Clinical and microbiologic features associated with novel swine-origin influenza A pandemic 2009 (H1N1) virus in children: a prospective cohort study.

BACKGROUND

Novel swine-origin influenza A pandemic 2009 (H1N1) virus (S-OIV) infection in the context of other respiratory viruses circulating in winter has not been studied.

METHODS

Clinical and microbiologic data were collected prospectively from 444 consecutive patients presenting with an influenza-like illness (ILI) to a large pediatric hospital at the beginning of the S-OIV outbreak in Australia.

RESULTS

Of 444 patients, 119 had polymerase chain reaction-confirmed S-OIV. Influenza A virus was detected by direct immunofluorescence in only 69 of these. Overall, inadequate respiratory samples were more common with rayon than flocked swabs (P = 0.01). The mean age of patients with S-OIV was higher than those with another cause of an ILI (10.2 vs. 6.4 years; P < 0.0001). The commonest symptoms in S-OIV were fever (93%) and cough (92%), followed by coryza (78%), sore throat (72%), headache (59%), myalgia (49%), vomiting (23%), and diarrhea (16%). Clinical features did not discriminate between patients with S-OIV and those with another ILI, except headache and myalgia, which were more common in children younger than 5 years who had S-OIV than those who did not (headache: P < 0.0001; myalgia: P = 0.0004). More patients with S-OIV had contact with a confirmed case but contact history had insufficient positive predictive value (44%) and negative predictive value (78%) for identifying S-OIV. Only 2% of the patients had a history of travel, and only 1 of these had S-OIV.

CONCLUSIONS

A clinical case definition is unlikely to be useful for discriminating patients with S-OIV from those with another cause of an ILI during winter. Direct immunofluorescence for influenza A cannot be used alone to reliably detect S-OIV.