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Original Study

Pneumonia Complicating Pandemic (H1N1) 2009

Risk Factors, Clinical Features, and Outcomes

Viasus, Diego MD; Paño-Pardo, José Ramón MD, PhD; Pachón, Jerónimo MD, PhD; Riera, Melchor MD, PhD; López-Medrano, Francisco MD, PhD; Payeras, Antoni MD, PhD; Fariñas, M. Carmen MD, PhD; Moreno, Asunción MD, PhD; Rodríguez-Baño, Jesús MD, PhD; Oteo, José Antonio MD, PhD; Martínez-Montauti, Joaquín MD, PhD; Torre-Cisneros, Julián MD, PhD; Segura, Ferrán MD, PhD; Gudiol, Francesc MD, PhD; Carratalà, Jordi MD, PhD for the Novel Influenza A(H1N1) Study Group of the Spanish Network for Research in Infectious Diseases (REIPI)

Author Information
doi: 10.1097/MD.0b013e31822e67a7
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Disease caused by influenza is usually limited to uncomplicated respiratory infection. When the influenza is complicated, it is associated with increased morbidity and mortality. The major complications of influenza are those involving the lower respiratory tract, mainly pneumonia (primary influenza pneumonia and concomitant/secondary bacterial pneumonia) and exacerbations of chronic pulmonary diseases.52

During previous influenza pandemics, pneumonia was a fearsome complication due to its frequency and high morbidity and mortality.40,49 During pandemic (H1N1) 2009, the reported frequency of pneumonia in hospitalized patients ranged between 23% and 66%.13,32,39 In addition, most patients requiring intensive care unit (ICU) admission had respiratory failure due mainly to primary influenza pneumonia.2,37,43,51 Furthermore, concomitant/secondary bacterial coinfection, although relatively infrequent, has been associated with poor prognosis.21 Nonetheless, clinical information about pneumonia in patients with influenza A (H1N1)v virus infection is particularly scarce.11,26,50 Significantly, data on risk factors for developing pneumonia in this setting are lacking.

We conducted the present study to determine the risk factors, clinical features, and outcome of pneumonia in a large cohort of adults hospitalized with pandemic (H1N1) 2009. We also compared clinical features and outcomes between patients with primary viral pneumonia and patients with concomitant/secondary bacterial pneumonia.


Setting, Patients, and Study Design

This prospective cohort study was carried out at 13 tertiary hospitals in Spain. All adults admitted to the hospital for at least 24 hours with confirmed influenza A (H1N1) virus infection from June 12 to November 10, 2009, were prospectively recruited and followed. A confirmed case was defined as a person with an influenza-like illness with confirmed laboratory pandemic influenza A (H1N1)v virus infection by real-time reverse-transcription polymerase chain reaction (RT-PCR) or viral culture.8 Pandemic influenza A (H1N1)v virus testing was performed at each institution. For the purposes of this study, patients were divided into 2 groups: patients with pneumonia (primary viral pneumonia or concomitant/secondary bacterial pneumonia) and patients without pneumonia (patients without infiltrates on chest X-rays). Patients in whom a chest X-ray examination was not performed were excluded. The study was approved by the Ethics Committee of the coordinating center, Hospital Universitari de Bellvitge, and informed consent was obtained from patients.

Clinical Assessment and Follow-Up

The microbiologic studies, hospital and ICU admission criteria, and treatment decisions were not standardized and were made by attending physicians. Patients were seen during their hospital stay by 1 or more of the investigators at each participating hospital, who recorded clinical data in a standardized, computer-assisted protocol. To stratify patients with pneumonia according to risk, we used community-acquired pneumonia (CAP) scores: pneumonia severity index (PSI) and CURB-65 (that is, confusion, urea, respiratory rate, blood pressure, and age ≥65 yr).24,38 For time calculations, the day of admission was considered to be hospital day 0, except for time to antiviral administration, in which the day of onset of symptoms was considered day 0. Completed protocols were sent to the coordinating center, and all data were carefully reviewed by 2 clinical investigators before the final validation.


