Introduction: In June 2009, the World Health Organization declared an influenza pandemic associated with the pandemic (H1N1) 2009 strain. It was summer in the northern hemisphere, and therefore travelling and vacation time, which also provided an increased opportunity for the dissemination of respiratory diseases.
Methodology: We reviewed the paper case report forms from all the patients with influenza-like illnesses with nasopharyngeal samples submitted for laboratory diagnosis of pandemic (H1N1) 2009 infection during the first wave of pandemic influenza that occurred between June and August 2009, in the central region of Portugal.
Results: From all the patients with influenza-like illnesses, one third was found positive for pandemic (H1N1) 2009. Individuals under the age of 29 (75%) were the most affected. Most of the patients (91%) presented with fever. A group of symptoms were positively correlated with the probability of pandemic (H1N1) 2009 infection: cough, epistaxis, lack of dyspnea or vomiting, fever, headache and myalgia.
Conclusions: During the first wave of the pandemic influenza, young individuals were the most affected, and in the ambulatory setting, presentation was of a mild febrile illness without complications.