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Purpose: Vaccine-associated enhanced disease (VAED) is a theoretical concern with new vaccines, although trials of authorized vaccines against SARS-CoV-2 have not identified markers for VAED. The purpose of this study was to detect any signals for VAED among adults vaccinated against coronavirus disease 2019 (COVID-19)., Methods: In this cross-sectional study, we assessed COVID-19 severity as a proxy for VAED among 400 adults hospitalized for COVID-19 from March through October 2021 at eight US healthcare systems. Primary outcomes were admission to an intensive care unit (ICU) and severe illness (score ≥6 on the World Health Organization [WHO] Clinical Progression Scale). We compared the risk of outcomes among those who had completed a COVID-19 vaccine primary series versus those who were unvaccinated. We incorporated inverse propensity weights for vaccination status in a doubly robust regression model to estimate the causal average treatment effect., Results: The causal risk ratio in vaccinated versus unvaccinated was 0.36 (95% confidence interval [CI], 0.15-0.94) for ICU admission and 0.46 (95% CI, 0.25-0.76) for severe illness., Conclusion: Among hospitalized patients, reduced disease severity in those vaccinated against COVID-19 supports the absence of VAED., (© 2024 John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

We describe humoral immune responses in 105 ambulatory patients with laboratory-confirmed SARS-CoV-2 Omicron variant infection. In dried blood spot (DBS) collected within 5 days of illness onset and during convalescence, we measured binding antibody (bAb) against ancestral spike protein receptor binding domain (RBD) and nucleocapsid (N) protein using a commercial multiplex bead assay. Geometric mean bAb concentrations against RBD increased by a factor of 2.5 from 1258 to 3189 units/mL and by a factor of 47 against N protein from 5.5 to 259 units/mL between acute illness and convalescence; lower concentrations were associated with greater geometric mean ratios. Paired DBS specimens may be used to evaluate humoral response to SARS-CoV-2 infection., (Published 2024. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Background: We estimated combined protection conferred by prior SARS-CoV-2 infection and COVID-19 vaccination against COVID-19-associated acute respiratory illness (ARI)., Methods: During SARS-CoV-2 Delta (B.1.617.2) and Omicron (B.1.1.529) variant circulation between October 2021 and April 2022, prospectively enrolled adult patients with outpatient ARI had respiratory and filter paper blood specimens collected for SARS-CoV-2 molecular testing and serology. Dried blood spots were tested for immunoglobulin-G antibodies against SARS-CoV-2 nucleocapsid (NP) and spike protein receptor binding domain antigen using a validated multiplex bead assay. Evidence of prior SARS-CoV-2 infection also included documented or self-reported laboratory-confirmed COVID-19. We used documented COVID-19 vaccination status to estimate vaccine effectiveness (VE) by multivariable logistic regression by prior infection status., Results: Four hundred fifty-five (29%) of 1577 participants tested positive for SARS-CoV-2 infection at enrollment; 209 (46%) case-patients and 637 (57%) test-negative patients were NP seropositive, had documented previous laboratory-confirmed COVID-19, or self-reported prior infection. Among previously uninfected patients, three-dose VE was 97% (95% confidence interval [CI], 60%-99%) against Delta, but not statistically significant against Omicron. Among previously infected patients, three-dose VE was 57% (CI, 20%-76%) against Omicron; VE against Delta could not be estimated., Conclusions: Three mRNA COVID-19 vaccine doses provided additional protection against SARS-CoV-2 Omicron variant-associated illness among previously infected participants., (© 2023 The Authors. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