Pneumonia was defined as the presence of a new infiltrate on a chest radiograph plus fever (temperature ≥38.0°C) and/or respiratory symptoms. Primary viral pneumonia was diagnosed in patients presenting pneumonia with negative respiratory and blood bacterial cultures and negative urine antigen tests. Concomitant/secondary bacterial pneumonia was diagnosed in patients with 1 or more positive cultures obtained from blood, normally sterile fluids, or sputum and/or a positive urinary antigen test. Bacterial diagnosis was considered definitive in the following situations: isolation of a respiratory pathogen in a usually sterile specimen, isolation of Legionella pneumophila in sputum, detection of Streptococcus pneumoniae or L. pneumophila serogroup 1 antigens in urine, 4-fold increase in the antibody titer, or seroconversion for atypical pathogens. Bacterial diagnosis was considered presumptive when a predominant microorganism was isolated from a purulent sample (presence of ≥25 polymorphonuclear leukocytes and ≤10 squamous cells per low power field [original magnification × 10]) with compatible Gram stain findings.5

Comorbidities were assessed by the Charlson Comorbidity Index.9 Other comorbidities such as immunosuppression, neuromuscular disorders, and sickle-cell disease were also recorded. For patients with weight and height available, obesity was defined as a body mass index (BMI) ≥30.0-39.9 and morbid obesity as a BMI ≥40. Tobacco abuse was recorded when a patient had smoked more than 10 cigarettes per day for at least 1 year preceding the study. Alcohol abuse was considered if alcohol intake was >3 standard drinks per day. Vaccination status was assessed from interviews with the patients or their relatives and from review of hospital and personal health records (vaccination card). A patient was considered to be pneumococcal-vaccinated if 23-valent polysaccharide pneumococcal vaccine had been administered in the 5 years before admission. A patient was considered to be influenza-vaccinated if seasonal influenza vaccine had been administered during the year before admission.

Complications were defined as any untoward circumstances occurring during hospitalization. The diagnosis of septic shock was based on a systolic blood pressure of <90 mm Hg and requirement of vasopressors.4Time to clinical stability in patients with pneumonia was evaluated as described elsewhere.28Severe disease was defined as the composite outcome of ICU admission or in-hospital mortality. In-hospital mortality was defined as death from any cause during hospitalization.

Microbiologic Studies

Two specific 1-step multiplex real-time RT-PCR were used for typing (A/B) and subtyping (H1/novel H1/H3/H5) influenza virus. The first assay consisted of primers and probes specific to the matrix (M1) gene of influenza A and influenza B viruses for typing. The second assay used primers and probes specific to the hemagglutinin gene of human H1, novel human H1, human H3, and avian H5 subtypes to identify the most prominent subtypes of influenza A virus capable of infecting humans (H1N1, pandemic H1N1, H3N2, and H5N1). Nontemplate controls and positive template controls for all primer/probe sets were included in each run. An additional third assay amplified a housekeeping gene (RNase P) of host cells to assess the correct progress of DNA extraction and to underline the absence of PCR inhibitors as internal control. The Real Time RT-PCR Protocol for Detection and Characterization of Influenza A (H1N1)v supplied by the Centers for Disease Control and Prevention (Atlanta, GA) was used to confirm our positive results.56

The investigation of bacterial pathogens in blood, normally sterile fluids, sputum, and other samples was performed by standard microbiologic procedures within the first 48 hours after admission. The finding of the S. pneumoniae antigen in urine was detected by using a rapid immunochromatographic assay (NOW Assay; Binax Inc, Portland, ME) or enzyme-linked immunosorbent assay (ELISA-Bartels, Bartels, Trinity Biotech, Wicklow, Ireland). L. pneumophila serogroup 1 antigen in urine was detected by an immunochromatographic method (NOW Legionella Urinary Antigen Test; Binax Inc). Standard serologic methods were used to determine antibodies against atypical agents.

For all microbiologic studies, except serologic methods against atypical agents (performed on admission and 3-4 weeks thereafter), the results were typically available in less than 24 hours (frequently less than 12 h).

Statistical Analysis

Descriptive statistical analyses were performed for all study variables. All proportions were calculated as percentages of the patients with available data. We compared patients with and without pneumonia and also patients with primary viral pneumonia and patients with concomitant/secondary bacterial pneumonia. To detect significant differences between groups, we used the chi-square test or Fisher exact test for categorical variables and the t test or Mann-Whitney test for continuous variables, when appropriate. In addition, 95% confidence intervals (CIs) of mean difference were reported for continuous variables. Receiver operating characteristic (ROC) curves and area under curves were generated to evaluate the predictive value of the CAP-specific scores. To determine factors potentially associated with the development of pneumonia and primary viral pneumonia, we performed a multivariate logistic analysis including significant variables in univariate analysis and clinically important variables whether they were significant or not. The relative risks were expressed as odds ratios (ORs) and 95% CIs. All p values reported are 2-tailed. Statistical significance was established at α = 0.05. We analyzed the results using SPSS version 15.0 (SPSS Inc, Chicago, IL).


