Background: Two waves of influenza (H1N1) 2009 were produced in Navarre in 2009, one in the summer and the other in the fall. We aim to compare the characteristics of the two epidemic waves.
Methods: We analysed individual influenza reports, virological confirmations in the primary care sentinel network, and hospitalised cases with confirmed influenza in Navarre. We compared the summer period (week 21 to 39 in 2009) with the fall-winter period (week 40 in 2009 to 20 in 2010).
Results: Two waves of influenza A(H1N1)2009 occurred during 2009, with peaks in July and November. In the summer (week 21 to 39) 4389 cases of influenza syndrome were reported, with young adults the most affected group (58% aged 15-44 years). The highest incidence was registered after the San Fermin fiesta (92 cases per 100,000 population in week 29), with immediate return to baseline levels. A second wave occurred in the fall, with rates up to 7 times higher (667 cases per 100,000 in week 45); they remained above the epidemic threshold for 9 weeks, with children aged 5-14 years (111 per 1000) the most affected group. In the peak of both waves the percentage of smears confirmed for influenza reached 60%. During the summer there were 66 admissions with confirmed influenza (H1N1) 2009, and 158 than in the fall. The proportion of cases requiring admission to hospital was higher in the summer (1.5%) than in fall (0.8%: p<0,0001).
Conclusion: Influenza circulation was much lower in the summer. Serious cases occurred in periods of both high and low incidence of influenza syndromes.