks of hospitalization associated with pre-existing conditions were estimated by combining the captured data with population prevalence estimates. A total of 2416 hospitalizations were reported up to 6 January 2010. Within the population, 4·7 people/100,000 were hospitalized with pandemic influenza A(H1N1). The estimated hospitalization rate of cases showed a U-shaped distribution with age. Chronic kidney disease, chronic neurological disease, chronic respiratory disease and immunosuppression were each associated with a 10- to 20-fold increased risk of hospitalization. Patients who received antiviral medication within 48 h of symptom onset were less likely to be admitted to critical care than those who received them after this time (adjusted odds ratio 0·64, 95% confidence interval 0·44-0·94, P=0·024). In England the risk of hospitalization with pandemic influenza A(H1N1) has been concentrated in the young and those with pre-existing conditions. By quantifying these risks, this study will prove useful in planning for the next winter in the northern and southern hemispheres, and for future pandemics.

Prevalence and clinical features of pneumonia in patients with laboratory-confirmed pandemic influenza A H1N1 2009 infection in South Korea

During the early stages of the pandemic influenza A H1N1 2009 (pH1N1) outbreak in South Korea, the government recommended antiviral therapy with laboratory confirmation in specialized hospitals. Hence we had a unique opportunity to test all patients suspected to have pH1N1, including those who initially presented with a mild illness, such as those who were not in at-risk groups and who had an uncomplicated illness. We therefore evaluated the proportion and clinical features of pneumonia in patients with laboratory-confirmed pH1N1. Of the 3253 patients who visited Asan Medical Center, Seoul, South Korea, between 24 August and 19 October 2009 for a suspected pH1N1 infection (temperature ≥ 37.5°C and at least 1 of the following symptoms: sore throat, cough, rhinorrhoea and nasal congestion), 553 (17%) were positive for pH1N1 by real-time reverse-transcriptase polymerase chain reaction. Chest radiographs were performed in 96 (17%) of the 553 patients. Of the 553 patients, 30 (5%; 95% confidence interval (CI) 4-8%) had pneumonia, including 26 (86%) with viral pneumonia and 4 (14%) with bacterial pneumonia. So, the proportion of pneumonia in patients with laboratory-confirmed pH1N1 was at least 5%. Multivariate analysis indicated that dyspnoea (odds ratio (OR) 57.8, p < 0.001), wheezing (OR 19.3, p = 0.02), vomiting (OR 18.5, p < 0.001) and diarrhoea (OR 11.0, p = 0.001) were independently associated with pneumonia. Antiviral therapy at >48 h after the onset of symptoms (OR 2.1, p = 0.09) tended to be more common in patients with pneumonia than in those without pneumonia.

Epidemiological, Demographic, and Molecular Characteristics of Laboratory-Confirmed Pandemic Influenza A (H1N1) Virus Infection in Turkey, May 15–November 30, 2009

A total of 19,973 clinical specimens obtained from suspected cases of pandemic influenza A virus infection were analyzed by real-time reverse transcription-polymerase chain reaction. Mutations in hemagglutinin (HA) gene and alteration at position 275 in neuraminidase (NA) gene of the randomly selected 29 isolates were detected by sequencing analysis. The virus RNA was detected in 47.3% of the clinical specimens. The pandemic flu cases increased from the 42nd week and peaked in the 46th week of 2009. This intensity continued to the end of the study period. Pandemic flu mainly affected children in the 5-14 year age group, without any gender predominance. The analyzed strains had >98.9% homology with vaccine strains and with each other. More than 37% of the isolates had mutation at position D222E/N on HA gene. There was no isolate harbored mutation at the position H275Y of the NA gene, indicating that the virus isolates currently circulating in Turkey are sensitive to oseltamivir.

Clinical Features, Risk Factors, and Complications Among Pediatric Patients With Pandemic Influenza A (H1N1)

The authors aimed to describe the epidemiological characteristics, clinical features, risk factors for severe disease, and complications in children with laboratory-confirmed pandemic influenza A (H1N1). H1N1 was confirmed by performing reverse-transcriptase polymerase chain reaction (RT-PCR) assay on oropharyngeal swab specimens. The medical charts of a subset of the evaluated patients were reviewed retrospectively; another subset was enrolled prospectively. A total of 51 patients (44 [86%] > 5 years) were identified to have laboratory-confirmed H1N1. Fever was the most common presenting symptom (92%). Of the 15 hospitalized patients, 4 had asthma, and 5 were overweight or obese. All but 1 of these 9 patients developed influenza-related complications. Overall, 10 of the 15 hospitalized patients (67%) developed an influenza-related complication (6 bronchitis and 4 pneumonia). In this cohort, most children with confirmed H1N1 infection experience an uncomplicated viral illness. Nevertheless, underlying asthma and obesity may aggravate their clinical course.