Patient Characteristics, Clinical Features,and Diagnostic Findings

During the study period, 585 patients (median age, 40 yr) required hospitalization. Chest radiography was obtained in 542. Of those 542 patients, 234 patients (43.1%) had pneumonia, of whom 210 underwent 1 or more bacterial microbiologic studies (sputum Gram stain and culture, 92; blood cultures, 157; S. pneumoniae urinary antigen test, 160; L. pneumophila urinary antigen test, 160; pleural fluid culture, 10; bronchoalveolar lavage/tracheal aspirate culture, 27; and serologic methods for atypical agents, 27). Pneumonia was primary viral in 174 of these patients and concomitant/secondary bacterial in 36.

The demographic and clinical characteristics of patients with and without pneumonia are shown in Tables 1 and 2. No significant differences were found between groups in terms of age and sex. No patients had received pandemic influenza A (H1N1)v virus vaccine. Patients with pneumonia were more frequently current smokers and heavy alcohol drinkers. Conversely, comorbid conditions were more common in patients without pneumonia, particularly asthma. Patients with pneumonia had longer time from symptom onset to hospitalization (95% CI of mean difference, 1.2-2.1 d). Similarly, these patients more frequently had shortness of breath, pleuritic chest pain, diarrhea, hypotension, tachypnea, and impaired consciousness at presentation.

Demographic Characteristics of Patients With Pandemic (H1N1) 2009 by Study Group
Clinical Characteristics of Patients With Pandemic (H1N1) 2009 by Study Group

Twenty-seven (11.7%) and 31 (13.6%) patients with pneumonia were classified into high-risk classes according to the PSI (groups IV-V) and CURB-65 (groups II-III) respectively. Area under ROC curve to predict severe disease for PSI was 0.77 (95% CI, 0.71-0.84), and for CURB-65 it was 0.74 (95% CI, 0.66-0.82).

Of the 174 patients with primary viral pneumonia, 22 of 149 with data available (14.8%) had received seasonal influenza vaccine, and 90 (51.7%) had comorbid conditions. Among the 36 patients with concomitant/secondary bacterial pneumonia, 10 (27.7%) had less than 3 days from symptom onset to hospital admission; none had received pneumococcal vaccine; and 19 (52.8%) patients had comorbid conditions.

The laboratory findings are detailed in Table 3. On admission, patients with pneumonia more frequently had leukopenia; hyponatremia; and elevated liver enzymes, lactate dehydrogenase, and C-reactive protein than patients without pneumonia. Respiratory failure was also more frequent in patients with pneumonia. Bilateral pneumonia occurred in 48.3% of patients, and pleural effusion in 9%. The most frequent radiographic finding in patients with primary viral pneumonia was interstitial infiltrates alone (29.2%). Conversely, the most frequent radiographic finding in patients with concomitant/secondary bacterial pneumonia was lobar alveolar infiltrates (38.9%). Multilobar alveolar infiltrates were found in 57 (32.7%) patients with primary viral pneumonia and in 14 (38.9%) patients with bacterial coinfection.

Diagnostic Findings of Patients With Pandemic (H1N1) 2009 by Study Group

Table 4 shows the causative agents of concomitant/secondary bacterial pneumonia. Four patients had more than 1 microorganism. Overall, S. pneumoniae was the most frequent pathogen. Among the subgroup of 28 patients with definitive bacterial pneumonia, urine antigen test was positive in 23 (all for S. pneumoniae); blood cultures in 6 (S. pneumoniae in 2 patients and methicillin-susceptible Staphylococcus aureus, S. sanguis, Haemophilus parainfluenzae, and Acinetobacter baumannii in 1 patient each); and pleural fluid culture in 1 patient (S. pyogenes).

Etiology in 36 Patients With Pandemic (H1N1) 2009 Complicated by Concomitant/Secondary Bacterial Pneumonia*

Among the 308 patients without pneumonia, 49 (15.9%) had physical signs of lower respiratory tract involvement. Of these patients, 39 (79.6%) had at least 1 microbiologic study. Four patients had presumptive bacterial coinfection (Pseudomonas aeruginosa in 2 patients with chronic pulmonary disease and in 1 renal transplant patient, and Stenotrophomonas maltophilia in 1 patient with chronic pulmonary disease).

