Influenza A H1N1 (2009): clinical spectrum of disease among adult patients admitted to a regional hospital in Singapore.

INTRODUCTION: The worldwide spread of Influenza A H1N1 (2009) has proceeded at an unprecedented rate, with the World Health Organization rapidly raising its influenza pandemic alert to phase six. We describe the disease spectrum of H1N1 (2009) to aid the triaging and identification of patients at risk. METHODS: This is a retrospective chart review of all confirmed H1N1 (2009) cases admitted to our institution between June and September 2009. RESULTS: The disease severity of the 153 patients studied was classified as mild (n is 75), moderate (n is 55) and severe (n is 23). 81 patients were female. The median age was 26 years. While comorbidities were more prevalent among patients with moderate-severe illness, 47.4 percent reported no pre-existing illness. Presenting complaints of breathlessness, tachycardia, low-pulse oximetry, higher leukocyte counts and C-reactive protein with low albumin levels were more commonly noted in moderate-severe illness (p-value less than 0.001). All patients received oseltamivir at a median of four days from illness onset. 18 required intensive care unit admission, with the majority (94.4 percent) within the first 24 hours of hospitalisation. The overall mortality rate was 4.6 percent. Median lengths of hospitalisation were four and nine days for moderate and severe cases, respectively. CONCLUSION: While the majority of H1N1 (2009) patients have mild illness, a subgroup can become critically ill. Prior good health is not necessarily a good discriminator against severe illness. The presence of dyspnoea, tachycardia and desaturation at triage should heighten the index of suspicion for H1N1 (2009)-related complications.