Severity of influenza A 2009 (H1N1) pneumonia is underestimated by routine prediction rules. Results from a prospective, population-based study

Abstract

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.

Figure 1. Epidemiology of influenza in Iceland, December 2008-December 2009, showing the number of reported cases of influenza-like illness (ILI) and confirmed influenza A 2009 (H1N1) (left y-axis) and weekly ILI incidence per 100 000 population (right y-axis).

In Iceland approximately 62% of all virologically confirmed cases and ILI were in Reykjavik . (Ref: http://www.influensa.is/pages/1505).

Figure 2. Etiologic causes of community acquired pneumonia (CAP) identified during the 12-month study, by quarters.

The proportion of total pneumonia admissions accounted for by each etiology for each quartile is shown. Influenza during the first and second quartiles was caused by seasonal influenza H3N2 whereas all influenza cases during the third and fourth quartiles were pandemic influenza (H1N1). Less frequently encountered pathogens listed as “other” included M. catarrhalis, S. aureus, C. pneumoniae, Legionella species, P. aeruginosa as well as various streptococcal species.