FROM SWINE TO HUMANS: THE A (H1N1) 2009 INFLUENZA PANDEMIC

On April 25(th) 2009, Mexico notified the WHO of an outbreak of an influenza-like disease. The pathogen responsible was rapidly identified as an influenza virus of swine origin, the A (H1N1) 2009 influenza virus. The virus quickly spread worldwide, leading the WHO to announce a phase 6 pandemic alert on June 11(th) 2009. The majority of cases were mild, but 2 to 10 % of cases required hospitalization for respiratory distress, especially in children under 5, pregnant women, and patients with cardiovascular and respiratory diseases, diabetes or immunosuppression. By April 11(th) 2010, 17,798 deaths were officially notified to the WHO, a very low number compared to the usual death toll of seasonal influenza. This low death rate is due to the presence of immune resistance in the population over 60, a most likely consequence of sequence homologies between the 1918 and 2009 H1N1 pandemic viruses. Several anti-A (H1N1) 2009 vaccines were prepared in 2009, with and without adjuvants, and all were proven safe and immunogenic in Phase I/II studies carried out in volunteers. However, the vaccination campaign met little enthusiasm, for reasons which will be reported. Swine appear once more as an important potential reservoir for influenza virus strains pathogenic for man, an observation which pleads for enhanced virological surveillance in this species.