2009 pandemic Influenza A (H1N1): clinical and laboratory characteristics in pediatric and adult patients and in patients with pulmonary involvement

Please cite this paper as: Lee and Liu et al. (2012) 2009 pandemic Influenza A (H1N1): clinical and laboratory characteristics in pediatric and adult patients and in patients with pulmonary involvement. Influenza and Other Respiratory Viruses 6(601), e152–e161. Background To better understand clinical and laboratory characteristics in children, adults, and patients with lung involvement suffering 2009 pandemic influenza A (H1N1). Methods A total of 442 patients with 2009 pandemic influenza A (H1N1) were retrospectively analyzed. Results Comparing to their adult counterpart (n = 55), pediatric patients (n = 387) had significantly higher frequencies of fever, rhinorrhea, cough, sore throat, nausea/vomiting, and longer length of fever; lower frequencies of chest pain and dyspnea; higher incidence of lymphopenia; and lower incidence of elevated serum C‐reactive protein. Among the 227 patients with radiographs available, lung involvement was found in 19 (8·4%) (52·6% consolidation and 47·4% interstitial infiltrations), including 18 children and one adult. One child with lung consolidation died of multiorgan failure. Significant findings in patients with lung involvement included predominant young age (≤10 years), prolonged fever, and delayed oseltamivir therapy (≥48 hours after onset of illness); higher frequencies of dyspnea, nausea/vomiting, and altered consciousness; and higher incidences of leukopenia, elevated serum creative kinase, and lactic dehydrogenase. Conclusions Among patients with 2009 pandemic influenza A (H1N1), we found significant difference in clinical manifestations between children and adults, and significant differences in clinical and laboratory manifestations between patients with lung involvement and those without. On the basis of data from this study and the existing literature, early treatment with oseltamivir is recommended for patients with 2009 pandemic influenza A (H1N1), regardless of age.     

Comparisons of oseltamivir‐resistant (H275Y) and concurrent oseltamivir‐susceptible seasonal influenza A(H1N1) virus infections in hospitalized adults, 2008–2009

In an observational cohort study, we found that adults hospitalized for oseltamivir‐resistant (H275Y) seasonal H1N1 influenza (n = 46) were older than those infected with oseltamivir‐susceptible strains (n = 31) [74(IQR 59–83) versus 64(IQR 48–76) years; P = 0·045], and most had major comorbidities (78% versus 65%). Disease severity and clinical outcomes were comparable between the two groups: radiographic pneumonia 40–42%, supplemental oxygen use 47–48%, critical illness 11–13%, median duration of hospitalization 5–6 days, death rate 6–9%. Failure to receive effective antiviral therapy was associated with progression to critical illness (23% versus 0%, P = 0·016) and death (20% versus 0%, P = 0·033) in hospitalized patients with seasonal H1N1 influenza.     

Pandemic (H1N1) 2009 influenza in Canadian pediatric cancer and hematopoietic stem cell transplant patients

Please cite this paper as: Tran et al. (2012) Pandemic (H1N1) 2009 influenza in Canadian pediatric cancer and hematopoietic stem cell transplant patients. Influenza and Other Respiratory Viruses 6(601), e105–e113. Background The impact of pandemic H1N1 influenza (pH1N1) virus in pediatric cancer is uncertain. The objectives of this study were to characterize the clinical course of pH1N1 and identify factors associated with severe outcomes. Methods We conducted a Canadian multicenter retrospective review of children with cancer and stem cell transplant (SCT) recipients who were diagnosed with laboratory‐confirmed pH1N1 infection between May 1, 2009 and January 31, 2010. Results We identified 100 (19 in wave 1 and 81 in wave 2) cases of pH1N1 infection. Median age was 8·7 years. 71% had a hematologic malignancy, and 20% received SCT. Median duration of fever and illness was 2 and 12·5 days, respectively. 51 (51·5%) were hospitalized for a median of 5 days, with no deaths and only 1 requiring admission to the intensive care unit. Radiologically confirmed pneumonia was diagnosed in 10 (10%). Interruption of chemotherapy or conditioning occurred in 43 patients. In multivariable analyses, age <5 years (relative to ≥10 years) and neutropenia were associated with hospitalization while neutropenia was associated with pneumonia. Despite oseltamivir use in 89%, viral shedding was prolonged (median, 46 days) and often persisted after symptom resolution. However, an extended treatment course (>5 days) correlated with shortened duration of viral shedding (P = 0·041). Conclusions pH1N1 infection in pediatric cancer and SCT patients infrequently caused complications but commonly interrupted cancer treatment. Persistent shedding of virus after illness resolution was common. Further research is needed to verify this finding as it could have implications for treatment guidelines and infection control practices.     

