Severity of influenza A 2009 (H1N1) pneumonia is underestimated by routine prediction rules. Results from a prospective, population-based study.
BACKGROUND
Characteristics of patients with community-acquired pneumonia (CAP) due to pandemic influenza A 2009 (H1N1) have been inadequately compared to CAP caused by other respiratory pathogens. The performance of prediction rules for CAP during an epidemic with a new infectious agent are unknown.


METHODS
Prospective, population-based study from November 2008-November 2009, in centers representing 70% of hospital beds in Iceland. Patients admitted with CAP underwent evaluation and etiologic testing, including polymerase chain reaction (PCR) for influenza. Data on influenza-like illness in the community and overall hospital admissions were collected. Clinical and laboratory data, including pneumonia severity index (PSI) and CURB-65 of patients with CAP due to H1N1 were compared to those caused by other agents.


RESULTS
Of 338 consecutive and eligible patients 313 (93%) were enrolled. During the pandemic peak, influenza A 2009 (H1N1) patients constituted 38% of admissions due to CAP. These patients were younger, more dyspnoeic and more frequently reported hemoptysis. They had significantly lower severity scores than other patients with CAP (1.23 vs. 1.61, P= .02 for CURB-65, 2.05 vs. 2.87 for PSI, P<.001) and were more likely to require intensive care admission (41% vs. 5%, P<.001) and receive mechanical ventilation (14% vs. 2%, P= .01). Bacterial co-infection was detected in 23% of influenza A 2009 (H1N1) patients with CAP.


CONCLUSIONS
Clinical characteristics of CAP caused by influenza A 2009 (H1N1) differ markedly from CAP caused by other etiologic agents. Commonly used CAP prediction rules often failed to predict admissions to intensive care or need for assisted ventilation in CAP caused by the influenza A 2009 (H1N1) virus, underscoring the importance of clinical acumen under these circumstances.
Faculty of Medicine, University of Iceland, Reykjavik, Iceland.
Agnar
A
Bjarnason
Gudlaug
G
Thorleifsdottir
Arthur
A
Löve
Janus F
JF
Gudnason
Hilmir
H
Asgeirsson
Kristinn L
KL
Hallgrimsson
Berglind S
BS
Kristjansdottir
Gunnsteinn
G
Haraldsson
Olafur
O
Baldursson
Karl G
KG
Kristinsson
Magnus
M
Gottfredsson
D000328:Adult; D000368:Aged; D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D000998:Antiviral Agents / Q000627:therapeutic use; D060085:Coinfection / Q000188:drug therapy / Q000382:microbiology / Q000821:virology; D017714:Community-Acquired Infections / Q000188:drug therapy / Q000453:epidemiology* / Q000382:microbiology; D015897:Comorbidity; D005260:Female; D006801:Humans; D007055:Iceland / Q000453:epidemiology; D053118:Influenza A Virus, H1N1 Subtype / Q000187:drug effects / Q000302:isolation & purification; D007251:Influenza, Human / Q000188:drug therapy / Q000453:epidemiology* / Q000821:virology; D007362:Intensive Care Units / Q000706:statistics & numerical data; D007902:Length of Stay; D008297:Male; D008875:Middle Aged; D009177:Mycoplasma pneumoniae / Q000187:drug effects / Q000302:isolation & purification; D053139:Oseltamivir / Q000627:therapeutic use; D058873:Pandemics; D010343:Patient Admission / Q000706:statistics & numerical data; D018410:Pneumonia, Bacterial / Q000188:drug therapy / Q000453:epidemiology* / Q000382:microbiology; D011446:Prospective Studies; D012720:Severity of Illness Index; D013296:Streptococcus pneumoniae / Q000187:drug effects / Q000302:isolation & purification
D000900:Anti-Bacterial Agents; D000998:Antiviral Agents; D053139:Oseltamivir