Background: US recommendations for COVID-19 vaccine boosters have expanded in terms of age groups covered and numbers of doses recommended, whereas evolution of Omicron sublineages raises questions about ongoing vaccine effectiveness., Methods: We estimated effectiveness of monovalent COVID-19 mRNA booster vaccination versus two-dose primary series during a period of Omicron variant virus circulation in a community cohort with active illness surveillance. Hazard ratios comparing SARS-CoV-2 infection between booster versus primary series vaccinated individuals were estimated using Cox proportional hazards models with time-varying booster status. Models were adjusted for age and prior SARS-CoV-2 infection. The effectiveness of a second booster among adults ≥50 years of age was similarly estimated., Results: The analysis included 883 participants ranging in age, from 5 to >90 years. Relative effectiveness was 51% (95% CI: 34%, 64%) favoring the booster compared with primary series vaccination and did not vary by prior infection status. Relative effectiveness was 74% (95% CI: 57%, 84%) at 15 to 90 days after booster receipt, but declined to 42% (95% CI: 16%, 61%) after 91 to 180 days, and to 36% (95% CI: 3%, 58%) after 180 days. The relative effectiveness of a second booster compared to a single booster was 24% (95% CI: -40% to 61%)., Conclusions: An mRNA vaccine booster dose added significant protection against SARS-CoV-2 infection, but protection decreased over time. A second booster did not add significant protection for adults ≥50 years of age. Uptake of recommended bivalent boosters should be encouraged to increase protection against Omicron BA.4/BA.5 sublineages., Competing Interests: HQM and DLM report research support from Seqirus outside the submitted work. JKM reports research support from Quidel and Seqirus outside the submitted work. All other authors report no potential conflicts., (© 2023 The Authors. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Background: We estimated SARS-CoV-2 Delta- and Omicron-specific effectiveness of two and three mRNA COVID-19 vaccine doses in adults against symptomatic illness in US outpatient settings., Methods: Between October 1, 2021, and February 12, 2022, research staff consented and enrolled eligible participants who had fever, cough, or loss of taste or smell and sought outpatient medical care or clinical SARS-CoV-2 testing within 10 days of illness onset. Using the test-negative design, we compared the odds of receiving two or three mRNA COVID-19 vaccine doses among SARS-CoV-2 cases versus controls using logistic regression. Regression models were adjusted for study site, age, onset week, and prior SARS-CoV-2 infection. Vaccine effectiveness (VE) was calculated as (1 - adjusted odds ratio) × 100%., Results: Among 3847 participants included for analysis, 574 (32%) of 1775 tested positive for SARS-CoV-2 during the Delta predominant period and 1006 (56%) of 1794 participants tested positive during the Omicron predominant period. When Delta predominated, VE against symptomatic illness in outpatient settings was 63% (95% CI: 51% to 72%) among mRNA two-dose recipients and 96% (95% CI: 93% to 98%) for three-dose recipients. When Omicron predominated, VE was 21% (95% CI: -6% to 41%) among two-dose recipients and 62% (95% CI: 48% to 72%) among three-dose recipients., Conclusions: In this adult population, three mRNA COVID-19 vaccine doses provided substantial protection against symptomatic illness in outpatient settings when the Omicron variant became the predominant cause of COVID-19 in the United States. These findings support the recommendation for a third mRNA COVID-19 vaccine dose., Competing Interests: Ana Florea reports unrelated institutional grant support for research from Gilead, GlaxoSmithKline, Moderna, and Pfizer. Carlos G. Grijalva reports consulting fees from Merck, Pfizer, and Sanofi Pasteur, and institutional grant support from the Agency for Health Care Research and Quality, Campbell Alliance/Syneos Health, the Food and Drug Administration, and the National Institutes of Health. Emily T. Martin reports institutional grant support from Merck. Arnold S. Monto reports personal fees from Sanofi and non‐financial support from Seqirus. Mary Patricia Nowalk reports unrelated institutional grant support and personal fees from Merck Sharp & Dohme and institutional investigator‐initiated grant support from Sanofi Pasteur. Sara Y. Tartof reports unrelated institutional grant support from Pfizer and GlaxoSmithKline. No other potential conflicts of interest were disclosed., (Published 2022. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Reduced COVID-19 vaccine effectiveness (VE) has been observed with increasing predominance of SARS-CoV-2 Delta (B.1.617.2) variant. Two-dose VE against laboratory-confirmed SARS-CoV-2 infection (symptomatic and asymptomatic) was estimated using Cox proportional hazards models with time-varying vaccination status in a prospective rural community cohort of 1266 participants aged ≥12 years. Between November 3, 2020 and December 7, 2021, VE was 56% for mRNA COVID-19 vaccines overall, 65% for Moderna, and 50% for Pfizer-BioNTech. VE when Delta predominated (June to December 2021) was 54% for mRNA COVID-19 vaccines overall, 59% for Moderna, and 52% for Pfizer-BioNTech., (© 2022 The Authors. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Background: Individuals in contact with persons with COVID-19 are at high risk of developing COVID-19; protection offered by COVID-19 vaccines in the context of known exposure is poorly understood., Methods: Symptomatic outpatients aged ≥12 years reporting acute onset of COVID-19-like illness and tested for SARS-CoV-2 between February 1 and September 30, 2021 were enrolled. Participants were stratified by self-report of having known contact with a COVID-19 case in the 14 days prior to illness onset. Vaccine effectiveness was evaluated using the test-negative study design and multivariable logistic regression., Results: Among 2229 participants, 283/451 (63%) of those reporting contact and 331/1778 (19%) without known contact tested SARS-CoV-2-positive. Adjusted vaccine effectiveness was 71% (95% confidence interval [CI], 49%-83%) among fully vaccinated participants reporting a known contact versus 80% (95% CI, 72%-86%) among those with no known contact (p-value for interaction = 0.2)., Conclusions: This study contributes to growing evidence of the benefits of vaccinations in preventing COVID-19 and support vaccination recommendations and the importance of efforts to increase vaccination coverage., (© Published 2022. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Individuals with type 2 diabetes mellitus experience high rates of influenza virus infection and complications. We compared the magnitude and duration of serologic response to trivalent influenza vaccine in adults aged 50-80 with and without type 2 diabetes mellitus. Serologic response to influenza vaccination was similar in both groups: greater fold-increases in antibody titer occurred among participants with lower pre-vaccination antibody titers. Waning of antibody titers was not influenced by diabetes status., (Published 2021. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

