[Main features of the new influenza virus a pandemic (H1N1)].

At the end of March 2009, a new influenza virus A (H1N1) of porcine origin was isolated in two children from California presenting flu-like clinical syndrome. This virus was initially disseminated in Mexico and US and then worldwide. Eight weeks later, it had reached 74 countries with almost 30,000 cases and had caused 145 deaths. The virus had also sustained community transmission in 6 countries. On June 11th, WHO stated the onset of a pandemic. The genetic combination of this virus is completely new, containing five segments of porcine origin, two avian, one human and a HA hemaglutinin adapted for human transmission, which is genetically and antigenically different compared with the H1N1 seasonal virus. Its transmissibility is slightly higher than the one observed in seasonal influenza and similar to previous pandemics. Its pathogenicity and virulence are low. Clinical manifestations are similar to seasonal influenza, with spontaneous resolution. Nevertheless, the variety of symptoms is large and range from asymptomatic to severe fatal pneumonia. The affected population is mainly young, aged under 30 years. Less than a half of the hospitalized patients in US and of the fatal cases in Mexico had concomitant chronic diseases or other baseline conditions. A specific monovalent vaccine against the virus is currently being produced in order to prevent and control the infection through the reduction of susceptible population.