Clinical features, complications and mortality in critically ill patients with 2009 influenza A(H1N1) in Sfax,Tunisia

Please cite this paper as: Damak et al.(2011) Clinical features, complications and mortality in critically ill patients with 2009 influenza A(H1N1) in Sfax,Tunisia. Influenza and Other Respiratory Viruses 5(4), 230–240 Purpose Africa, as the rest of the world, was touched by the 2009 pandemic influenza A(H1N1). In the literature, a few publications covering this subject emerged from this continent. We prospectively describe baseline characteristics, treatment and outcomes of consecutive critically ill patients with confirmed 2009 influenza A(H1N1) in the intensive care unit (ICU) of Sfax hospital. Methods From 29 November 2009 through 21 January 2010, 32 patients with confirmed 2009 influenza A(H1N1) were admitted to our ICU. We prospectively analysed data and outcomes of these patients and compared survivors and dead patients to identify any predictors of death. Results Patients were young (mean, 36·1 [SD], 20·7 years) and 21 (65·6%) of whom had co‐morbidities. During ICU care, 29 (90·6%) patients had respiratory failure; among these, 15 (46·9%) patients required invasive ventilation with a median duration of 9 (IQR 3–12) days. In our experience, respiratory dysfunction can remain isolated but may also be associated with other dysfunctions or complications, such as, septic shock, seizures, myasthenia gravis exacerbation, Guillan–Barre syndrome, acute renal failure, nosocomial infections and biological disturbances. The nine patients (28·1%) who died had greater initial severity of illness (SAPS II and sequential organ failure assessment (SOFA) scores) but also a higher SOFA score and increasing severity of organ dysfunction during their ICU evolution. Conclusion Critical illness from the 2009 influenza A(H1N1) in Sfax occurred in young individuals and was associated with severe acute respiratory and additional organ system failure. SAPS II and SOFA scores at ICU admission, and also during evolution, constitute a good predictor of death.     

Outcome of pandemic H1N1 infections in hematopoietic stem cell transplant recipients

During 2009, a new strain of A/H1N1 influenza appeared and became pandemic. A prospective study was performed to collect data regarding risk factors and outcome of A/H1N1 in hematopoietic stem cell transplant recipients. Only verified pandemic A/H1N1 influenza strains were included: 286 patients were reported, 222 allogeneic and 64 autologous recipients. The median age was 38.3 years and the median time from transplant was 19.4 months. Oseltamivir was administered to 267 patients and 15 patients received zanamivir. One hundred and twenty-five patients (43.7%) were hospitalized. Ninety-three patients (32.5%) developed lower respiratory tract disease. In multivariate analysis, risk factors were age (OR 1.025; 1.01-1.04; P=0.002) and lymphopenia (OR 2.49; 1.33-4.67; P<0.001). Thirty-three patients (11.5%) required mechanical ventilation. Eighteen patients (6.3%) died from A/H1N1 infection or its complications. Neutropenia (P=0.03) and patient age (P=0.04) were significant risk factors for death. The 2009 A/H1N1 influenza pandemic caused severe complications in stem cell transplant recipients.     

Emergence of 2009A/H1N1 cases in a tertiary care hospital in New Delhi, India

Please cite this paper as: Broor et al. (2011) Emergence of 2009A/H1N1 cases in a tertiary care hospital in New Delhi, India. Influenza and Other Respiratory Viruses 5(6), e552–e557. Objective To determine virologic and epidemiologic characteristics of pandemic (H1N1) 2009 at All India Institute of Medical Sciences (AIIMS) a tertiary care hospital in New Delhi, India. Methods Nasal and throat swabs from patients with febrile acute respiratory illness (FARI) from August to December 2009 (n = 1401) were tested for 2009A/H1N1 and seasonal influenza A viruses by real‐time RT‐PCR. Results Of 1401 samples tested, 475 (33·9%) were positive for influenza A, of these majority (412; 87%) were 2009A/H1N1, whereas the remaining 63 (13%) were seasonal influenza A (49 were A/H3 and 14 were A/H1). While co‐circulation of 2009A/H1N1 and A/H3 was observed in August–September, subsequent months had exclusive pandemic influenza activity (October–December 2009). Pandemic 2009A/H1N1 emergence did not follow typical seasonal influenza seasonality in New Delhi, which normally peaks in July–August, but instead showed bimodal peaks in weeks 39 and 48 in 2009. The percent of specimens testing positive for 2009A/H1N1 influenza virus was found to be highest in >5‐ to 18‐year age group (41·2%; OR = 2·3; CI = 1·6–3·2; P = 0·00). Conclusions Taken together, our data provide high prevalence of pandemic 2009A/H1N1 in urban New Delhi with bimodal peaks in weeks 39 and 48 and highest risk group being the children of school‐going age (aged >5–18).     