The association of influenza vaccine and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection was assessed by test-negative design using data collected for a study of outpatient COVID-19-like illness with onset dates from June to September 2020. Multivariable logistic regression models examined the association between receipt of 2019-2020 influenza vaccine and PCR-confirmed SARS-CoV-2 with adjustment for potential confounders. Receipt of influenza vaccine during the 2019-2020 influenza season was not associated with increased odds of SARS-CoV-2 infection in adults (aOR 0.83, 95% CI 0.63 to 1.10) or children (aOR 0.92, 95% CI 0.47 to 1.80)., (© 2021 The Authors. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

We developed and evaluated a model to predict serious outcomes among 243 adults ≥60 years old with medically attended respiratory illness and laboratory-confirmed respiratory syncytial virus (RSV); 47 patients had a serious outcome defined as hospital admission, emergency department (ED) visit, or pneumonia diagnosis. The model used logistic regression with penalized maximum likelihood estimation. The reduced penalized model included age ≥ 75 years, ≥1 ED visit in prior year, crackles/rales, tachypnea, wheezing, new/increased sputum, and new/increased dyspnea. The optimal score cutoff yielded sensitivity and specificity of 66.0% and 81.6%. This prediction model provided moderate utility for identifying older adults with elevated risk of complicated RSV illness., (© 2020 The Authors. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

Background: Asthma was associated with influenza hospitalizations in children during the 2009 pandemic, but it is unclear if asthma is associated with serious illness during seasonal epidemics. Little is known regarding the effect of vaccination on influenza severity in children with asthma., Methods: Children aged 5-17 years in a community cohort presenting with acute respiratory illness were prospectively enrolled and tested for influenza from 2007-08 through 2017-18 (excluding the 2009-10 pandemic season). Data from the electronic health record were extracted to determine asthma status and serious outcomes associated with influenza infection. A serious outcome was defined as hospitalization, emergency department visit, and/or pneumonia diagnosis within 30 days of symptom onset. Multivariable logistic regression models were used to assess asthma status and effect of vaccination on odds of a serious outcome., Results: One thousand seven hundred and sixty four medically-attended influenza infections among school-aged children were included. Asthma was confirmed in 287 (16%) children. A serious influenza-associated outcome occurred in 104 (6%) children. The odds of a serious outcome did not differ between those with confirmed asthma and those without asthma [adjusted odds ratio (aOR): 1.35, 95% confidence interval (CI): (0.77-2.35), P = .3]. The effect of vaccination on serious outcomes was not modified by asthma status [aOR for children without asthma: 0.55 (95% CI: 0.28-1.07), children with asthma: 1.39 (95% CI: 0.53-3.69); interaction P-value = .12]., Conclusions: Asthma was not a risk factor for serious illness among children with influenza. Additional studies are needed to better understand the role of influenza vaccination in preventing serious outcomes among children with asthma., (© 2019 The Authors. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