Treatment and Clinical Outcomes

The treatment and clinical outcomes are shown in Table 5. Patients with pneumonia had a longer time between symptom onset and oseltamivir administration than patients without pneumonia (95% CI of mean difference, 1.4-2.5 d). Patients with pneumonia more frequently received antibiotic therapy. No significant differences were observed regarding the number of patients receiving corticosteroids. The median length of hospital stay was significantly longer for patients with pneumonia than for those without (95% CI of mean difference, 2.9-5.7 d respectively). Patients with pneumonia were more likely to present complications such as shock, nosocomial infections, and heart complications, and more frequently required ICU admission and mechanical ventilation than patients who did not have pneumonia.

Treatment and Clinical Outcomes of Patients With Pandemic (H1N1) 2009 by Study Group

In-hospital mortality was higher in patients with pneumonia than in those without (5.2% vs. 0%; p < 0.001). Twelve of 234 patients with pneumonia died (8 had primary viral pneumonia and 4 had concomitant/secondary bacterial pneumonia). The median time from hospital admission to death was 11.5 days (range, 1-33 d). Among the 8 patients with primary viral pneumonia who died, 5 were aged less than 50 years, 7 had comorbid conditions, 7 had multilobar/bilateral pneumonia, and 6 required ICU admission and mechanical ventilation. The microorganisms documented from the 4 patients with bacterial pneumonia who died were S. pneumoniae (urine antigen test), Staphylococcus aureus (blood cultures), H. influenzae (sputum culture), and A. baumannii (sputum culture). Three of these patients were aged less than 50 years, all had comorbid conditions and multilobar/bilateral pneumonia, and 3 required ICU admission.

Risk Factors for Pneumonia

The results of multivariate logistic regression analysis for factors potentially associated with the development of pneumonia are summarized in Table 6. After adjustment, the absence of comorbid conditions was found to be an independent risk factor for pneumonia (OR, 2.07; 95% CI, 1.32-3.24), whereas early oseltamivir therapy (≤48 h) was found to be a protective factor (OR, 0.29; 95% CI, 0.19-0.46). Absence of comorbid conditions and early oseltamivir therapy were also independent factors associated with primary viral pneumonia (data not shown).

Risk Factors for Pneumonia in Patients With Pandemic (H1N1) 2009: Multivariate Analysis

Comparison of Patients With Primary Viral and Concomitant/Secondary Bacterial Pneumonia

At hospital presentation, patients with primary viral pneumonia less frequently had chronic liver disease (5.2% vs. 13.9%; p = 0.06), purulent sputum (23.6% vs. 44.4%; p = 0.01), and tachycardia (heart rate >90 beats per min) (58.7% vs. 84.4%; p = 0.006) than patients with concomitant/secondary bacterial pneumonia. Among laboratory and radiographic findings, patients with primary viral pneumonia less frequently presented leukocytosis (leukocytes >12,000/mm3) (13.8% vs. 25%; p = 0.09), C-reactive protein levels >80 mg/L (39.4% vs. 63.6%; p = 0.03), and pleural effusion (5.2% vs. 22.2%; p = 0.002). Conversely, interstitial bilateral infiltrates in chest X-rays were more common in patients with primary viral pneumonia (25.9% vs. 11.2%; p = 0.05).

No significant differences were found in the time from symptom onset to antiviral administration (median, 5 d for both groups; p = 0.39), time to clinical stability (median, 3 d for both groups; p = 0.26) and length of hospital stay (median, 7 d for both groups; p = 0.11) between groups. Compared with patients with primary viral pneumonia, patients with bacterial pneumonia more frequently had respiratory complications (mechanical ventilation, empyema, and acute respiratory distress syndrome) (17.8% vs. 33.3%; p = 0.03). ICU admission (23.6% vs. 33.3%; p = 0.21) and in-hospital mortality (4.6% vs. 11.1%; p = 0.12) did not differ significantly between groups, respectively.


In the current prospective multicenter study, we found that pneumonia was a frequent complication among adults hospitalized for pandemic (H1N1) 2009. Most patients had primary viral pneumonia; concomitant/secondary bacterial pneumonia was less common. Pneumonia was associated with increased morbidity and mortality. Significantly, we documented that early oseltamivir therapy was a protective factor against this complication.