Detection of 2009 pandemic influenza A(H1N1) virus Infection in different age groups by using rapid influenza diagnostic tests

Please cite this paper as: Gao et al. (2011) Detection of 2009 pandemic influenza A(H1N1) virus Infection in different age groups by using rapid influenza diagnostic tests. Influenza and Other Respiratory Viruses 6(3), e30–e34. Background The performance of rapid influenza diagnostic tests (RIDTs) in detecting influenza A(H1N1) 2009 has varied widely. Evaluations of RIDTs among infected individuals across all age groups have not been described in depth. Objectives Determine RIDT clinical sensitivity in comparison with influenza detection using real‐time RT‐PCR among patients infected with influenza A(H1N1) 2009 across all age groups. Study design This study analyzed respiratory specimens received by the New Hampshire Public Health Laboratories (NHPHL) from September 1, 2009, through December 31, 2009. RIDT performance was evaluated among different age groups of patients determined to be infected with influenza A (H1N1) 2009, and the association between age and RIDT sensitivity was determined. Results Of 1373 specimens examined, 269 tested positive for influenza A(H1N1) 2009 by real‐time RT‐PCR (rRT‐PCR) and had RIDT results available. Overall clinical sensitivity and specificity of RIDTs were 53·9 and 98·5%, respectively. By age group, clinical sensitivity was 85·7% in patients <2 years old, 60·3% in patients between 2‐ and 39 years old, and 33·3% in patients aged 40 and older. Logistic regression analysis indicated that increasing age was negatively associated with RIDT performance. Conclusion Rapid influenza diagnostic test sensitivity decreased significantly with increasing age. Findings from this study may impact a clinician’s interpretation of RIDT test results and ultimately have implications in clinical decision‐making.     

Clinical, laboratory and radiologic characteristics of 2009 pandemic influenza A/H1N1 pneumonia: primary influenza pneumonia versus concomitant/secondary bacterial pneumonia

Please cite this paper as: Song et al. (2011). Clinical, laboratory and radiologic characteristics of 2009 pandemic influenza A/H1N1 pneumonia: primary influenza pneumonia versus concomitant/secondary bacterial pneumonia. Influenza and Other Respiratory Viruses 5(6), e535–e543. Background Although influenza virus usually involves the upper respiratory tract, pneumonia was seen more frequently with the 2009 pandemic influenza A/H1N1 than with seasonal influenza. Methods From September 1, 2009, to January 31, 2010, a specialized clinic for patients (aged ≥15 years) with ILI was operated in Korea University Guro Hospital. RT‐PCR assay was performed to diagnose 2009 pandemic influenza A/H1N1. A retrospective case–case–control study was performed to determine the predictive factors for influenza pneumonia and to discriminate concomitant/secondary bacterial pneumonia from primary influenza pneumonia during the 2009–2010 pandemic. Results During the study period, the proportions of fatal cases and pneumonia development were 0·12% and 1·59%, respectively. Patients with pneumonic influenza were less likely to have nasal symptoms and extra‐pulmonary symptoms (myalgia, headache, and diarrhea) compared to patients with non‐pneumonic influenza. Crackle was audible in just about half of the patients with pneumonic influenza (38·5% of patients with primary influenza pneumonia and 53·3% of patients with concomitant/secondary bacterial pneumonia). Procalcitonin, C‐reactive protein (CRP), and lactate dehydrogenase were markedly increased in patients with influenza pneumonia. Furthermore, procalcitonin (cutoff value 0·35 ng/ml, sensitivity 81·8%, and specificity 66·7%) and CRP (cutoff value 86·5 mg/IU, sensitivity 81·8%, and specificity 59·3%) were discriminative between patients with concomitant/secondary bacterial pneumonia and patients with primary influenza pneumonia. Conclusions Considering the subtle manifestations of 2009 pandemic influenza A/H1N1 pneumonia in the early stage, high clinical suspicion is required to detect this condition. Both procalcitonin and CRP would be helpful to differentiate primary influenza pneumonia from concomitant/secondary bacterial pneumonia.     