Over five seasons, we determined the proportion of outpatients with laboratory-confirmed, influenza-associated illness who were hospitalized within 30 days following the outpatient visit. Overall, 136 (1.7%) of 7813 influenza-positive patients were hospitalized a median of 4 days after an outpatient visit. Patients aged ≥ 65 years and those with high-risk conditions were at increased risk of hospitalization. After controlling for age and high-risk conditions, vaccination status and infecting influenza virus type were not associated with hospitalization risk among adults., (Published 2018. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

Self-reported influenza vaccination is generally accurate for the current season, but the accuracy of self-report for vaccination in prior seasons is largely unknown. This study evaluated the accuracy of self-report for current and prior season influenza vaccination among patients with medically attended acute respiratory illness enrolled in a study of influenza vaccine effectiveness during the 2014-15 influenza season. It demonstrates there is a greater potential for exposure misclassification when prior season vaccinations are ascertained by self-report. Percent agreement between self-report and final status was high for both current and prior season vaccination: 97.7% and 93.2%, respectively., (© 2018 The Authors. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

Background: Population-based estimates of influenza-associated outpatient visits including both pandemic and interpandemic seasons are uncommon. Comparisons of such estimates with laboratory-confirmed rates of outpatient influenza are rare., Objective: To estimate influenza-associated outpatient visits in 6 US integrated healthcare delivery organizations enrolling ~7.7 million persons., Methods: Using negative binomial regression methods, we modeled rates of influenza-associated visits with ICD-9-CM-coded pneumonia or acute respiratory outpatient visits during 2001-10. These estimated counts were added to visits coded specifically for influenza to derive estimated rates. We compared these rates with those observed in 2 contemporaneous studies recording RT-PCR-confirmed influenza outpatient visits., Results: Outpatient rates estimated with pneumonia visits were 39 (95% confidence interval [CI], 30-70) and 203 (95% CI, 180-240) per 10 000 person-years, respectively, for interpandemic and pandemic seasons. Corresponding rates estimated with respiratory visits were 185 (95% CI, 161-255) and 542 (95% CI, 441-823) per 10 000 person-years. During the pandemic, children aged 2-17 years had the largest increase in rates (when estimated with pneumonia visits, from 64 [95% CI, 50-121] to 381 [95% CI, 366-481]). Rates estimated with pneumonia visits were consistent with rates of RT-PCR-confirmed influenza visits during 4 of 5 seasons in 1 comparison study. In another, rates estimated with pneumonia visits during the pandemic for children and adults were consistent in timing, peak, and magnitude., Conclusions: Estimated rates of influenza-associated outpatient visits were higher in children than adults during pre-pandemic and pandemic seasons. Rates estimated with pneumonia visits plus influenza-coded visits were similar to rates from studies using RT-PCR-confirmed influenza., (Published 2017. This article is a U.S. Government work and is in the public domain in the USA. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.)