To date, few studies have analyzed the clinical features of patients with pneumonia complicating influenza A (H1N1)v virus infection.11,26,50 We found that pneumonia affected mainly younger adult patients. Healthy people and those with comorbid conditions were affected equally, although most patients with pneumonia who required ICU admission and died had comorbid conditions. As in previous reports,15,40,49 the main comorbid conditions in patients with pneumonia were chronic respiratory disease, diabetes mellitus, and chronic heart disease. Current smokers and heavy alcohol drinkers had a higher risk of pneumonia in the present study. Interestingly, we found no significant delay in the time from symptom onset to antiviral therapy between patients with comorbidities and those without (data not shown). Therefore, 1 of the salient features of pandemic (H1N1) 2009 complicated by pneumonia is the involvement of a young and previously healthy population. In this regard, the absence of comorbid conditions was found to be an independent risk factor for pneumonia among subjects admitted to a hospital in the present study. This concurs with data from the 1918 pandemic and from the H5N1 avian influenza, in which healthy young adults were found to be especially susceptible to more severe disease and poor outcomes because their immune systems overreacted and caused a severe inflammatory response.23

It has long been recognized that influenza infection is closely associated with an increased incidence of bacterial pneumonia.36 Morens et al45 reported that the majority of deaths in the 1918-1919 influenza pandemic likely resulted from secondary bacterial pneumonia caused by common upper respiratory-tract bacteria. During the current influenza pandemic, bacterial pneumonia was infrequent: in Mexico19 and California,32 the prevalence of bacterial coinfection was 0.4% and 4%, respectively. However, Shieh et al53 recently documented in studies of autopsy specimens that bacterial coinfection was present in 26 of 100 patients with fatal pandemic (H1N1) 2009. In the current study, the prevalence of concomitant/secondary bacterial pneumonia was 6% (17.2% among patients with pneumonia), and we found no evidence of definitive bacterial coinfection in patients without pneumonia.

Given the relatively low frequency of bacterial coinfection and the high use of antibacterial therapy in patients with influenza A (H1N1)v virus infection, researchers have aimed to determine the clinical features and factors associated with concomitant/secondary bacterial pneumonia.14,20 Cunha15 recently reported that pleuritic chest pain, purulent sputum, focal segmental/lobar infiltrates, and pleural effusion indicate bacterial coinfection. Some studies have found that patients with bacterial coinfection27 or patients with CAP of bacterial origin31 had higher levels of procalcitonin and C-reactive protein compared with patients with influenza A (H1N1)v virus pneumonia. Likewise, bacterial pneumonia differs because it is often associated with pleural effusion, lymphadenopathy, and lobar consolidations in radiologic images. Conversely, the characteristic imaging finding in primary viral pneumonia is ground-glass opacities with areas of consolidation.1,3,54 In the current study, compared with patients with primary viral pneumonia, patients with bacterial pneumonia were more likely to have chronic liver disease, purulent sputum, tachycardia, pleural effusion, leukocytosis, and C-reactive protein levels higher than 80 mg/L at hospital admission. On the other hand, interstitial bilateral infiltrates in chest X-rays were more frequent in patients with primary viral pneumonia.

As in previous reports,7,53 we found that S. pneumoniae was the most frequent causative organism of concomitant/secondary bacterial pneumonia (26 of 36 cases). During the current pandemic, the United States Advisory Committee on Immunization Practices provided interim guidance on which groups should be vaccinated with 23-valent pneumococcal polysaccharide vaccine.6 In the present study, although 75% of patients with concomitant/secondary bacterial pneumonia had 1 or more criteria for pneumococcal vaccination, none had been vaccinated. Significantly, none of the 23 patients who had received pneumococcal vaccination developed bacterial coinfection. These data reinforce the importance of vaccination against this organism during influenza pandemics. Although there is little evidence of vaccine protection for pneumonia,29,44 it has been shown that vaccination may be effective in preventing invasive pneumococcal disease.44 In addition, some studies have found that pneumococcal polysaccharide vaccine has an effectiveness in preventing hospitalizations for pneumonia16 and reduces the rates of ICU admission and in-hospital mortality in immunosuppressed and nonimmunosuppressed patients with CAP or invasive pneumococcal disease.25,30,34,48