Outbreak of pandemic 2009 influenza A/H1N1 infection in the hematology ward: fatal clinical outcome of hematopoietic stem cell transplant recipients and emergence of the H275Y neuraminidase mutation

We report an outbreak of pandemic 2009 influenza A/H1N1 virus (2009 H1N1) infection that occurred in the hematology ward of our institution during the 2010-2011 influenza season. A total of seven hospitalized patients with hematologic tumors, including five recipients of hematopoietic stem cell transplantation (HSCT), successively developed rapid influenza detection test (RIDT)-positive influenza A; four patients had laboratory-confirmed 2009 H1N1 infection. Three HSCT recipients required mechanical ventilation support and two were admitted to the intensive care unit; they died of progressive respiratory failure despite receiving available anti-viral drugs. We implemented outbreak-control measures including transferal of RIDT-positive patients to a single-patient room and chemoprophylaxis with oseltamivir. We note that the H275Y neuraminidase mutation was detected in respiratory specimens from three patients, who were administered therapeutic or prophylactic dosages of oseltamivir. The present report demonstrates that the nosocomial 2009 H1N1 outbreak in the hematology ward led to fatal clinical outcomes and the emergence of a resistant virus at a markedly high rate.     

Pandemic influenza A (2009 H1N1) in children with acute lymphoblastic leukaemia

Pandemic influenza A (2009‐H1N1) usually results in mild clinical illness, but in some individuals it can be life‐threatening. There are no reports of this disease among paediatric patients with acute lymphoblastic leukaemia (ALL). We report ten consecutive patients with ALL and pandemic influenza treated in a single institution. Median age was 7 years (range: 3–12). All were treated with oseltamivir. There were no deaths. Two patients under intensive chemotherapy developed pneumonia and one required ventilatory support. ALL patients under maintenance treatment had mild disease. In conclusion, in our series only patients under intensive treatment developed a moderate to severe disease.     

Patterns in influenza antiviral medication use before and during the 2009 H1N1 pandemic,Vaccine Safety Datalink Project, 2000–2010

Please cite this paper as: Greene et al. (2012) Patterns in influenza antiviral medication use before and during the 2009 H1N1 pandemic, Vaccine Safety Datalink Project, 2000‐2010. Influenza and Other Respiratory Viruses 6(601), e143–e151. Background U.S. recommendations for using influenza antiviral medications changed in response to viral resistance (to reduce adamantane use) and during the 2009 H1N1 pandemic (to focus on protecting high‐risk patients). Little information is available on clinician adherence to these recommendations. We characterized population‐based outpatient antiviral medication usage, including diagnosis and testing practices, before and during the pandemic. Methods Eight medical care organizations in the Vaccine Safety Datalink Project provided data on influenza antiviral medication dispensings from January 2000 through June 2010. Dispensing rates were explored in relation to changes in recommendations and influenza diagnosis and laboratory testing frequencies. Factors associated with oseltamivir dispensings in pandemic versus pre‐pandemic periods were identified using multivariable logistic regression. Results Antiviral use changed coincident with recommendations to avoid adamantanes in 2006, to use alternatives to oseltamivir in 2008, and to use oseltamivir during the pandemic. Of 38,019 oseltamivir dispensings during the pandemic, 31% were to patients not assigned an influenza diagnosis, and 97% were to patients not tested for influenza. Oseltamivir was more likely to be dispensed in pandemic versus pre‐pandemic periods to patients <25 years old and to those with underlying conditions, including chronic pulmonary disease or pregnancy (P < 0·0001 for each factor in multivariable analysis). Conclusions Antiviral medication usage patterns suggest that clinicians followed recommendations to change antiviral prescribing based on resistance and to focus on high‐risk patients during the pandemic. Medications were commonly dispensed to patients without influenza diagnoses and tests, suggesting that antiviral dispensings may offer useful supplemental data for monitoring influenza incidence.     