Background: Acute respiratory illnesses (ARIs) are common in school-aged children, but few studies have assessed school absenteeism due to specific respiratory viruses., Objective: To evaluate school absenteeism among children with medically attended ARI due to common viruses., Methods: We analyzed follow-up surveys from children seeking care for acute respiratory illness who were enrolled in the influenza vaccine effectiveness study at Marshfield Clinic during the 2012-2013 through 2014-2015 influenza seasons. Archived influenza-negative respiratory swabs were retested using multiplex RT-PCR to detect 16 respiratory virus targets. Negative binomial and logistic regression models were used to examine the association between school absence and type of respiratory viruses; endpoints included mean days absent from school and prolonged (>2 days) absence. We examined the association between influenza vaccination and school absence among children with RT-PCR-confirmed influenza., Results: Among 1027 children, 2295 days of school were missed due to medically attended ARIs; influenza accounted for 39% of illness episodes and 47% of days missed. Mean days absent were highest for influenza (0.96-1.19) and lowest for coronavirus (0.62). Children with B/Yamagata infection were more likely to report prolonged absence than children with A/H1N1 or A/H3N2 infection [OR (95% CI): 2.1 (1.0, 4.5) and 1.7 (1.0, 2.9), respectively]. Among children with influenza, vaccination status was not associated with prolonged absence., Conclusions: School absenteeism due to medically attended ARIs varies by viral infection. Influenza B infections accounted for the greatest burden of absenteeism., (© 2016 The Authors. Influenza and Other Respiratory Viruses Published by John Wiley & Sons Ltd.)

Purpose: The changes in each year in influenza vaccine antigenic components as well as vaccine administration patterns may pose new risks of adverse events following immunization (AEs). To evaluate the safety of influenza vaccines annually administered to people ≥ 6 months, we conducted weekly post licensure surveillance for seven pre-specified adverse events following receipt of influenza vaccines during the 2013-2014 and 2014-2015 seasons in the Vaccine Safety Datalink (VSD)., Methods: We used both a historically-controlled cohort design with the Poisson-based maximized sequential probability ratio test (maxSPRT) and a self-controlled risk interval (SCRI) design with the binomial-based maxSPRT. For each adverse event outcome, we defined the risk interval on the basis of biologic plausibility and prior literature. For the historical cohort design, numbers of expected adverse events were calculated from the prior seven seasons, adjusted for age and site. For the SCRI design, a comparison window was defined either before vaccination or after vaccination, depending on each specific outcome., Results: An elevated risk of febrile seizures 0-1 days following trivalent inactivated influenza vaccine (IIV3) was identified in children aged 6-23 months during the 2014-2015 season using the SCRI design. We found the relative risk (RR) of febrile seizures following concomitant administration of IIV3 and PCV13 was 5.3 with a 95% CI 1.87-14.75. Without concomitant PCV 13 administration, the estimated risk decreased and was no longer statistically significant (RR: 1.4; CI: 0.54 - 3.61)., Conclusion: No increased risks, other than for febrile seizures, were identified in influenza vaccine safety surveillance during 2013-2014 and 2014-2015 seasons in the VSD. Copyright © 2016 John Wiley & Sons, Ltd., (Copyright © 2016 John Wiley & Sons, Ltd.)

Purpose: We conducted weekly surveillance for pre-specified adverse events following receipt of the 2012-2013 influenza vaccines in the Vaccine Safety Datalink (VSD)., Methods: For each outcome, risk intervals (i.e., period after vaccination with a potentially increased risk) were defined on the basis of biologic plausibility and prior literature. Seizures following inactivated influenza vaccine (IIV) were monitored in children in three age groups (6-23 months, 24-59 months, and 5-17 years) using a self-controlled risk interval design. We also monitored for Guillain-Barré syndrome, encephalitis, and anaphylaxis following IIV in patients ≥6 months of age using a cohort design with historical controls. In the risk intervals following live attenuated influenza vaccine (LAIV), we collected weekly counts of Guillain-Barré syndrome, encephalitis, and anaphylaxis in patients ages 2-49. Among LAIV vaccinees, numbers of expected events based on rates in historical controls were calculated, adjusted for age and site., Results: At the end of surveillance, approximately 3.6 million first doses of IIV and 250 000 first doses of LAIV had been administered in the VSD. No elevated risks were identified in risk intervals following 2012-2013 IIV, as compared with a self-matched control interval or to historical controls. For each outcome, fewer than three events occurred in the risk interval following 2012-2013 LAIV, and we thus were unable to estimate measures of relative risks., Conclusions: No increased risk was identified for any of the pre-specified outcomes following 2012-2013 influenza vaccinations in the VSD. Published 2014. This article is a U.S. Government work and is in the public domain in the USA., (Published 2014. This article is a U.S. Government work and is in the public domain in the USA.)