In the present study, pneumonia was associated with high morbidity, as assessed by the length of hospital stay and the rates of ICU admission and in-hospital complications. Likewise, mortality occurred only in patients with pneumonia, but it was lower than that found in previous pandemics.40,49 Unlike previous pandemic viruses, the influenza A (H1N1)v virus does not code for the virulence factor PB1-F2. In mice, PB1-F2 increases the severity of primary viral and secondary bacterial infections,41,57 and has been associated with the high pathogenicity of avian H5N1 and the 1918 pandemic strain.12 The absence of a full-length PB1-F2 protein has been suggested as 1 possible determinant of the low virulence of the influenza A (H1N1)v virus. In addition, improvements in the nutritional status, housing, and availability of health care and antiviral therapy might explain some of the apparent reduction in case fatality from 1 pandemic to the next.17 It is noteworthy that we found no significant differences in ICU admission and mortality between patients with concomitant/secondary bacterial pneumonia and patients with primary viral pneumonia, although mortality was 3 times more frequent in patients with bacterial pneumonia. This finding is in agreement with a previous study,21 which found an association between concomitant/secondary bacterial coinfection and poor prognosis.

CAP-specific scores demonstrated moderate usefulness for predicting severe disease in patients with pandemic (H1N1) 2009 complicated by pneumonia. However, high-risk classes of these scores only identify 30% of patients with severe disease. Consistent with these data, Muller et al47 found that currently existing pneumonia severity scores may not be adequately predictive of both in-hospital mortality and the need for ICU admission in patients with influenza. A limitation of these scores is that age is the variable with more weight, and, as we noted above, most patients affected by pneumonia during the current pandemic were younger adults. In addition, other risk factors for severe pandemic (H1N1) 2009 such as obesity46 were not included in these scores. A tool is required that properly identifies patients at risk of severe outcomes and avoids unnecessary hospitalizations during influenza pandemics.

We found that the prevalence of pneumonia increased with the time from symptom onset to antiviral administration (≤2 d: 20.4%; 3-5 d: 32.7%; and ≥6 d: 60.7%; p = <0.001 chi-square for trend). After adjustment for confounders in a multivariate logistic regression analysis, we found that early oseltamivir therapy (≤48 h) decreased the risk for developing pneumonia. This finding concurs with those of previous studies reporting lower influenza-associated complications in healthy and high-risk patients when given early antiviral therapy.18,35,55 In animal models during influenza infection,42 treatment with neuraminidase inhibitors has been associated with a decrease in the incidence of bacterial pneumonia and an improvement in survival. Early administration of antiviral treatment has been related with a lower risk of respiratory failure10 and mortality32 during the current pandemic. However, Falagas et al22 and Jefferson et al33 recently reported in 2 meta-analyses that data about the effectiveness of neuraminidase inhibitors on individual respiratory influenza complications and mortality are still insufficient. This was attributed to the limited numbers of published trials and the low number of mortality events that occurred during trials.

The strengths of the present study are its prospective and multicenter design, the large number of patients included, and the comprehensive clinical and microbiologic data collection. Importantly, more than 1 bacterial microbiologic study was performed in approximately 85% of patients with primary viral pneumonia. However, some limitations should be noted. Hospital admission criteria were not standardized, and some pregnant women or certain patients with comorbidities were probably hospitalized in spite of having only mild disease. Moreover, the diagnostic sensitivity of microbiologic studies may have been reduced by prior antibiotic treatment. Other tests used to identify bacterial infections in patients with pneumonia, such as polymerase chain reaction or immunochemistry methods in lung tissue, were not performed.

In conclusion, pneumonia is a frequent complication among adults hospitalized for pandemic (H1N1) 2009 and causes significant morbidity. Most hospitalized patients develop primary viral pneumonia. Mortality in pandemic (H1N1) 2009 is low, but occurs mainly in patients with pneumonia. Early oseltamivir therapy is a protective factor for this complication.


Other members of the Novel Influenza A(H1N1) Study Group: Jordi Niubo, Alejandro Martín-Quirós, María Romero-Gómez, Guillermo Ruíz-Carrascoso, Juan Carlos Figueira, María Concepción Prados, Elisa Cordero, Emilio García-Cabrera, Antoni Campins, José María Aguado, Juan Vila, Aroa Villoslada, Mercedes García-Gasalla, José Luis González-Fernández, Manuel Gutiérrez-Cuadra, María Victoria Sanjuan, José Antonio Parra, Laura Linares, Irma Hoyo, María Ángeles Marcos, Tomás Pumarola, Marina de Cueto, Ángel Domínguez, Juan Gálvez, Laura Pérez-Martínez, José R. Blanco, Mercedes Sanz, Luis Metola, Valvenera Ibarra, Lucía Ortega, Rosario Lara, Manuel Causse, Juan Gutiérrez-Aroca, Gemma Navarro, Eva María González.


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