Community‐acquired respiratory viruses and co‐infection among patients of Ontario sentinel practices,April 2009 to February 2010

Please cite this paper as: Peci et al. (2012) Community‐acquired respiratory viruses and co‐infection among patients of Ontario Sentinel practices, April 2009 to February 2010. Influenza and Other Respiratory Viruses 7(4), 559–566. Background Respiratory viruses are known to cocirculate but this has not been described in detail during an influenza pandemic. Objectives To describe respiratory viruses, including co‐infection and associated attributes such as age, sex or comorbidity, in patients presenting with influenza‐like illness to a community sentinel network, during the pandemic A(H1N1)pdm09 in Ontario, Canada. Methods Respiratory samples and epidemiologic details were collected from 1018 patients with influenza‐like illness as part of respiratory virus surveillance and a multiprovincial case–control study of influenza vaccine effectiveness. Results At least one virus was detected in 668 (65·6%) of 1018 samples; 512 (50·3%) had single infections and 156 (15·3%) co‐infections. Of single infections, the most common viruses were influenza A in 304 (59·4%) samples of which 275 (90·5%) were influenza A(H1N1)pdm09, and enterovirus/rhinovirus in 149 (29·1%) samples. The most common co‐infections were influenza A and respiratory syncytial virus B, and influenza A and enterovirus/rhinovirus. In multinomial logistic regression analyses adjusted for age, sex, comorbidity, and timeliness of sample collection, single infection was less often detected in the elderly and co‐infection more often in patients <30 years of age. Co‐infection, but not single infection, was more likely detected in patients who had a sample collected within 2 days of symptom onset as compared to 3–7 days. Conclusions Respiratory viral co‐infections are commonly detected when using molecular techniques. Early sample collection increases likelihood of detection of co‐infection. Further studies are needed to better understand the clinical significance of viral co‐infection.     

Pandemic 2009 influenza A (H1N1) infection among 2009 Hajj Pilgrims from Southern Iran: a real‐time RT‐PCR‐based study

Please cite this paper as: Ziyaeyan et al. (2012) Pandemic 2009 influenza A H1N1 infection among 2009 Hajj Pilgrims from Southern Iran: a real‐time RT‐PCR‐based study. Influenza and Other Respiratory Viruses 6(601), e80–e84. Background Hajj is a mass gathering undertaken annually in Mecca, Saudi Arabia. The 2009 Hajj coincided with both the pandemic influenza A/H1N1 2009 (A(H1N1)pdm09) and seasonal types of influenza A viruses. The interaction between pandemic influenza and Hajj could cause both a high level of mortality among the pilgrims and the spread of infection in their respective countries upon their return home. Objective The present study attempted to determine the point prevalence of A(H1N1)pdm09 among returning Iranian pilgrims, most of whom had been vaccinated for seasonal influenza but not A(H1N1)pdm09. Methods Pharyngeal swabs were collected from 305 pilgrims arriving at the airport in Shiraz, Iran. RNA was extracted from the samples and A(H1N1)pdm09 and other seasonal influenza A viruses were detected using TaqMan real‐time PCR. For A(H1N1)pdm09‐positive samples, the sensitivity to oseltamivir was also evaluated. Results Subjects included 132 (43·3%) men and 173 (56·7%) women, ranging in age from 24 to 65 years. The A(H1N1)pdm09 virus was detected in five (1·6%) pilgrims and other influenza A viruses in eight (2·6%). All the A(H1N1)pdm09 were sensitive to oseltamivir. Conclusions Only five cases were found to be positive for A(H1N1)pdm09, and it seems unlikely that the arrival of infected pilgrims to their homelands would cause an outbreak of a new wave of infection there. Thus, the low morbidity and mortality rates among the pilgrims could be attributed to the characteristics of A(H1N1)pdm09, which causes morbidity and mortality in a way similar to the seasonal influenza infections, absence of high‐risk individuals among the Iranian pilgrims, and the instructions given to them about contact and hand hygiene, and respiratory etiquette.     