Background: The relationship between obesity and susceptibility to influenza infection in humans is unclear. Morbidly obese people were at an increased risk of complications from 2009 pandemic H1N1 influenza [A(H1N1)pdm09]., Objective: The goal of this study was to determine whether medically attended, laboratory-confirmed influenza is independently associated with obesity in adults with acute respiratory illness., Patients/methods:   Adults ≥20 years with a medical encounter for acute respiratory illness were recruited from a population cohort during the 2007-2008 (n = 903), 2008-2009 (n = 869), and 2009 pandemic (n = 851) season. Nasopharyngeal swabs were tested for influenza by real-time reverse-transcription polymerase chain reaction. Body mass index (BMI) was calculated using data from the electronic medical record. Logistic regression evaluated the association between influenza and obesity, adjusting for gender, vaccination, age, and high-risk medical condition., Results: Influenza was detected in 50% of patients in 2007-2008, 15% in 2008-2009, and 14% during the 2009 pandemic. Predominant seasonal viruses in this population were A/H3N2 in 2007-2008, and A/H1N1 and B in 2008-2009. Mean (±SD) BMI was 30·58 (±7·31) in patients with influenza and 30·93 (±7·55) in test-negative controls during all seasons. Mean BMI of patients with influenza did not vary by season. After adjusting for confounders, neither obesity nor extreme obesity were associated with influenza by season or for all years combined (OR 0·95: 95% CI 0·75, 1·20 and 1·10: 0·80, 1·52, respectively, for obesity and extreme obesity, all years)., Conclusions: Obesity was not associated with medically attended influenza among adults with acute respiratory illness in this population., (© 2012 Blackwell Publishing Ltd.)

Nineteen patients with oseltamivir-resistant seasonal influenza A (H1N1) infections were randomized to receive oseltamivir or placebo. Nasopharyngeal swabs were obtained, and clinical and virologic outcomes were compared, stratified by early or late treatment. Neuraminidase inhibition assay and pyrosequencing for H275Y confirmed resistance. Twelve (63%) patients received oseltamivir; 8 (67%) received late treatment. Seven (37%) patients received placebo; 6 (86%) presented >48 hours after onset. Time to 50% decrease in symptom severity, complete symptom resolution, and first negative culture were shortest among the early treatment group. While sample size prohibits a strong conclusion, future studies should evaluate for similar trends., (© 2011 Blackwell Publishing Ltd.)

Background: There are few prospectively collected data comparing illnesses caused by different subtypes of influenza. We compared the clinical presentation and outcomes of subjects with primarily outpatient-attended influenza A and B infections during four consecutive influenza seasons (2004-2005 through 2007-2008)., Methods: Patients were prospectively enrolled and tested for influenza following an encounter for acute respiratory illness. Influenza infections were confirmed by culture or reverse transcription polymerase chain reaction; subtype was determined for a sample of influenza A isolates each season. Clinical characteristics of influenza A and B infections were compared across and within individual seasons., Results: We identified 901 cases of influenza A and 284 cases of influenza B; 98% of cases were identified through an outpatient medical encounter. Thirty-six percent of patients with each strain had received seasonal influenza vaccine prior to illness onset. There were no consistent differences in symptoms associated with influenza A and B. Influenza A infection was associated with earlier care seeking compared with influenza B during the 2005-2006 and 2007-2008 seasons, when H3N2 was the dominant type A virus, and in a combined analysis that included all seasons. Twenty-six (2·2%) of 1185 cases were diagnosed with radiographically confirmed pneumonia, and 59 (5%) of 1185 patients were hospitalized within 30 days of illness onset., Conclusions: Over four influenza seasons, aside from shorter intervals from illness onset to clinical encounter for infections with the A(H3N2) subtype, clinical symptoms and outcomes were similar for patients with predominantly outpatient-attended influenza A and B infections., (© 2011 Blackwell Publishing Ltd.)