Concurrent comparison of epidemiology, clinical presentation and outcome between adult patients suffering from the pandemic influenza A (H1N1) 2009 virus and the seasonal influenza A virus infection

Abstract

Purpose of study: The demographics, clinical features and outcome of patients with pandemic influenza A (H1N1) 2009 infection were compared with a concurrent cohort of patients with seasonal influenza A infection.

Study design: The clinical and microbiological data of hospitalised adult patients admitted between 29 June and 28 October 2009, with pandemic A (H1N1) 2009 or seasonal influenza A infection, were analysed.

Results: A total of 186 patients including 69 pandemic A (H1N1) and 117 seasonal influenza were analysed. The majority (75%) under 50 years of age had pandemic A (H1N1). Compared with seasonal influenza, pandemic A (H1N1) patients were younger (median age 47 years vs 76 years, p<0.001), less likely to have lower respiratory tract symptoms (46.4% vs 66.7%, p=0.007), but more likely to be obese (5.8% vs 0%, p=0.018), pregnant (7.2% vs 0.9%, p=0.027) or have no underlying predisposing factors (24.6% vs 5.1%, p<0.001). Patients with pandemic A (H1N1) were more likely to receive oseltamivir (91.3% vs 40.2%, p<0.001), but less likely to receive antibiotics (75.4% vs 90.6%, p=0.005). Respiratory failure was the reason for intensive care unit admission for all four patients with pandemic A (H1N1), but only for one of three patients with seasonal influenza. There were no statistical significant differences in the rate of intensive care unit admission or death.

Conclusions: In addition to age, several clinical parameters were different between pandemic A (H1N1) and seasonal influenza. However, since both seasonal and pandemic influenza can lead to significant morbidity and mortality, the impact of pre-existing seasonal influenza should not be underestimated during the pandemic period.

Figure 1

Age distribution of hospitalised patients infected with pandemic influenza A (H1N1) virus and seasonal influenza virus.

Figure 2

Chest radiograph of a patient with good past health, showing right lower zone consolidation.

Figure 3

Chest radiographs of a patient admitted for diabetic ketoacidosis. (A) Chest radiograph was taken on admission, showing early left lower zone infiltration. (B) Rapid progression to bilateral diffuse panlobar involvement (chest radiograph taken 20 h after the radiograph shown in panel A).

Figure 4

Chest radiographs of a patient with congestive heart failure and end stage renal failure due to diabetic nephropathy. (A) Chest radiograph taken on admission, showing acute pulmonary oedema. (B) Persistent right lower zone infiltrates after diuresis.