Pandemic H1N1 influenza-associated hospitalizations in children in Madrid, Spain

Please cite this paper as: del Rosal et al. (2011) Pandemic H1N1 influenza‐associated hospitalizations in children in Madrid, Spain. Influenza and Other Respiratory Viruses 5(6), e544–e551. Objective To describe the epidemiological and clinical characteristics of children hospitalized with 2009 pandemic influenza (pH1N1) in Madrid, Spain. Patients/Methods We included patients less than 14 years of age admitted to one of 18 hospitals in Madrid, Spain, between May 1 and November 30, 2009 and diagnosed with pH1N1 by polymerase chain reaction. A retrospective chart review was conducted and data were compared by age, presence of high‐risk medical conditions, and pediatric intensive care unit (PICU) admission. Results A total of 517 pH1N1 cases were included for final analysis. One hundred and forty‐two patients (27·5%) had predisposing underlying illnesses, with immunosuppression (36 children, 7%) and moderate persistent asthma (34, 6·6%) being the most common ones. Patients with underlying medical conditions had longer hospital stays [median 5, interquartile range (IQR) 3–8 days, versus median 4, IQR 3–6, P < 0·001] and required intensive care (20·4% versus 5·9%, P < 0·001) and mechanical ventilation more frequently than previously healthy children. Globally, intensive care was required for 51 patients (10%) and invasive mechanical ventilation for 12 (2%). Pediatric intensive care unit admission was significantly associated with abnormal initial chest X‐ray [Odds Ratio (OR) 3·5, 95% confidence interval (CI) 1·5–8·5], underlying neurological condition (OR 3·1, CI 1·2–7·5) and immunosuppression (OR 2·9, 1·2–6·8). Five patients (0·9%) died; two with severe neurological disease, two with leukemia, and one with a malignant solid tumor. Conclusions Children with underlying medical conditions experienced more severe pH1N1 disease. Risk factors for admission to the PICU included underlying neurological conditions, immunosuppression and abnormal initial chest X‐ray.     

Seroprevalence of pandemic (H1N1) 2009 influenza and effectiveness of 2010/2011 influenza vaccine during 2010/2011 season in Beijing, China

Please cite this paper as: Yang et al. (2011) Seroprevalence of pandemic (H1N1) 2009 influenza and effectiveness of 2010/2011 influenza vaccine during 2010/2011 season in Beijing, China. Influenza and Other Respiratory Viruses 6(6), 381–388. Background In the post‐pandemic period, pandemic (H1N1) 2009 virus was expected to circulate seasonally and was introduced into trivalent influenza vaccine during 2010/2011 season in the Northern Hemisphere. Objectives The aim of this study was to examine the evolution of herd immunity against pandemic (H1N1) 2009 virus in Beijing, China, during 2010/2011 season and effectiveness of the 2010/2011 trivalent vaccine. Methods Two serological surveys were conducted before and after 2010/2011 season in Beijing. A case–control study was used to investigate vaccine effectiveness against influenza‐like illness (ILI) and lower respiratory tract infection (LRI). Results A total of 4509 and 4543 subjects participated in the pre‐ and post‐season surveys, respectively. The standardized seroprevalence of pandemic (H1N1) 2009 influenza increased from 22·1% pre‐season to 24·3% post‐season (P < 0·001). Significant elevation in seroprevalence appeared in the ≥60 years age‐group (P < 0·001), but not in others. The 2010/2011 trivalent vaccine contributed to the higher post‐seasonal seroprevalence in unvaccinated individuals (P = 0·024), but not in those vaccinated with monovalent pandemic vaccine (P = 0·205), as well as in those without prior immunity versus those with immunity. The adjusted effectiveness of the 2010/2011 trivalent vaccine was 79% protection against ILI (95% CI, 61–89%) and 95% against LRI (95% CI: 59–99%). Conclusions A slight increase in herd immunity against pandemic (H1N1) 2009 influenza was observed in Beijing, China, during the 2010/2011 season. Prior vaccination and immunity had a suppressive impact on immune response toward this novel influenza virus, elicited by 2010/2011 trivalent vaccine. This trivalent vaccine conferred good protection against ILI and LRI.     

Risk factors for pandemic H1N1 2009 infection in healthcare personnel of four general hospitals

To characterize an outbreak of pandemic H1N1 2009 among healthcare personnel (HCP), we conducted a cross-sectional survey of HCP who had worked in four general hospitals during the outbreak. More than one-quarter of responding HCP (27.6%) had influenza-like illness (ILI) during the outbreak. The estimated infection rate of pandemic H1N1 2009 was 9.1% in the study of HCP. Independent risk factors for ILI were female gender, <40 years of age, the presence of chronic diseases associated with influenza complications, having family members with ILI or pandemic H1N1 2009, and working in influenza outpatient, influenza inpatient, non-influenza outpatient or emergency departments. During the outbreak of pandemic H1N1 2009, HCP frequently had ILI or the influenza infection. The development of the influenza infection in HCP was associated with some of their baseline characteristics, occupational risk factors, and non-occupational ones during the outbreak.